U.S. patent application number 13/750493 was filed with the patent office on 2014-07-31 for standby biasing of electrochemical sensor to reduce sensor stabilization time during measurement.
This patent application is currently assigned to Google Inc.. The applicant listed for this patent is Google Inc.. Invention is credited to Brian Otis, Nathan Pletcher.
Application Number | 20140209481 13/750493 |
Document ID | / |
Family ID | 51221752 |
Filed Date | 2014-07-31 |
United States Patent
Application |
20140209481 |
Kind Code |
A1 |
Pletcher; Nathan ; et
al. |
July 31, 2014 |
Standby Biasing Of Electrochemical Sensor To Reduce Sensor
Stabilization Time During Measurement
Abstract
An eye-mountable device includes an electrochemical sensor
embedded in a polymeric material configured for mounting to a
surface of an eye. The electrochemical sensor applies a
stabilization voltage between a working electrode and a reference
electrode to allow the amperometric current to stabilize before
powering measurement electronics configured to measure the
amperometric current and communicate the measured amperometric
current. The electrochemical sensor consumes less power while
applying the stabilization voltage than during the measurement. The
measurement is initiated in response to receiving a measurement
signal at an antenna in the eye-mountable device.
Inventors: |
Pletcher; Nathan; (Mountain
View, CA) ; Otis; Brian; (Sunnyvale, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Google Inc. |
Mountian View |
CA |
US |
|
|
Assignee: |
Google Inc.
Mountain View
CA
|
Family ID: |
51221752 |
Appl. No.: |
13/750493 |
Filed: |
January 25, 2013 |
Current U.S.
Class: |
205/777.5 ;
204/403.14 |
Current CPC
Class: |
A61B 5/0004 20130101;
A61B 5/1486 20130101; A61B 2560/0219 20130101; A61B 5/14507
20130101; G01N 27/3271 20130101; A61B 5/686 20130101; A61B
2560/0209 20130101; A61B 2560/0214 20130101; A61B 5/0031 20130101;
A61B 5/1495 20130101; A61B 5/14532 20130101; A61B 5/1459 20130101;
A61B 5/742 20130101; A61B 5/7264 20130101; A61B 5/6821 20130101;
A61B 5/14865 20130101 |
Class at
Publication: |
205/777.5 ;
204/403.14 |
International
Class: |
C12Q 1/00 20060101
C12Q001/00 |
Claims
1. A method comprising: applying a stabilization voltage between a
working electrode and a reference electrode in an eye-mountable
device, wherein the stabilization voltage is sufficient to cause an
analyte to undergo an electrochemical reaction at the working
electrode; while the stabilization voltage is being applied,
wirelessly receiving a measurement signal at an antenna in the
eye-mountable device; responsive to receiving the measurement
signal, activating measurement electronics in the eye-mountable
device to transition the measurement electronics from a standby
mode to an active mode, wherein the measurement electronics consume
more power in the active mode than in the standby mode; and during
the active mode, operating the measurement electronics to (i)
measure an amperometric current through the working electrode,
wherein the amperometric current is related to the analyte, and
(ii) wirelessly communicate the measured amperometric current via
the antenna.
2. The method according to claim 1, further comprising: after
wirelessly communicating the measured amperometric current via the
antenna, de-activating the measurement electronics to transition
the measurement electronics from the active mode to the standby
mode.
3. The method according to claim 1, further comprising
intermittently activating the measurement electronics to wirelessly
communicate a series of amperometric current values measured
through the working electrode, wherein each of the amperometric
current values is measured following a stabilization period during
which the stabilization voltage is applied to allow current through
the working electrode caused by the stabilization voltage to reach
a stable value prior to measurement.
4. The method according to claim 3, wherein the measurement
electronics are intermittently activated with a duty cycle of less
than 10 percent.
5. The method according to claim 1, further comprising: during the
active mode, applying a measurement voltage between the working
electrode and the reference electrode, wherein the measurement
voltage generates the amperometric current.
6. The method according to claim 5, wherein the stabilization
voltage is supplied by an auxiliary power supply and the
measurement voltage is supplied by a primary power supply.
7. The method according to claim 5, wherein the stabilization
voltage and the measurement voltage are approximately equal.
8. The method according to claim 5, wherein the stabilization
voltage is within 20 percent of the measurement voltage.
9. The method according to claim 1, further comprising: during the
active mode, harvesting energy from radio frequency (RF) radiation
received at the antenna to power the measurement electronics.
10. The method according to claim 1, further comprising: applying
the stabilization voltage during a stabilization period, wherein
the stabilization period has a duration sufficient to allow current
through the working electrode to reach a stable value prior to
activating the control electronics such that the amperometric
current measured by the measurement electronics is not affected by
transient variations.
11. The method according to claim 1, wherein the received
measurement signal includes radio frequency radiation for powering
an energy harvesting power supply, and wherein the method further
comprises: rectifying electrical signals on the antenna caused by
the received radio frequency radiation to thereby generate a supply
voltage; and applying the generated supply voltage to the
measurement electronics to power the measurement electronics.
12. The method according to claim 1, further comprising: prior to
receiving a subsequent measurement signal, validating the measured
amperometric current by operating the measurement electronics in
the active mode to measure a second amperometric current value
through the working electrode and wirelessly communicate the second
amperometric current value via the antenna.
13. A method comprising: during a stabilization period, wirelessly
transmitting, by a reader, a stabilization signal to an
eye-mountable device comprising a working electrode, stabilization
electronics, measurement electronics, and an antenna, wherein the
stabilization signal is configured to cause the stabilization
electronics to apply a stabilization voltage between the working
electrode and the reference electrode, wherein the stabilization
voltage is sufficient to cause an analyte to undergo an
electrochemical reaction at the working electrode; during a
measurement period following the stabilization period, wirelessly
transmitting, by the reader, a measurement signal to the
eye-mountable device, wherein the measurement signal is configured
to (i) cause the measurement electronics to measure an amperometric
current through the working electrode, wherein the amperometric
current is related to the analyte, (ii) cause the measurement
electronics to wirelessly communicate the measured amperometric
current via the antenna, and (iii) supply power for powering the
measurement electronics; and receiving, by the reader, an
indication of the measured amperometric current wirelessly
communicated from the eye-mountable device.
14. The method according to claim 13, further comprising: during an
idle period following the measurement period, the reader
discontinuing transmission of wireless signals to the eye-mountable
device.
15. The method according to claim 13, further comprising:
obtaining, by the reader, a series of amperometric current values
wirelessly communicated from the eye-mountable device, by:
transmitting a stabilization signal to the eye-mountable device to
cause the stabilization electronics to apply the stabilization
voltage between the working electrode and the reference electrode;
transmitting a measurement signal to the sensing platform to cause
the measurement electronics to measure an amperometric current
through the working electrode and wirelessly communicate the
measured amperometric current; and receiving an indication of the
measured amperometric current; and wherein each transmission of the
measurement signal immediately follows a transmission of the
stabilization signal such that each of the series of amperometric
current values is measured following a stabilization period during
which the stabilization voltage is applied between the working
electrode and the reference electrode to allow current through the
working electrode caused by the stabilization voltage to reach a
stable value prior to measurement.
16. The method according to claim 15, wherein the measurement
signal is transmitted with a duty cycle of less than 10
percent.
17. The method according to claim 13, further comprising:
receiving, by the reader, an indication of a second measured
amperometric current obtained during the measurement period;
comparing the values of the two measured amperometric currents; and
determining, based on the comparison, if the current through the
working electrode of the electrochemical sensor reached a stable
value prior to the measurement period.
18. An eye-mountable device comprising: a transparent polymeric
material having a concave surface and a convex surface, wherein the
concave surface is configured to be removably mounted over a
corneal surface and the convex surface is configured to be
compatible with eyelid motion when the concave surface is so
mounted; an antenna; an electrochemical sensor that includes a
working electrode and a reference electrode; stabilization
electronics operable to apply a stabilization voltage between the
working electrode and the reference electrode, wherein the
stabilization voltage is sufficient to cause an analyte to undergo
an electrochemical reaction at the working electrode; measurement
electronics, wherein the measurement electronics, when activated,
are configured to: (i) apply a measurement voltage between the
working electrode and the reference electrode, (ii) measure an
amperometric current through the working electrode, wherein the
amperometric current is related to the analyte, and (iii) use the
antenna to communicate the measured amperometric current; and a
controller, wherein the controller is configured to operate the
stabilization electronics to apply the stabilization voltage during
a stabilization period and to activate the measurement electronics
during a measurement period in response to receiving a measurement
signal via the antenna.
19. The device according to claim 18, wherein the controller is
further configured to de-activate the measurement electronics
during the stabilization period.
20. The device according to claim 18, further comprising a power
supply configured to harvest radiation incident on the antenna to
power the measurement electronics during the measurement
period.
21. The device according to claim 20, wherein the power supply
includes a rectifier configured to harvest energy from electrical
signals on the antenna caused by incident radio frequency
radiation.
22. The device according to claim 20, further comprising an
auxiliary power supply configured to power the stabilization
electronics during the stabilization period.
23. The device according to claim 22, wherein the auxiliary power
supply includes a photovoltaic cell configured to harvest energy
from incident light.
Description
BACKGROUND
[0001] Unless otherwise indicated herein, the materials described
in this section are not prior art to the claims in this application
and are not admitted to be prior art by inclusion in this
section.
[0002] An electrochemical amperometric sensor measures a
concentration of an analyte by measuring a current generated
through electrochemical oxidation or reduction reactions of the
analyte at a working electrode of the sensor. A reduction reaction
occurs when electrons are transferred from the electrode to the
analyte, whereas an oxidation reaction occurs when electrons are
transferred from the analyte to the electrode. The direction of the
electron transfer is dependent upon the electrical potentials
applied to the working electrode. A counter electrode and/or
reference electrode is used to complete a circuit with the working
electrode and allow the generated current to flow. When the working
electrode is appropriately biased, the output current can be
proportional to the reaction rate, so as to provide a measure of
the concentration of the analyte surrounding the working
electrode.
[0003] In some examples, a reagent is localized proximate the
working electrode to selectively react with a desired analyte. For
example, glucose oxidase can be fixed near the working electrode to
react with glucose and release hydrogen peroxide, which is then
electrochemically detected by the working electrode to indicate the
presence of glucose. Other enzymes and/or reagents can be used to
detect other analytes.
SUMMARY
[0004] Some embodiments of the present disclosure provide a method
including applying a stabilization voltage between a working
electrode and a reference electrode in an eye-mountable device. The
stabilization voltage can be sufficient to cause an analyte to
undergo an electrochemical reaction at the working electrode. The
method can include wirelessly receiving a measurement signal at an
antenna in the eye-mountable device while the stabilization voltage
is being applied. The method can include activating measurement
electronics in the eye-mountable device to transition the
measurement electronics from a standby mode to an active mode
responsive to receiving the measurement signal. The measurement
electronics can consume more power in the active mode than in the
standby mode. The method can include, during the active mode,
operating the measurement electronics to (i) measure an
amperometric current through the working electrode, wherein the
amperometric current is related to the analyte, and (ii) wirelessly
communicate the measured amperometric current via the antenna.
[0005] Some embodiments of the present disclosure provide a method
including wirelessly transmitting a stabilization signal by a
reader during a stabilization period. The stabilization signal can
be transmitted to an eye-mountable device comprising a working
electrode, stabilization electronics, measurement electronics, and
an antenna. The stabilization signal can be configured to cause the
stabilization electronics to apply a stabilization voltage between
the working electrode and the reference electrode. The
stabilization voltage can be sufficient to cause an analyte to
undergo an electrochemical reaction at the working electrode. The
method can include a measurement signal by a reader during a
measurement period following the stabilization period. The
measurement signal can be transmitted to the eye-mountable device.
The measurement signal can be configured to (i) cause the
measurement electronics to measure an amperometric current through
the working electrode, (ii) cause the measurement electronics to
wirelessly communicate the measured amperometric current via the
antenna, and (iii) supply power for powering the measurement
electronics. The amperometric current can be related to the
analyte. The method can include receiving an indication of the
measured amperometric current by the reader. The indication of the
measured amperometric current can be wirelessly communicated from
the eye-mountable device.
[0006] Some embodiments of the present disclosure provide an
eye-mountable device including a transparent polymeric material, an
antenna, an electrochemical sensor, stabilization electronics,
measurement electronics, and a controller. The transparent
polymeric material can have a concave surface and a convex surface.
The concave surface can be configured to be removably mounted over
a corneal surface and the convex surface can be configured to be
compatible with eyelid motion when the concave surface is so
mounted. The electrochemical sensor can include a working electrode
and a reference electrode. The stabilization electronics can be
operable to apply a stabilization voltage between the working
electrode and the reference electrode. The stabilization voltage
can be sufficient to cause an analyte to undergo an electrochemical
reaction at the working electrode. The measurement electronics can
be configured, when activated, to (i) apply a measurement voltage
between the working electrode and the reference electrode, (ii)
measure an amperometric current through the working electrode, and
(iii) use the antenna to communicate the measured amperometric
current. The amperometric current can be related to the analyte.
The controller can be configured to operate the stabilization
electronics to apply the stabilization voltage during a
stabilization period and to activate the measurement electronics
during a measurement period in response to receiving a measurement
signal via the antenna.
[0007] These as well as other aspects, advantages, and
alternatives, will become apparent to those of ordinary skill in
the art by reading the following detailed description, with
reference where appropriate to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a block diagram of an example system that includes
an eye-mountable device in wireless communication with an external
reader.
[0009] FIG. 2A is a bottom view of an example eye-mountable
device.
[0010] FIG. 2B is a side view of the example eye-mountable device
shown in FIG. 2A.
[0011] FIG. 2C is a side cross-section view of the example
eye-mountable device shown in FIGS. 2A and 2B while mounted to a
corneal surface of an eye.
[0012] FIG. 2D is a side cross-section view enhanced to show the
tear film layers surrounding the surfaces of the example
eye-mountable device when mounted as shown in FIG. 2C.
[0013] FIG. 3 is a functional block diagram of an example system
for electrochemically measuring a tear film analyte
concentration.
[0014] FIG. 4A is a flowchart of an example process for operating
an amperometric sensor in an eye-mountable device to measure a tear
film analyte concentration.
[0015] FIG. 4B is a flowchart of an example process for operating
an external reader to interrogate an amperometric sensor in an
eye-mountable device to measure a tear film analyte
concentration.
[0016] FIG. 5A is a functional block diagram of an example
electrochemical sensor system including a dual mode power
supply.
[0017] FIG. 5B is a flowchart of an example process for operating
the example electrochemical sensor of FIG. 5A with a startup bias
mode prior to obtaining a measurement.
[0018] FIG. 5C is a functional block diagram of the example
electrochemical sensor shown in FIG. 5A operating in standby
mode.
[0019] FIG. 5D is a functional block diagram of the example
electrochemical sensor shown in FIG. 5A operating in active
mode.
[0020] FIGS. 6A-6D illustrate sensor voltage, sensor current,
electronics supply voltages, and power consumption for an example
measurement cycle.
[0021] FIGS. 7A-7E illustrate sensor voltage, sensor current,
electronics supply voltages, incident radiation, and power
consumption for an example repeated measurement cycle.
[0022] FIG. 8A is a functional block diagram of an example
electrochemical sensor system including a measurement electronics
power supply and a standby bias power supply.
[0023] FIG. 8B is a functional block diagram of an example
embodiment where the standby bias power supply includes a
photovoltaic cell.
[0024] FIG. 8C is a flowchart of an example process for operating
the example electrochemical sensor of FIG. 8A with a startup bias
mode prior to obtaining a measurement.
[0025] FIGS. 9A-9E illustrate sensor voltage, sensor current,
electronics supply voltages, incident radiation, and power
consumption for an example repeated measurement cycle.
[0026] FIG. 10A is a block diagram of an ophthalmic electrochemical
sensor system operated by an external reader to obtain a series of
amperometric current measurements over time.
[0027] FIG. 10B is a block diagram of the ophthalmic
electrochemical sensor system described in connection with FIG.
10A.
[0028] FIG. 10C is a flowchart of an example process for operating
the ophthalmic electrochemical sensor system shown in FIG. 10A.
[0029] FIG. 11 depicts a computer-readable medium configured
according to an example embodiment.
DETAILED DESCRIPTION
[0030] In the following detailed description, reference is made to
the accompanying figures, which form a part hereof. In the figures,
similar symbols typically identify similar components, unless
context dictates otherwise. The illustrative embodiments described
in the detailed description, figures, and claims are not meant to
be limiting. Other embodiments may be utilized, and other changes
may be made, without departing from the scope of the subject matter
presented herein. It will be readily understood that the aspects of
the present disclosure, as generally described herein, and
illustrated in the figures, can be arranged, substituted, combined,
separated, and designed in a wide variety of different
configurations, all of which are explicitly contemplated
herein.
[0031] I. Overview
[0032] An ophthalmic sensing platform or implantable sensing
platform can include a sensor, control electronics and an antenna
all situated on a substrate embedded in a polymeric material. The
polymeric material can be incorporated in an ophthalmic device,
such as an eye-mountable device or an implantable medical device.
The control electronics can operate the sensor to perform readings
and can operate the antenna to wirelessly communicate the readings
from the sensor to an external reader via the antenna.
[0033] In some examples, the polymeric material can be in the form
of a round lens with a concave curvature configured to mount to a
corneal surface of an eye. The substrate can be embedded near the
periphery of the polymeric material to avoid interference with
incident light received closer to the central region of the cornea.
The sensor can be arranged on the substrate to face inward, toward
the corneal surface, so as to generate clinically relevant readings
from near the surface of the cornea and/or from tear fluid
interposed between the polymeric material and the corneal surface.
Additionally or alternatively, the sensor can be arranged on the
substrate to face outward, away from the corneal surface and toward
the layer of tear fluid coating the surface of the polymeric
material exposed to the atmosphere. In some examples, the sensor is
entirely embedded within the polymeric material. For example, an
electrochemical sensor that includes a working electrode and a
reference electrode can be embedded in the polymeric material and
situated such that the sensor electrodes are less than 10
micrometers from the polymeric surface configured to mount to the
cornea. The sensor can generate an output signal indicative of a
concentration of an analyte that diffuses through the lens material
to the sensor electrodes.
[0034] The ophthalmic sensing platform can be powered via radiated
energy harvested at the sensing platform. Power can be provided by
light energizing photovoltaic cells included on the sensing
platform. Additionally or alternatively, power can be provided by
radio frequency energy harvested from the antenna. A rectifier
and/or regulator can be incorporated with the control electronics
to generate a stable DC voltage to power the sensing platform from
the harvested energy. The antenna can be arranged as a loop of
conductive material with leads connected to the control
electronics. In some embodiments, such a loop antenna can also
wirelessly communicate the sensor readings to an external reader by
modifying the impedance of the loop antenna so as to modify
backscatter radiation from the antenna.
[0035] Tear fluid contains a variety of inorganic electrolytes
(e.g., Ca.sup.2+, Mg.sup.2+, Cl.sup.-), organic components (e.g.,
glucose, lactate, proteins, lipids, etc.), and so on that can be
used to diagnose health states. An ophthalmic sensing platform
configured to measure one or more of these analytes can thus
provide a convenient non-invasive platform useful in diagnosing
and/or monitoring health states. For example, an ophthalmic sensing
platform can be configured to sense glucose and can be used by
diabetic individuals to measure/monitor their glucose levels.
[0036] In some embodiments of the present disclosure, when a
voltage is first applied to electrodes in an electrochemical
sensor, a large initial amperometric current may be generated due
to build-up of analyte at the electrode during the time that no
voltage is applied. Once the initial analyte build-up is consumed,
the electrochemical reaction rate settles at a steady state value
(e.g., where analyte diffusion compensates for electrochemical
analyte consumption), at which point the reaction rate is
approximately proportionate to the analyte concentration. Thus,
when non-continuously (i.e., intermittently) sampling an analyte
concentration, each reading may require a stabilization time to
pass before the amperometric current settles at the steady state
value.
[0037] Some embodiments of the present disclosure therefore provide
systems and methods for intermittently sampling an electrochemical
sensor by first applying voltage to electrochemical sensor
electrodes to allow the current to stabilize, then reading the
current. Such an intermittent measurement scheme reduces total
power consumption, because measurement electronics are only powered
while a measurement reading is being performed, and not during the
initial stabilization period. During the stabilization period, a
voltage is applied across the electrochemical sensor electrodes
without also powering the measurement electronics. In some
examples, a single power supply system can operate in both a high
power setting (during the measurement) and a low setting (during
the stabilization). In other examples, two separate power supplies
can power the electrochemical sensor in a high power setting and
low power setting, respectively. For example, a first power supply
can apply voltage between the sensor electrodes during the
stabilization period preceding a measurement. A second power
supply, alone or in combination with the first power supply, can
then power the measurement electronics during a measurement event
to both sense the stabilized amperometric current and communicate
the results.
[0038] The techniques described herein for intermittently measuring
a pre-stabilized amperometric current from pre-charged sensor
electrodes reduces total power consumption in an electrochemical
sensor, relative to a system that powers measurement electronics
while the sensor current reaches a stable value. The technique can
be employed in applications with strict power budgets, such as in
implantable medical devices or in electrochemical sensors included
in an eye-mountable device.
[0039] The sensing platform can be powered by an energy harvesting
system to capture energy from incident radiation, rather than by
internal energy storage devices requiring more space. For example,
power can be provided by light energizing photovoltaic cells
included on the sensing platform. Power may also be provided by
radio frequency (RF) energy harvested via a loop antenna. A
rectifier and/or regulator can be incorporated with the control
electronics to generate a stable DC voltage to power the sensing
platform from the harvested RF energy. Furthermore, the control
electronics can wirelessly communicate the sensor readings to an
external reader by modifying the impedance of the loop antenna so
as to characteristically modify the backscatter from the
antenna.
[0040] In addition to the DC voltage for powering the control
electronics (i.e., the measurement and communication circuitry),
the energy harvesting system can also generate a voltage to apply
to the sensor electrodes of the electrochemical sensor without also
powering the measurement electronics. The voltage applied to the
sensor electrodes may be referred to as a stabilization voltage and
may be used to pre-charge the sensor electrodes prior to performing
an amperometric current measurement. The eventual amperometric
current measurement thereby avoids the transient effects in the
amperometric current described above that occur immediately after
applying voltage across the sensor electrodes.
[0041] The energy harvesting system can therefore operate in a
standby mode in which the stabilization voltage is applied across
sensor electrodes without also powering the control electronics.
The system can also operate in a measurement mode in which a DC
voltage is supplied to the control electronics to cause the sensing
platform to perform a current measurement and communicate the
result. During the high power mode, the voltage across the sensor
electrodes can be maintained through the control electronics, such
as by a potentiostat that simultaneously applies a voltage across
the electrodes and measures the resulting amperometric current
through the working electrode. Moreover, the energy harvesting
system may include multiple energy harvesting devices, with one
dedicated to providing a stabilization voltage and another
dedicated to powering the sensing platform for measurement and
communication. For example, an antenna may be used to harvest
energy from incident radio frequency radiation and a photovoltaic
cell may be used to harvest energy from incident light. In one
example, the photovoltaic cell may be used to provide the
stabilization voltage during the low power mode, and the antenna
may be used to power the control electronics (e.g., a potentiostat
and a backscatter communication circuit) during the high power
mode.
[0042] An external reader can radiate radio frequency radiation to
power the sensor via the energy harvesting system. The external
reader may thereby control the operation of the sensing platform by
controlling the supply of power to the sensing platform. In some
examples, the external reader can operate to intermittently
interrogate the sensing platform to provide a reading by radiating
sufficient radiation to power the sensing platform to obtain a
measurement and communicate the result. The external reader can
also store the sensor results communicated by the sensing platform.
In this way, the external reader can acquire a series of analyte
concentration measurements over time without continuously powering
the sensing platform.
[0043] In some embodiments of the present disclosure, the external
reader is configured to cause the sensing platform to operate
according to the sensor electrode pre-charge technique described
herein. For example, the external reader can first send a
stabilization signal to the sensing platform to initiate a low
power stabilization mode, then send a measurement signal to the
sensing platform to initiate a measurement. Following the
measurement, the external reader may cease radiating entirely, and
the sensing platform can enter an idle mode until the external
reader sends the next stabilization signal to pre-charge the
sensor.
[0044] In examples where the sensing platform enters the
stabilization mode and the measurement mode in response to control
signals from the external reader, the duration of the stabilization
mode may be controlled by the external reader. That is, the period
during which the sensor electrodes are pre-charged prior to
obtaining a measurement can be controlled by the external reader by
adjusting the time between the initiation of the stabilization mode
and the initiation of the measurement mode. The reader can thus set
the stabilization period to allow sufficient time for the
amperometric current to reach a steady value and may be determined
based on the duty cycle of the system, the duration of an idle
period between subsequent measurement modes, and/or empirically
determined factors.
[0045] In some examples, the sensing platform can communicate two
sensor readings to allow the reader to determine whether the
amperometric current is at a steady value. For example, if the two
sensor readings are approximately equal, the reader may determine
that the measured current is at its steady value, and therefore the
reading can be used to estimate the analyte concentration level.
The reader may also conclude that the duration of the stabilization
time preceding such a steady-state reading was sufficient to allow
the current to reach a stable value and may therefore employ a
similar duration under similar circumstances. On the other hand, if
the two sensor readings are not approximately equal, the reader may
determine that the measured current is still undergoing transient
variations and has not yet reached a steady state level. In such a
case, the reader may conclude that the duration of the
stabilization time was insufficient to allow the current to reach a
stable value.
[0046] II. Example Ophthalmic Electronics Platform
[0047] FIG. 1 is a block diagram of a system 100 that includes an
eye-mountable device 110 in wireless communication with an external
reader 180. The exposed regions of the eye-mountable device 110 are
made of a polymeric material 120 formed to be contact-mounted to a
corneal surface of an eye. A substrate 130 is embedded in the
polymeric material 120 to provide a mounting surface for a power
supply 140, a controller 150, bio-interactive electronics 160, and
a communication antenna 170. The bio-interactive electronics 160
are operated by the controller 150. The power supply 140 supplies
operating voltages to the controller 150 and/or the bio-interactive
electronics 160. The antenna 170 is operated by the controller 150
to communicate information to and/or from the eye-mountable device
110. The antenna 170, the controller 150, the power supply 140, and
the bio-interactive electronics 160 can all be situated on the
embedded substrate 130. Because the eye-mountable device 110
includes electronics and is configured to be contact-mounted to an
eye, it is also referred to herein as an ophthalmic electronics
platform.
[0048] To facilitate contact-mounting, the polymeric material 120
can have a concave surface configured to adhere ("mount") to a
moistened corneal surface (e.g., by capillary forces with a tear
film coating the corneal surface). Additionally or alternatively,
the eye-mountable device 110 can be adhered by a vacuum force
between the corneal surface and the polymeric material due to the
concave curvature. While mounted with the concave surface against
the eye, the outward-facing surface of the polymeric material 120
can have a convex curvature that is formed to not interfere with
eye-lid motion while the eye-mountable device 110 is mounted to the
eye. For example, the polymeric material 120 can be a substantially
transparent curved polymeric disk shaped similarly to a contact
lens.
[0049] The polymeric material 120 can include one or more
biocompatible materials, such as those employed for use in contact
lenses or other ophthalmic applications involving direct contact
with the corneal surface. The polymeric material 120 can optionally
be formed in part from such biocompatible materials or can include
an outer coating with such biocompatible materials. The polymeric
material 120 can include materials configured to moisturize the
corneal surface, such as hydrogels and the like. In some
embodiments, the polymeric material 120 can be a deformable
("non-rigid") material to enhance wearer comfort. In some
embodiments, the polymeric material 120 can be shaped to provide a
predetermined, vision-correcting optical power, such as can be
provided by a contact lens.
[0050] The substrate 130 includes one or more surfaces suitable for
mounting the bio-interactive electronics 160, the controller 150,
the power supply 140, and the antenna 170. The substrate 130 can be
employed both as a mounting platform for chip-based circuitry
(e.g., by flip-chip mounting to connection pads) and/or as a
platform for patterning conductive materials (e.g., gold, platinum,
palladium, titanium, copper, aluminum, silver, metals, other
conductive materials, combinations of these, etc.) to create
electrodes, interconnects, connection pads, antennae, etc. In some
embodiments, substantially transparent conductive materials (e.g.,
indium tin oxide) can be patterned on the substrate 130 to form
circuitry, electrodes, etc. For example, the antenna 170 can be
formed by forming a pattern of gold or another conductive material
on the substrate 130 by deposition, photolithography,
electroplating, etc. Similarly, interconnects 151, 157 between the
controller 150 and the bio-interactive electronics 160, and between
the controller 150 and the antenna 170, respectively, can be formed
by depositing suitable patterns of conductive materials on the
substrate 130. A combination of microfabrication techniques
including, without limitation, the use of photoresists, masks,
deposition techniques, and/or plating techniques can be employed to
pattern materials on the substrate 130. The substrate 130 can be a
relatively rigid material, such as polyethylene terephthalate
("PET") or another material configured to structurally support the
circuitry and/or chip-based electronics within the polymeric
material 120. The eye-mountable device 110 can alternatively be
arranged with a group of unconnected substrates rather than a
single substrate. For example, the controller 150 and a bio-sensor
or other bio-interactive electronic component can be mounted to one
substrate, while the antenna 170 is mounted to another substrate
and the two can be electrically connected via the interconnects
157.
[0051] In some embodiments, the bio-interactive electronics 160
(and the substrate 130) can be positioned away from the center of
the eye-mountable device 110 and thereby avoid interference with
light transmission to the central, light-sensitive region of the
eye. For example, where the eye-mountable device 110 is shaped as a
concave-curved disk, the substrate 130 can be embedded around the
periphery (e.g., near the outer circumference) of the disk. In some
embodiments, however, the bio-interactive electronics 160 (and the
substrate 130) can be positioned in or near the central region of
the eye-mountable device 110. Additionally or alternatively, the
bio-interactive electronics 160 and/or substrate 130 can be
substantially transparent to incoming visible light to mitigate
interference with light transmission to the eye. Moreover, in some
embodiments, the bio-interactive electronics 160 can include a
pixel array 164 that emits and/or transmits light to be received by
the eye according to display instructions. Thus, the
bio-interactive electronics 160 can optionally be positioned in the
center of the eye-mountable device so as to generate perceivable
visual cues to a wearer of the eye-mountable device 110, such as by
displaying information (e.g., characters, symbols, flashing
patterns, etc.) on the pixel array 164.
[0052] The substrate 130 can be shaped as a flattened ring with a
radial width dimension sufficient to provide a mounting platform
for the embedded electronics components. The substrate 130 can have
a thickness sufficiently small to allow the substrate 130 to be
embedded in the polymeric material 120 without influencing the
profile of the eye-mountable device 110. The substrate 130 can have
a thickness sufficiently large to provide structural stability
suitable for supporting the electronics mounted thereon. For
example, the substrate 130 can be shaped as a ring with a diameter
of about 10 millimeters, a radial width of about 1 millimeter
(e.g., an outer radius 1 millimeter larger than an inner radius),
and a thickness of about 50 micrometers. The substrate 130 can
optionally be aligned with the curvature of the eye-mounting
surface of the eye-mountable device 110 (e.g., convex surface). For
example, the substrate 130 can be shaped along the surface of an
imaginary cone between two circular segments that define an inner
radius and an outer radius. In such an example, the surface of the
substrate 130 along the surface of the imaginary cone defines an
inclined surface that is approximately aligned with the curvature
of the eye mounting surface at that radius.
[0053] The power supply 140 is configured to harvest ambient energy
to power the controller 150 and bio-interactive electronics 160.
For example, a radio-frequency energy-harvesting antenna 142 can
capture energy from incident radio radiation. Additionally or
alternatively, solar cell(s) 144 ("photovoltaic cells") can capture
energy from incoming ultraviolet, visible, and/or infrared
radiation. Furthermore, an inertial power scavenging system can be
included to capture energy from ambient vibrations. The energy
harvesting antenna 142 can optionally be a dual-purpose antenna
that is also used to communicate information to the external reader
180. That is, the functions of the communication antenna 170 and
the energy harvesting antenna 142 can be accomplished with the same
physical antenna.
[0054] A rectifier/regulator 146 can be used to condition the
captured energy to a stable DC supply voltage 141 that is supplied
to the controller 150. For example, the energy harvesting antenna
142 can receive incident radio frequency radiation. Varying
electrical signals on the leads of the antenna 142 are output to
the rectifier/regulator 146. The rectifier/regulator 146 rectifies
the varying electrical signals to a DC voltage and regulates the
rectified DC voltage to a level suitable for operating the
controller 150. Additionally or alternatively, output voltage from
the solar cell(s) 144 can be regulated to a level suitable for
operating the controller 150. The rectifier/regulator 146 can
include one or more energy storage devices to mitigate high
frequency variations in the ambient energy gathering antenna 142
and/or solar cell(s) 144. For example, one or more energy storage
devices (e.g., a capacitor, an inductor, etc.) can be connected in
parallel across the outputs of the rectifier 146 to regulate the DC
supply voltage 141 and configured to function as a low-pass
filter.
[0055] The controller 150 is turned on when the DC supply voltage
141 is provided to the controller 150, and the logic in the
controller 150 operates the bio-interactive electronics 160 and the
antenna 170. The controller 150 can include logic circuitry
configured to operate the bio-interactive electronics 160 so as to
interact with a biological environment of the eye-mountable device
110. The interaction could involve the use of one or more
components, such an analyte bio-sensor 162, in bio-interactive
electronics 160 to obtain input from the biological environment.
Additionally or alternatively, the interaction could involve the
use of one or more components, such as pixel array 164, to provide
an output to the biological environment.
[0056] In one example, the controller 150 includes a sensor
interface module 152 that is configured to operate analyte
bio-sensor 162. The analyte bio-sensor 162 can be, for example, an
amperometric electrochemical sensor that includes a working
electrode and a reference electrode. A voltage can be applied
between the working and reference electrodes to cause an analyte to
undergo an electrochemical reaction (e.g., a reduction and/or
oxidation reaction) at the working electrode. The electrochemical
reaction can generate an amperometric current that can be measured
through the working electrode. The amperometric current can be
dependent on the analyte concentration. Thus, the amount of the
amperometric current that is measured through the working electrode
can provide an indication of analyte concentration. In some
embodiments, the sensor interface module 152 can be a potentiostat
configured to apply a voltage difference between working and
reference electrodes while measuring a current through the working
electrode.
[0057] In some instances, a reagent can also be included to
sensitize the electrochemical sensor to one or more desired
analytes. For example, a layer of glucose oxidase ("GOD") proximal
to the working electrode can catalyze glucose oxidation to generate
hydrogen peroxide (H.sub.2O.sub.2). The hydrogen peroxide can then
be electro-oxidized at the working electrode, which releases
electrons to the working electrode, resulting in an amperometric
current that can be measured through the working electrode.
##STR00001##
[0058] The current generated by either reduction or oxidation
reactions is approximately proportionate to the reaction rate.
Further, the reaction rate is dependent on the rate of analyte
molecules reaching the electrochemical sensor electrodes to fuel
the reduction or oxidation reactions, either directly or
catalytically through a reagent. In a steady state, where analyte
molecules diffuse to the electrochemical sensor electrodes from a
sampled region at approximately the same rate that additional
analyte molecules diffuse to the sampled region from surrounding
regions, the reaction rate is approximately proportionate to the
concentration of the analyte molecules. The current measured
through the working electrode thus provides an indication of the
analyte concentration.
[0059] The controller 150 can optionally include a display driver
module 154 for operating a pixel array 164. The pixel array 164 can
be an array of separately programmable light transmitting, light
reflecting, and/or light emitting pixels arranged in rows and
columns. The individual pixel circuits can optionally include
liquid crystal technologies, microelectromechanical technologies,
emissive diode technologies, etc. to selectively transmit, reflect,
and/or emit light according to information from the display driver
module 154. Such a pixel array 164 can also optionally include more
than one color of pixels (e.g., red, green, and blue pixels) to
render visual content in color. The display driver module 154 can
include, for example, one or more data lines providing programming
information to the separately programmed pixels in the pixel array
164 and one or more addressing lines for setting groups of pixels
to receive such programming information. Such a pixel array 164
situated on the eye can also include one or more lenses to direct
light from the pixel array to a focal plane perceivable by the
eye.
[0060] The controller 150 can also include a communication circuit
156 for sending and/or receiving information via the antenna 170.
The communication circuit 156 can optionally include one or more
oscillators, mixers, frequency injectors, etc. to modulate and/or
demodulate information on a carrier frequency to be transmitted
and/or received by the antenna 170. In some examples, the
eye-mountable device 110 is configured to indicate an output from a
bio-sensor by modulating an impedance of the antenna 170 in a
manner that is perceivable by the external reader 180. For example,
the communication circuit 156 can cause variations in the
amplitude, phase, and/or frequency of backscatter radiation from
the antenna 170, and such variations can be detected by the reader
180.
[0061] The controller 150 is connected to the bio-interactive
electronics 160 via interconnects 151. For example, where the
controller 150 includes logic elements implemented in an integrated
circuit to form the sensor interface module 152 and/or display
driver module 154, a patterned conductive material (e.g., gold,
platinum, palladium, titanium, copper, aluminum, silver, metals,
combinations of these, etc.) can connect a terminal on the chip to
the bio-interactive electronics 160. Similarly, the controller 150
is connected to the antenna 170 via interconnects 157.
[0062] It is noted that the block diagram shown in FIG. 1 is
described in connection with functional modules for convenience in
description. However, embodiments of the eye-mountable device 110
can be arranged with one or more of the functional modules
("sub-systems") implemented in a single chip, integrated circuit,
and/or physical component. For example, while the
rectifier/regulator 146 is illustrated in the power supply block
140, the rectifier/regulator 146 can be implemented in a chip that
also includes the logic elements of the controller 150 and/or other
features of the embedded electronics in the eye-mountable device
110. Thus, the DC supply voltage 141 that is provided to the
controller 150 from the power supply 140 can be a supply voltage
that is provided to components on a chip by rectifier and/or
regulator components located on the same chip. That is, the
functional blocks in FIG. 1 shown as the power supply block 140 and
controller block 150 need not be implemented as physically
separated modules. Moreover, one or more of the functional modules
described in FIG. 1 can be implemented by separately packaged chips
electrically connected to one another.
[0063] Additionally or alternatively, the energy harvesting antenna
142 and the communication antenna 170 can be implemented with the
same physical antenna. For example, a loop antenna can both harvest
incident radiation for power generation and communicate information
via backscatter radiation.
[0064] The external reader 180 includes an antenna 188 (or a group
of more than one antennae) to send and receive wireless signals 171
to and from the eye-mountable device 110. The external reader 180
also includes a computing system with a processor 186 in
communication with a memory 182. The memory 182 is a non-transitory
computer-readable medium that can include, without limitation,
magnetic disks, optical disks, organic memory, and/or any other
volatile (e.g. RAM) or non-volatile (e.g. ROM) storage system
readable by the processor 186. The memory 182 can include a data
storage 183 to store indications of data, such as sensor readings
(e.g., from the analyte bio-sensor 162), program settings (e.g., to
adjust behavior of the eye-mountable device 110 and/or external
reader 180), etc. The memory 182 can also include program
instructions 184 for execution by the processor 186 to cause the
external reader 180 to perform processes specified by the
instructions 184. For example, the program instructions 184 can
cause external reader 180 to provide a user interface that allows
for retrieving information communicated from the eye-mountable
device 110 (e.g., sensor outputs from the analyte bio-sensor 162).
The external reader 180 can also include one or more hardware
components for operating the antenna 188 to send and receive the
wireless signals 171 to and from the eye-mountable device 110. For
example, oscillators, frequency injectors, encoders, decoders,
amplifiers, filters, etc. can drive the antenna 188 according to
instructions from the processor 186.
[0065] The external reader 180 can be a smart phone, digital
assistant, or other portable computing device with wireless
connectivity sufficient to provide the wireless communication link
171. The external reader 180 can also be implemented as an antenna
module that can be plugged in to a portable computing device, such
as in an example where the communication link 171 operates at
carrier frequencies not commonly employed in portable computing
devices. In some instances, the external reader 180 is a
special-purpose device configured to be worn relatively near a
wearer's eye to allow the wireless communication link 171 to
operate with a low power budget. For example, the external reader
180 can be integrated in a piece of jewelry such as a necklace,
earring, etc. or integrated in an article of clothing worn near the
head, such as a hat, headband, etc.
[0066] In an example where the eye-mountable device 110 includes an
analyte bio-sensor 162, the system 100 can be operated to monitor
the analyte concentration in tear film on the surface of the eye.
Thus, the eye-mountable device 110 can be configured as a platform
for an ophthalmic analyte bio-sensor. The tear film is an aqueous
layer secreted from the lacrimal gland to coat the eye. The tear
film is in contact with the blood supply through capillaries in the
structure of the eye and includes many biomarkers found in blood
that are analyzed to characterize a person's health condition(s).
For example, the tear film includes glucose, calcium, sodium,
cholesterol, potassium, other biomarkers, etc. The biomarker
concentrations in the tear film can be systematically different
than the corresponding concentrations of the biomarkers in the
blood, but a relationship between the two concentration levels can
be established to map tear film biomarker concentration values to
blood concentration levels. For example, the tear film
concentration of glucose can be established (e.g., empirically
determined) to be approximately one tenth the corresponding blood
glucose concentration. Although another ratio relationship and/or a
non-ratio relationship may be used. Thus, measuring tear film
analyte concentration levels provides a non-invasive technique for
monitoring biomarker levels in comparison to blood sampling
techniques performed by lancing a volume of blood to be analyzed
outside a person's body. Moreover, the ophthalmic analyte
bio-sensor platform disclosed here can be operated substantially
continuously to enable real time monitoring of analyte
concentrations.
[0067] To perform a reading with the system 100 configured as a
tear film analyte monitor, the external reader 180 can emit radio
frequency radiation 171 that is harvested to power the
eye-mountable device 110 via the power supply 140. Radio frequency
electrical signals captured by the energy harvesting antenna 142
(and/or the communication antenna 170) are rectified and/or
regulated in the rectifier/regulator 146 and a regulated DC supply
voltage 147 is provided to the controller 150. The radio frequency
radiation 171 thus turns on the electronic components within the
eye-mountable device 110. Once turned on, the controller 150
operates the analyte bio-sensor 162 to measure an analyte
concentration level. For example, the sensor interface module 152
can apply a voltage between a working electrode and a reference
electrode in the analyte bio-sensor 162. The applied voltage can be
sufficient to cause the analyte to undergo an electrochemical
reaction at the working electrode and thereby generate an
amperometric current that can be measured through the working
electrode. The measured amperometric current can provide the sensor
reading ("result") indicative of the analyte concentration. The
controller 150 can operate the antenna 170 to communicate the
sensor reading back to the external reader 180 (e.g., via the
communication circuit 156). The sensor reading can be communicated
by, for example, modulating an impedance of the communication
antenna 170 such that the modulation in impedance is detected by
the external reader 180. The modulation in antenna impedance can be
detected by, for example, backscatter radiation from the antenna
170.
[0068] In some embodiments, the system 100 can operate to
non-continuously ("intermittently") supply energy to the
eye-mountable device 110 to power the controller 150 and
electronics 160. For example, radio frequency radiation 171 can be
supplied to power the eye-mountable device 110 long enough to carry
out a tear film analyte concentration measurement and communicate
the results. For example, the supplied radio frequency radiation
can provide sufficient power to apply a potential between a working
electrode and a reference electrode sufficient to induce
electrochemical reactions at the working electrode, measure the
resulting amperometric current, and modulate the antenna impedance
to adjust the backscatter radiation in a manner indicative of the
measured amperometric current. In such an example, the supplied
radio frequency radiation 171 can be considered an interrogation
signal from the external reader 180 to the eye-mountable device 110
to request a measurement. By periodically interrogating the
eye-mountable device 110 (e.g., by supplying radio frequency
radiation 171 to temporarily turn the device on) and storing the
sensor results (e.g., via the data storage 183), the external
reader 180 can accumulate a set of analyte concentration
measurements over time without continuously powering the
eye-mountable device 110.
[0069] FIG. 2A is a bottom view of an example eye-mountable
electronic device 210 (or ophthalmic electronics platform). FIG. 2B
is an aspect view of the example eye-mountable electronic device
shown in FIG. 2A. It is noted that relative dimensions in FIGS. 2A
and 2B are not necessarily to scale, but have been rendered for
purposes of explanation only in describing the arrangement of the
example eye-mountable electronic device 210. The eye-mountable
device 210 is formed of a polymeric material 220 shaped as a curved
disk. The polymeric material 220 can be a substantially transparent
material to allow incident light to be transmitted to the eye while
the eye-mountable device 210 is mounted to the eye. The polymeric
material 220 can be a biocompatible material similar to those
employed to form vision correction and/or cosmetic contact lenses
in optometry, such as polyethylene terephthalate ("PET"),
polymethyl methacrylate ("PMMA"), polyhydroxyethylmethacrylate
("polyHEMA"), silicone hydrogels, combinations of these, etc. The
polymeric material 220 can be formed with one side having a concave
surface 226 suitable to fit over a corneal surface of an eye. The
opposite side of the disk can have a convex surface 224 that does
not interfere with eyelid motion while the eye-mountable device 210
is mounted to the eye. A circular outer side edge 228 connects the
concave surface 224 and convex surface 226.
[0070] The eye-mountable device 210 can have dimensions similar to
a vision correction and/or cosmetic contact lenses, such as a
diameter of approximately 1 centimeter, and a thickness of about
0.1 to about 0.5 millimeters. However, the diameter and thickness
values are provided for explanatory purposes only. In some
embodiments, the dimensions of the eye-mountable device 210 can be
selected according to the size and/or shape of the corneal surface
of the wearer's eye.
[0071] The polymeric material 220 can be formed with a curved shape
in a variety of ways. For example, techniques similar to those
employed to form vision-correction contact lenses, such as heat
molding, injection molding, spin casting, etc. can be employed to
form the polymeric material 220. While the eye-mountable device 210
is mounted in an eye, the convex surface 224 faces outward to the
ambient environment while the concave surface 226 faces inward,
toward the corneal surface. The convex surface 224 can therefore be
considered an outer, top surface of the eye-mountable device 210
whereas the concave surface 226 can be considered an inner, bottom
surface. The "bottom" view shown in FIG. 2A is facing the concave
surface 226. From the bottom view shown in FIG. 2A, the outer
periphery 222, near the outer circumference of the curved disk is
curved to extend out of the page, whereas the central region 221,
near the center of the disk is curved to extend into the page.
[0072] A substrate 230 is embedded in the polymeric material 220.
The substrate 230 can be embedded to be situated along the outer
periphery 222 of the polymeric material 220, away from the central
region 221. The substrate 230 does not interfere with vision
because it is too close to the eye to be in focus and is positioned
away from the central region 221 where incident light is
transmitted to the eye-sensing portions of the eye. Moreover, the
substrate 230 can be formed of a transparent material to further
mitigate effects on visual perception.
[0073] The substrate 230 can be shaped as a flat, circular ring
(e.g., a disk with a centered hole). The flat surface of the
substrate 230 (e.g., along the radial width) is a platform for
mounting electronics such as chips (e.g., via flip-chip mounting)
and for patterning conductive materials (e.g., via microfabrication
techniques such as photolithography, deposition, plating, etc.) to
form electrodes, antenna(e), and/or interconnections. The substrate
230 and the polymeric material 220 can be approximately
cylindrically symmetric about a common central axis. The substrate
230 can have, for example, a diameter of about 10 millimeters, a
radial width of about 1 millimeter (e.g., an outer radius 1
millimeter greater than an inner radius), and a thickness of about
50 micrometers. However, these dimensions are provided for example
purposes only, and in no way limit the present disclosure. The
substrate 230 can be implemented in a variety of different form
factors, similar to the discussion of the substrate 130 in
connection with FIG. 1 above.
[0074] A loop antenna 270, controller 250, and bio-interactive
electronics 260 are disposed on the embedded substrate 230. The
controller 250 can be a chip including logic elements configured to
operate the bio-interactive electronics 260 and the loop antenna
270. The controller 250 is electrically connected to the loop
antenna 270 by interconnects 257 also situated on the substrate
230. Similarly, the controller 250 is electrically connected to the
bio-interactive electronics 260 by an interconnect 251. The
interconnects 251, 257, the loop antenna 270, and any conductive
electrodes (e.g., for an electrochemical analyte bio-sensor, etc.)
can be formed from conductive materials patterned on the substrate
230 by a process for precisely patterning such materials, such as
deposition, photolithography, etc. The conductive materials
patterned on the substrate 230 can be, for example, gold, platinum,
palladium, titanium, carbon, aluminum, copper, silver,
silver-chloride, conductors formed from noble materials, metals,
combinations of these, etc.
[0075] As shown in FIG. 2A, which is a view facing the concave
surface 226 of the eye-mountable device 210, the bio-interactive
electronics module 260 is mounted to a side of the substrate 230
facing the concave surface 226. Where the bio-interactive
electronics module 260 includes an analyte bio-sensor, for example,
mounting such a bio-sensor on the substrate 230 to be close to the
concave surface 226 allows the bio-sensor to sense analyte
concentrations in tear film near the surface of the eye. However,
the electronics, electrodes, etc. situated on the substrate 230 can
be mounted to either the "inward" facing side (e.g., situated
closest to the concave surface 226) or the "outward" facing side
(e.g., situated closest to the convex surface 224). Moreover, in
some embodiments, some electronic components can be mounted on one
side of the substrate 230, while other electronic components are
mounted to the opposing side, and connections between the two can
be made through conductive materials passing through the substrate
230.
[0076] The loop antenna 270 is a layer of conductive material
patterned along the flat surface of the substrate to form a flat
conductive ring. In some instances, the loop antenna 270 can be
formed without making a complete loop. For instances, the antenna
270 can have a cutout to allow room for the controller 250 and
bio-interactive electronics 260, as illustrated in FIG. 2A.
However, the loop antenna 270 can also be arranged as a continuous
strip of conductive material that wraps entirely around the flat
surface of the substrate 230 one or more times. For example, a
strip of conductive material with multiple windings can be
patterned on the side of the substrate 230 opposite the controller
250 and bio-interactive electronics 260. Interconnects between the
ends of such a wound antenna (e.g., the antenna leads) can then be
passed through the substrate 230 to the controller 250.
[0077] FIG. 2C is a side cross-section view of the example
eye-mountable electronic device 210 while mounted to a corneal
surface 22 of an eye 10. FIG. 2D is a close-in side cross-section
view enhanced to show the tear film layers 40, 42 surrounding the
exposed surfaces 224, 226 of the example eye-mountable device 210.
It is noted that relative dimensions in FIGS. 2C and 2D are not
necessarily to scale, but have been rendered for purposes of
explanation only in describing the arrangement of the example
eye-mountable electronic device 210. For example, the total
thickness of the eye-mountable device can be about 200 micrometers,
while the thickness of the tear film layers 40, 42 can each be
about 10 micrometers, although this ratio may not be reflected in
the drawings. Some aspects are exaggerated to allow for
illustration and facilitate explanation.
[0078] The eye 10 includes a cornea 20 that is covered by bringing
the upper eyelid 30 and lower eyelid 32 together over the top of
the eye 10. Incident light is received by the eye 10 through the
cornea 20, where light is optically directed to light sensing
elements of the eye 10 (e.g., rods and cones, etc.) to stimulate
visual perception. The motion of the eyelids 30, 32 distributes a
tear film across the exposed corneal surface 22 of the eye 10. The
tear film is an aqueous solution secreted by the lacrimal gland to
protect and lubricate the eye 10. When the eye-mountable device 210
is mounted in the eye 10, the tear film coats both the concave and
convex surfaces 224, 226 with an inner layer 40 (along the concave
surface 226) and an outer layer 42 (along the convex layer 224).
The tear film layers 40, 42 can be about 10 micrometers in
thickness and together account for about 10 microliters.
[0079] The tear film layers 40, 42 are distributed across the
corneal surface 22 and/or the convex surface 224 by motion of the
eyelids 30, 32. For example, the eyelids 30, 32 raise and lower,
respectively, to spread a small volume of tear film across the
corneal surface 22 and/or the convex surface 224 of the
eye-mountable device 210. The tear film layer 40 on the corneal
surface 22 also facilitates mounting the eye-mountable device 210
by capillary forces between the concave surface 226 and the corneal
surface 22. In some embodiments, the eye-mountable device 210 can
also be held over the eye in part by vacuum forces against corneal
surface 22 due to the concave curvature of the eye-facing concave
surface 226.
[0080] As shown in the cross-sectional views in FIGS. 2C and 2D,
the substrate 230 can be inclined such that the flat mounting
surfaces of the substrate 230 are approximately parallel to the
adjacent portion of the concave surface 226. As described above,
the substrate 230 is a flattened ring with an inward-facing surface
232 (closer to the concave surface 226 of the polymeric material
220) and an outward-facing surface 234 (closer to the convex
surface 224). The substrate 230 can have electronic components
and/or patterned conductive materials mounted to either or both
mounting surfaces 232, 234. As shown in FIG. 2D, the
bio-interactive electronics 260, controller 250, and conductive
interconnect 251 are mounted on the inward-facing surface 232 such
that the bio-interactive electronics 260 are relatively closer in
proximity to the corneal surface 22 than if they were mounted on
the outward-facing surface 234.
[0081] III. An Ophthalmic Electrochemical Analyte Sensor
[0082] FIG. 3 is a functional block diagram of a system 300 for
electrochemically measuring a tear film analyte concentration. The
system 300 includes an eye-mountable device 310 with embedded
electronic components powered by an external reader 340. The
eye-mountable device 310 includes an antenna 312 for capturing
radio frequency radiation 341 from the external reader 340. The
eye-mountable device 310 includes a rectifier 314, an energy
storage 316, and regulator 318 for generating power supply voltages
330, 332 to operate the embedded electronics. The eye-mountable
device 310 includes an electrochemical sensor 320 with a working
electrode 322 and a reference electrode 323 driven by a sensor
interface 321. The eye-mountable device 310 includes hardware logic
324 for communicating results from the sensor 320 to the external
reader 340 by modulating the impedance of the antenna 312. An
impedance modulator 325 (shown symbolically as a switch in FIG. 3)
can be used to modulate the antenna impedance according to
instructions from the hardware logic 324. Similar to the
eye-mountable devices 110, 210 discussed above in connection with
FIGS. 1 and 2, the eye-mountable device 310 can include a mounting
substrate embedded within a polymeric material configured to be
mounted to an eye.
[0083] The electrochemical sensor 320 can be situated on a mounting
surface of such a substrate proximate the surface of the eye (e.g.,
corresponding to the bio-interactive electronics 260 on the
inward-facing side 232 of the substrate 230) to measure analyte
concentration in a tear film layer interposed between the
eye-mountable device 310 and the eye (e.g., the inner tear film
layer 40 between the eye-mountable device 210 and the corneal
surface 22). In some embodiments, however, an electrochemical
sensor can be situated on a mounting surface of such a substrate
distal the surface of the eye (e.g., corresponding to the
outward-facing side 234 of the substrate 230) to measure analyte
concentration in a tear film layer coating the exposed surface of
the eye-mountable device 310 (e.g., the outer tear film layer 42
interposed between the convex surface 224 of the polymeric material
210 and the atmosphere and/or closed eyelids).
[0084] With reference to FIG. 3, the electrochemical sensor 320
measures analyte concentration by applying a voltage between the
electrodes 322, 323 that is sufficient to cause products of the
analyte catalyzed by the reagent to electrochemically react (e.g.,
a reduction and/or oxidization reaction) at the working electrode
322. The electrochemical reactions at the working electrode 322
generate an amperometric current that can be measured at the
working electrode 322. The sensor interface 321 can, for example,
apply a reduction voltage between the working electrode 322 and the
reference electrode 323 to reduce products from the
reagent-catalyzed analyte at the working electrode 322.
Additionally or alternatively, the sensor interface 321 can apply
an oxidization voltage between the working electrode 322 and the
reference electrode 323 to oxidize the products from the
reagent-catalyzed analyte at the working electrode 322. The sensor
interface 321 measures the amperometric current and provides an
output to the hardware logic 324. The sensor interface 321 can
include, for example, a potentiostat connected to both electrodes
322, 323 to simultaneously apply a voltage between the working
electrode 322 and the reference electrode 323 and measure the
resulting amperometric current through the working electrode
322.
[0085] The rectifier 314, energy storage 316, and voltage regulator
318 operate to harvest energy from received radio frequency
radiation 341. The radio frequency radiation 341 causes radio
frequency electrical signals on leads of the antenna 312. The
rectifier 314 is connected to the antenna leads and converts the
radio frequency electrical signals to a DC voltage. The energy
storage 316 (e.g., capacitor) is connected across the output of the
rectifier 314 to filter out high frequency components of the DC
voltage. The regulator 318 receives the filtered DC voltage and
outputs both a digital supply voltage 330 to operate the hardware
logic 324 and an analog supply voltage 332 to operate the
electrochemical sensor 320. For example, the analog supply voltage
can be a voltage used by the sensor interface 321 to apply a
voltage between the sensor electrodes 322, 323 to generate an
amperometric current. The digital supply voltage 330 can be a
voltage suitable for driving digital logic circuitry, such as
approximately 1.2 volts, approximately 3 volts, etc. Reception of
the radio frequency radiation 341 from the external reader 340 (or
another source, such as ambient radiation, etc.) causes the supply
voltages 330, 332 to be supplied to the sensor 320 and hardware
logic 324. While powered, the sensor 320 and hardware logic 324 are
configured to generate and measure an amperometric current and
communicate the results.
[0086] The sensor results can be communicated back to the external
reader 340 via backscatter radiation 343 from the antenna 312. The
hardware logic 324 receives the output current from the
electrochemical sensor 320 and modulates (325) the impedance of the
antenna 312 in accordance with the amperometric current measured by
the sensor 320. The antenna impedance and/or change in antenna
impedance is detected by the external reader 340 via the
backscatter signal 343. The external reader 340 can include an
antenna front end 342 and logic components 344 to decode the
information indicated by the backscatter signal 343 and provide
digital inputs to a processing system 346. The external reader 340
associates the backscatter signal 343 with the sensor result (e.g.,
via the processing system 346 according to a pre-programmed
relationship associating impedance of the antenna 312 with output
from the sensor 320). The processing system 346 can then store the
indicated sensor results (e.g., tear film analyte concentration
values) in a local memory and/or an external memory (e.g., by
communicating with the external memory through a network).
[0087] In some embodiments, one or more of the features shown as
separate functional blocks can be implemented ("packaged") on a
single chip. For example, the eye-mountable device 310 can be
implemented with the rectifier 314, energy storage 316, voltage
regulator 318, sensor interface 321, and the hardware logic 324
packaged together in a single chip or controller module. Such a
controller can have interconnects ("leads") connected to the loop
antenna 312 and the sensor electrodes 322, 323. Such a controller
operates to harvest energy received at the loop antenna 312, apply
a voltage between the electrodes 322, 323 sufficient to develop an
amperometric current, measure the amperometric current, and
indicate the measured current via the antenna 312 (e.g., through
the backscatter radiation 343).
[0088] FIG. 4A is a flowchart of a process 400 for operating an
amperometric sensor in an eye-mountable device to measure a tear
film analyte concentration. Radio frequency radiation is received
at an antenna in an eye-mountable device including an embedded
electrochemical sensor (402). Electrical signals due to the
received radiation are rectified and regulated to power the
electrochemical sensor and associated controller (404). For
example, a rectifier and/or regulator can be connected to the
antenna leads to output a DC supply voltage for powering the
electrochemical sensor and/or controller. A voltage sufficient to
cause electrochemical reactions at the working electrode is applied
between a working electrode and a reference electrode on the
electrochemical sensor (406). An amperometric current is measured
through the working electrode (408). For example, a potentiostat
can apply a voltage between the working and reference electrodes
while measuring the resulting amperometric current through the
working electrode. The measured amperometric current is wirelessly
indicated with the antenna (410). For example, backscatter
radiation can be manipulated to indicate the sensor result by
modulating the antenna impedance.
[0089] FIG. 4B is a flowchart of a process 420 for operating an
external reader to interrogate an amperometric sensor in an
eye-mountable device to measure a tear film analyte concentration.
Radio frequency radiation is transmitted to an electrochemical
sensor mounted in an eye from the external reader (422). The
transmitted radiation is sufficient to power the electrochemical
sensor with energy from the radiation for long enough to perform a
measurement and communicate the results (422). For example, the
radio frequency radiation used to power the electrochemical sensor
can be similar to the radiation 341 transmitted from the external
reader 340 to the eye-mountable device 310 described in connection
with FIG. 3 above. The external reader then receives backscatter
radiation indicating the measurement by the electrochemical analyte
sensor (424). For example, the backscatter radiation can be similar
to the backscatter signals 343 sent from the eye-mountable device
310 to the external reader 340 described in connection with FIG. 3
above. The backscatter radiation received at the external reader is
then associated with a tear film analyte concentration (426). In
some cases, the analyte concentration values can be stored in the
external reader memory (e.g., in the processing system 346) and/or
a network-connected data storage.
[0090] For example, the sensor result (e.g., the measured
amperometric current) can be encoded in the backscatter radiation
by modulating the impedance of the backscattering antenna. The
external reader can detect the antenna impedance and/or change in
antenna impedance based on a frequency, amplitude, and/or phase
shift in the backscatter radiation. The sensor result can then be
extracted by associating the impedance value with the sensor result
by reversing the encoding routine employed within the eye-mountable
device. Thus, the reader can map a detected antenna impedance value
to an amperometric current value. The amperometric current value is
approximately proportionate to the tear film analyte concentration
with a sensitivity (e.g., scaling factor) relating the amperometric
current and the associated tear film analyte concentration. The
sensitivity value can be determined in part according to
empirically derived calibration factors, for example.
[0091] IV. Example Electrochemical Sensor
[0092] FIG. 5A is a functional block diagram of an example
electrochemical sensor system 500 including a dual mode power
supply 520. The electrochemical sensor system 500 can also include
a working electrode 502, a reference electrode 504, an antenna 522,
and measurement and communication electronics 524. The dual mode
power supply 520 is electrically connected to the measurement and
control electronics 524 to supply power (e.g., a DC supply
voltage). For expediency, the measurement and control electronics
524 is alternately referred to herein as the "measurement
electronics" or the "measurement module." The dual mode power
supply 520 is also electrically connected to the sensor electrodes
502, 504 to apply a bias voltage (e.g., Vbias) across the sensor
electrodes 502, 504. Thus, the working electrode 502 may be
connected to both the measurement and communication electronics 524
and the dual mode power supply 520 at a node 510. Similarly, the
reference electrode 504 may be connected to both the measurement
and communication electronics 524 and the dual mode power supply at
a node 512. Although, it is noted that functional block diagram of
the system 500 shown in FIG. 5A illustrates separate functional
modules, which are not necessarily implemented as physically
distinct modules. For example, the dual mode power supply 520 and
measurement and communication electronics 524 can be packaged in a
common chip that includes terminals connected to the antenna 522
and the sensor electrodes 502, 504. Further, while not specifically
illustrated, it is noted that a reagent layer can be provided on or
near the working electrode 502 to sensitize the electrochemical
sensor to an analyte of interest. For example, glucose oxidase may
be fixed around the working electrode 502 (e.g., by incorporating
glucose oxidase in a gel or medium) to cause the electrochemical
sensor system 500 to detect glucose.
[0093] The dual mode power supply 520 is configured to provide
power to the electrochemical sensor system 500 both in a standby
mode and an active measurement mode. For example, during an active
measurement mode, the dual mode power supply 520 can provide a DC
supply voltage to the measurement and communication electronics 524
to thereby activate the measurement and control electronics 524.
The DC supply voltage can be, for example, a DC voltage sufficient
to turn on the measurement and control electronics 524. The
measurement and control electronics 524 can be configured to
measure an amperometric current through the working electrode 502
and use the antenna 522 to communicate the measured amperometric
current. Thus, providing the DC supply voltage from the dual mode
power supply 520 causes the system 500 to operate to obtain a
measurement and wirelessly communicate the result.
[0094] The dual mode power supply 520 can be configured to provide
power to the electrochemical sensor system 500 both in a standby
mode and an active measurement mode. For example, in the standby
mode, a bias voltage can be applied across the sensor electrodes
502, 504 to generate an amperometric current. However, while in the
standby mode, the measurement and communication electronics 524 can
be unpowered (e.g., no DC supply voltage conveyed from the dual
mode power supply 520) in order to consume a relatively low level
of power during the standby mode. In the active measurement mode,
the measurement and communication electronics 524 can be turned on
by providing an adequate DC supply voltage from the dual mode power
supply 520.
[0095] In some embodiments, the dual mode power supply 520 can be
similar to the voltage regulator and/or rectifier 314, 318
described in connection with FIG. 3 that outputs both an analog
voltage 332 to the sensor interface 321, and a DC supply voltage
330 to the circuit logic 324. With reference to the system 500 in
FIG. 5, the bias voltage (e.g., Vbias) applied across the sensor
electrodes 502, 504 may be analogous to the analog voltage output
of the energy harvesting system, while the DC supply voltage
provided to the measurement and communication electronics 524 can
be analogous to the digital voltage output of the energy harvesting
system. Thus, some embodiments of the dual mode power supply 520
may include a rectifier, a low-pass filter (e.g., one or more
capacitors), and/or voltage regulation/conditioning modules that
may be similar in some respects to the rectifier 314, energy
storage 316, and/or voltage regulator/conditioner 318 described in
connection with FIG. 3 above.
[0096] The measurement and communication electronics 524 are shown
and described in connection with FIG. 5A as a functional module
that receives a DC supply voltage, obtains an amperometric current
measurement measured through the working electrode, and then
operates the antenna 522 to communicate the measured current.
However, the measurement and communication electronics may include
one or more of the functional modules shown and described in
connection with FIG. 3 above, such as a sensor interface (e.g., a
potentiostat), an antenna interface (e.g., a backscatter radiation
modulator, one or more oscillators, etc.), and/or logic elements
configured to cause the module 524 to function as described.
Moreover, while the measurement and communication electronics are
shown and described as a single physical module, it is noted that
the measurement and communication electronics 524 can include a
combination of one or more modules, or can be combined with other
modules (e.g., rectifier, regulator and/or other related power
supply modules) in a single physical implementation, such as an
integrated circuit or chip.
[0097] In some examples, the dual mode power supply 520 is
configured to switch between the standby mode and the active
measurement mode based on signals received at the antenna 522. For
example, the antenna 522, which can be an energy harvesting antenna
similar to those described above in connection with FIGS. 2 and 3,
can receive a low-level radio frequency radiation (e.g., radiated
from an external reader), sufficient to generate a bias voltage
across the sensor electrodes 502, 504. The dual mode power supply
520 can receive voltage fluctuations on the leads of the antenna
522 and generate one or both of a bias voltage (e.g., Vbias) or a
DC supply voltage by harvesting the energy in the voltage
fluctuations on the antenna leads. For example, the dual mode power
supply 520 can rectify the radiation-induced voltage fluctuations
and can filter (or otherwise regulate/condition) the voltage to
generate a voltage output to supply to the sensor electrodes 502,
504 and/or the measurement and communication electronics 524.
[0098] In some embodiments, the dual mode power supply 520 is
configured to detect the power of the received radiation and
generate the DC supply voltage (e.g., to initiate the active
measurement mode) only if the received radiation includes
sufficient power to allow the DC supply voltage to be generated.
Thus, the dual mode power supply 520 may automatically detect
whether there is sufficient power in the received radiation to
generate a DC supply voltage and generate the DC supply voltage
only if enough power is available. In other words, the amount of
power in the received radiation may, by itself, control the mode of
operation of the dual mode power supply 520. On the other hand, in
some embodiments, the received radiation can include indications
embedded in the signal to initiate the active measurement mode or
the standby bias mode. For example, the received radiation can
include a binary indicator that can be interpreted by the dual mode
power supply 520 (and/or related receiver electronics) to indicate
whether the system 500 is in the standby bias mode or the active
measurement mode. Thus, the system 500 may be operated to switch
between standby bias mode and active measurement mode based on an
embedded indicator in the received radiation, but without regard to
the power of the received radiation. For instance, if the external
reader (or other radio frequency radiation source) is located in
close proximity, the received radiation may reach a high power
level, but the radiation may still indicate that the system 500 is
to be operated in the standby bias mode.
[0099] Operating the system 500 in the standby bias mode helpfully
allows the system to circumvent costly energy consumption during a
period of amperometric current stabilization that occurs
immediately after first applying a bias voltage across the sensor
electrodes 502, 504. While the bias voltage is applied, analytes
present in the sensor region 501 electrochemically react at the
working electrode 502 and are thereby electrochemically consumed.
Thus, in a steady state operation, the analyte concentration in the
sensor region 501 is balanced between the electrochemical
consumption at the working electrode 502 and the diffusion of
additional analyte into the sensor region 501 from surrounding
areas. When the diffusion rate approximately balances the
electrochemical consumption, the amperometric current reaches a
stable value, which provides a good estimation of the analyte
concentration at or near the sensor region 501. However,
immediately after the bias voltage is first applied, the sensor
system 500 is not in a steady state, and the electrochemical
consumption rate is not balanced by the diffusion of additional
analyte to the sensor region 501. Instead, immediately after the
bias voltage is first applied, the sensor region 501 is filled with
a relatively large amount of analyte, because the analyte is not
being consumed at the working electrode 502, and so the initial
amperometric current reading can be relatively greater than the
eventual stable value. Once the relatively high concentration of
analyte is consumed, the amperometric current stabilizes at a
stable value where the electrochemical consumption balances analyte
diffusion from surrounding areas.
[0100] As a result, some embodiments of the present disclosure
provide techniques for avoiding sensor measurements obtained while
the sensor is still stabilizing at its steady state amperometric
current value. That is, techniques provided herein allow sensors to
operate to only obtain measurements after the sensor is pre-charged
with a bias voltage that allows the sensor to reach its steady
state amperometric current value. In some embodiments, the sensor
is pre-charged by applying a bias voltage to the sensor for a
period of time sufficient to stabilize the amperometric current
(e.g., in a standby bias voltage mode). The standby bias mode can
immediately precede obtaining an amperometric current reading
(e.g., in an active measurement mode). In such an example, the bias
voltage is applied intermittently for a duration sufficient to
achieve stabilization, and each application of the bias voltage is
followed immediately by powering the measurement and communication
electronics 524 to obtain a measurement and communicate the
results. In some embodiments, the sensor electrodes are pre-charged
substantially continuously to allow the sensor to continuously
achieve its steady state level, but the measurement electronics are
then powered only for short durations to intermittently obtain a
measurement and communicate the results.
[0101] In a measurement scheme where power is limited (e.g., an
electrochemical sensor in an ophthalmic or implantable device that
is inductively powered by harvested radiation), the system may be
operated intermittently. FIG. 5B is a flowchart of an example
process 530 for operating the example electrochemical sensor system
500 of FIG. 5A with a startup bias mode prior to obtaining a
measurement. The dual mode power supply 520 applies a stabilization
voltage (e.g., Vbias) between the working electrode 502 and the
reference electrode 504 (532). While the bias voltage is being
applied, the measurement and communication electronics 524 may be
unpowered (e.g., no DC supply voltage is provided) or in a
low-power state. The operation scheme of block 532 can be referred
to herein as a standby bias voltage mode. The bias voltage can be
applied for a sufficient duration to allow the amperometric current
caused by the electrochemical reactions at the working electrode
502 to reach a steady state value. The duration may be referred to
herein for convenience as a stabilization time (e.g., t.sub.stab).
In some embodiments, the application of the bias voltage can be
initiated by receiving a signal from an external reader via the
antenna 532 that indicates the standby bias mode (e.g., according
to a power level of the received radiation and/or an embedded
message in the signal).
[0102] A measurement signal can be received at the antenna 522 to
indicate initiation of the active measurement mode (534). The
measurement signal can be indicated by a message embedded (e.g.,
encoded) in a signal received at the antenna 522 and/or by a power
level of the received radiation. For instance, the radio frequency
radiation can increase to provide sufficient power to be harvested
at the dual mode power supply 520 to allow for generation of both
the bias voltage and the DC supply voltage to turn on the
measurement and communication electronics 524. The received
measurement signal in block 534 may thus cause the system 500 to
activate the measurement and communication electronics 524 to
transition the measurement and communication electronics 524 from
the standby bias mode to the active measurement mode (536). For
example, the measurement and communication electronics 524 can then
be turned on by generating a DC supply voltage in the dual mode
power supply 520 and providing the supply voltage to the
measurement and communication electronics 524. Once transitioned to
the active mode, the measurement and communication electronics 524
can measure the amperometric current through the working electrode
502 and communicate the sensor results through the antenna 522
(538). While the measurement and communication electronics 524 may
be powered down (e.g., turned off) during the standby bias mode to
minimize the power consumed during the standby bias mode, some
embodiments may include the measurement and communication
electronics 524 consuming a low level of power during the standby
bias mode. The measurement and communication electronics 524
consume a greater amount of energy/power, while in the active
measurement mode (e.g., as described in block 538) than in the
standby mode (e.g., as described in block 532).
[0103] FIG. 5C is a functional block diagram of the example
electrochemical sensor shown in FIG. 5A operating in standby mode.
For purposes of explanation only, the various modules and
interconnections are shown illustrated with dashed lines to
illustrate modules that are turned off (or in low-level power mode)
and interconnections that are inactive (or in low-level power
mode). Thus, in standby mode, the antenna 522 receives radiation,
which generates low-level power input 540 to the dual mode power
supply 520. The low-level power input 540 may include an embedded
message instructing the system 500 to operate in standby mode or
may include an amount of power that is insufficient to cause the
system 500 to operate in the active measurement mode. The dual mode
power supply 520 receives the low-level input 540 and rectifies
and/or regulates the received input to generate the bias voltage
(e.g., Vbias) that is output directly to the sensor electrodes 502,
504. The measurement and communication electronics 524 receive a
power input 542 that is at a low level or zero level. Thus,
measurement and communication electronics 524 are either turned off
or in a standby, low-level power state.
[0104] Thus, in the standby mode the bias voltage is applied across
the sensor electrodes 502, 504 without powering the measurement and
communication electronics 524. The measurement and communication
electronics 524 therefore do not modulate the antenna impedance
(544), and the antenna 522 does not communicate any results via
backscatter radiation. For clarity, the dashed modules and
interconnects indicate inactive and/or low-level power mode modules
and/or interconnects while the bold modules and interconnects
indicate active modules and/or interconnects during the standby
mode. In the standby mode, the dual mode power supply 520 receives
energy from the antenna 522 and applies a bias voltage to the
sensor electrodes 502, 504 (as indicated by the bold lines), but
the measurement and communication electronics 524 are not operated
to obtain a measurement of the amperometric currents or communicate
the sensor results (as indicated by the dashed lines).
[0105] FIG. 5D is a functional block diagram of the example
electrochemical sensor shown in FIG. 5A operating in an active
measurement mode. Similar to the description in connection with
FIG. 5C, the various modules and interconnects shown in FIG. 5D are
shown in bold to indicate they are in an activated mode, while the
dashed modules and interconnects indicate inactivated features. In
the active measurement mode, the antenna 522 receives an active
measurement mode signal that generates an active mode input 550 to
the dual mode power supply. The active mode input 550 can be
indicated by an embedded message in the received radiation and/or
by a power level of the received radiation being sufficient to
generate a DC supply voltage to power to the measurement and
communication electronics 524. The active mode input 550 causes the
dual mode power supply 520 to generate a DC supply voltage 552 that
activates the measurement and communication electronics 524. The
measurement and communication electronics 524 then operate the
sensor electrodes 502, 504 to stabilize the voltage across the
electrodes 502, 504 while measuring the amperometric current
through the working electrode 502. The measurement and
communication electronics 524 then modulate the antenna impedance
(554) to cause the antenna 522 to communicate the sensor result
(e.g., according to the modulation of the backscatter radiation
from the antenna 522).
[0106] In some examples, the measurement and communication
electronics 524 apply a voltage across the sensor electrodes 502,
504 that is different from the standby bias voltage Vbias. For
example, the measurement and communication electronics 524 may
apply a more accurate voltage across the sensor electrodes 502, 504
than the Vbias output of the dual mode power supply 520. Thus,
while the voltage across the electrodes in the standby mode (e.g.,
Vbias) is generally approximately equal to the voltage across the
electrodes 502, 504 while in the measurement mode (e.g., Vmeas),
there may be a difference of approximately 20%. Generally, the
voltage applied during the standby bias mode is selected to be
sufficient to allow the amperometric current to reach a stable
value, even if not as precise as the sensor voltage that is applied
by the measurement and communication electronics 524 during the
active measurement mode (e.g., Vmeas).
[0107] The measurement and communication electronics 524 may
include, for example, a potentiostat configured to apply a voltage
across the sensor electrodes 502, 504 while measuring the
amperometric current through the working electrode 502. In the
example illustrated in FIG. 5D, the dual mode power supply 520 is
not used to provide the bias voltage during the active measurement
mode (e.g., the bias voltage outputs from the dual mode power
supply 520 may be disconnected or turned off), but this is only one
embodiment provided for example purposes. In some examples, the
bias voltage may be applied across the sensor electrodes 502, 504
by the measurement and communication electronics 524 and/or the
dual mode power supply 520 during the active measurement mode. For
clarity, the dashed modules and interconnects indicate inactive
and/or low-level power mode modules and/or interconnects while the
bold modules and interconnects indicate active modules and/or
interconnects during the standby mode. In the active measurement
mode, the dual mode power supply 520 receives energy from the
antenna 522 and provides a supply voltage to the measurement and
communication electronics 524 (as indicated by the bold lines), and
the measurement and communication electronics 524 obtain an
amperometric current measurement through the working electrode 502
and communicate the results through the antenna 522.
[0108] FIGS. 6A-6D illustrate sensor voltage, sensor current,
electronics supply voltages, and power consumption for an example
measurement cycle. The example shown in FIGS. 6A-6D illustrates an
example operation scheme where the measurement and communication
electronics are powered intermittently (i.e., non-continuously) to
obtain a series of measurements over time without continuously
powering the system. The measurement cycle is initiated by applying
the bias voltage (e.g., Vbias) across the sensor electrodes 502,
504, which is indicated in FIG. 6A. The bias voltage is applied for
a duration t.sub.bias, which is the duration of the entire
measurement cycle. The amperometric current due to electrochemical
reactions at the working electrode 502 is shown in FIG. 6B, which
shows an initial spike in the sensor current immediately after
applying the bias voltage. The sensor current stabilizes at a value
labeled i.sub.stab after a time t.sub.stab. The value of i.sub.stab
is the amperometric current value that reflects a steady state
balance between analyte consumption by electrochemical reactions at
the working electrode 502 and analyte diffusion into the sensor
region 501. The duration t.sub.stab is the time required for the is
amperometric current to reach i.sub.stab, and may be a
pre-determined (e.g., pre-programmed) value and/or may be
dynamically determined (e.g., by a processor in an external reader)
based on the duty cycle of the measurement system (e.g., the
fraction of time the system is in the measurement mode), previous
and/or predicted values of i.sub.stab, value of Vbias, duration of
the immediately preceding standby bias voltage mode, etc.
[0109] The supply voltage provided to the measurement and
communication electronics 524 is shown in FIG. 6C. After t.sub.stab
(e.g., after the amperometric current reaches a steady state value)
the DC supply voltage is turned on (e.g., set to Von, a voltage
sufficient to turn on the measurement and communication electronics
524). The DC supply voltage is turned on long enough to power the
measurement and control electronics to obtain an amperometric
current measurement and communicate the results (e.g., a time
period t.sub.meas). The duration of the measurement period
t.sub.meas may be pre-determined (e.g., pre-programmed). The
measurement mode (i.e., turning on the measurement and
communication electronics 524) can be initiated in response to
receiving a signal through the antenna 522 that includes an active
measurement mode indicator.
[0110] FIG. 6D shows the power consumption of the system 500 over
the full measurement cycle, which includes both the standby mode
and the active measurement mode. During the standby mode (e.g.,
while the bias voltage is applied and the amperometric current
stabilizes without measuring or communicating sensor results), the
power consumption is at a low level that reflects the dual mode
power supply 520 generating the bias voltage, but not the DC supply
voltage. During the active measurement mode (e.g., while the
voltage Von is provided to the measurement and communication
electronics), the power consumption is at a high level. In
comparison to an operation scheme where the measurement and
communication electronics are powered for the entire measurement
cycle, including the stabilization period, rather than only
following the standby bias voltage mode, the operation scheme
including the standby bias voltage mode consumes much less power
over time. In particular, in an operation scheme where the
measurement and communication electronics are powered for the
entire duration of the stabilization period, the power consumption
of the system may be at the high level (indicated in FIG. 6D as the
active mode) for the entire duration of the measurement cycle
(e.g., the period t.sub.bias shown in FIG. 6A).
[0111] FIGS. 7A-7E illustrate sensor voltage, sensor current,
electronics supply voltages, incident radiation, and power
consumption for an example repeated measurement cycle. FIG. 7A
shows the voltage applied across the sensor electrodes over time.
FIG. 7B shows the sensor current over time. FIG. 7C shows the
supply voltage provided to the measurement and communication
electronics 524 over time. FIG. 7D shows the power of the incident
radio frequency radiation over time that is needed to power the
system by harvesting energy from the radiation. FIG. 7E shows the
power consumption of the system over time when operated in the
intermittent measurement scheme. Various time points in the
operation scheme are labeled in FIG. 7E, but apply to all of the
timing diagrams of FIGS. 7A-7E. At time t0 low level radiation is
received at the antenna 522 sufficient to generate the bias voltage
Vbias in the dual mode power supply 520. The dual mode power supply
520 accordingly generates the bias voltage and applies Vbias across
the sensor electrodes 502, 504. The sensor current stabilizes at
the current i.sub.stab over a period of time given by
t.sub.stab.
[0112] After a time period given by t.sub.startup, the incident
radiation increases to a high level at time t1. The high level
radiation includes sufficient power that, when the energy is
harvested by the dual mode power supply 520, the dual mode power
supply can supply a DC supply voltage (e.g., the voltage Von) to
the measurement and communication electronics 524. The high level
radiation (shown in FIG. 7D) thus causes the system 500 to
transition to the active measurement mode by causing the dual mode
power supply 520 to generate a DC supply voltage to the measurement
and communication electronics 524, which causes the measurement and
communication electronics 524 to measure the amperometric current
(i.e., the pre-stabilized current i.sub.stab) through the working
electrode 502 and communicate the results through the antenna 522
(e.g., by modulating the antenna impedance to adjust the
backscatter radiation in a manner that indicates the sensor
result). The active measurement mode continues for a duration
t.sub.meas, until time t2, at which point the incident radiation
goes to zero (as shown in FIG. 7D), and the system 500 can
optionally turn off entirely (e.g., no bias voltage across the
sensor electrodes 502, 504 and no supply voltage to the measurement
and communication electronics 524). Thus, time t2 marks the end of
a single measurement operation during which the system is put in
standby mode to charge the bias voltage on the sensor electrodes
and allow the current level to stabilize (standby mode from time t0
to t1) and then the measurement and control electronics and powered
just long enough to obtain a measurement and communication the
results (active measurement mode from time t1 to t2).
[0113] At time t3, which may occur a period t.sub.off after the
time t2, a new measurement operation can be initiated by receiving
low power-level radiation at the antenna (as shown in FIG. 7D). The
low level radiation causes the system to enter standby mode. Thus,
the dual mode power supply 520 applies a bias voltage across the
sensor electrodes 502, 504 without powering the measurement and
communication electronics 524. The system can undergo standby mode
for a period t.sub.standby, to allow the sensor current to
stabilize. Then, at time t4, the active measurement mode is
initiated once again when the incident radiation returns to a high
power level, which causes the dual mode power supply 520 to provide
a DC supply voltage to the measurement and communication
electronics 524. In turn the measurement and communication
electronics 524 measure the amperometric current through the
working electrode 502 and communication the sensor result through
the antenna 522. The active mode ends at time t5, and the
intermittent measurement operation can continue thereafter by
repeating the off mode, standby mode, and active measurement mode
in turn to obtain a time series of amperometric sensor current
readings.
[0114] The duration of the standby mode from time t0 to t1 (e.g.,
the duration t.sub.startup) can be a pre-programmed duration for
allowing the current level to stabilize when the timing of the
previous application of the bias voltage and/or the previous
current reading is unknown or uncertain. Thus, the value of
t.sub.startup may be a relatively large duration that assumes a
worst case for time required to achieve a stable amperometric
current. However, the duration of the subsequent standby mode from
time t3 to t4 (e.g., the duration t.sub.standby) can be dynamically
determined based on the duration of the off mode (e.g., the time
t.sub.off) and/or the value of the previously measured amperometric
current (e.g., i.sub.stab), and may optionally be shorter than
t.sub.startup. In some examples, an external reader that provides
the incident radiation and receives the sensor results communicated
back through the antenna may be configured to dynamically determine
the duration of t.sub.startup (e.g., via a computing system
associated with the external reader). For example, an external
reader can control the system 500 to transition between the standby
mode, active measurement mode, and/or off mode according to the
radiation it emits toward the sensor system 500.
[0115] For example, when t.sub.off is large relative to the initial
stabilization time, the system may substantially return to its
initial state prior to the next measurement operation and the
duration of t.sub.standby may be approximately the same as
t.sub.startup. However, where t.sub.off is small relative to the
initial stabilization time, the system may not fully return to its
initial state prior to the next application of Vbias (e.g., at time
t3) and so the duration required to achieve stabilization may be
relatively less than the duration used initially. For example,
where t.sub.off is small, the sensor region 501 may not be fully
repopulated with analyte prior to initiation of the next
measurement operation at time t3 and so relatively little time is
required to allow the system to achieve steady state again.
Moreover, the value of the previously measured amperometric current
itself may additionally or alternatively be used to dynamically
adjust the duration of the standby mode (e.g., the duration
t.sub.standby).
[0116] The duration of the full measurement cycle is shown as the
time between subsequent measurements (e.g., the time t.sub.cyc
between times t1 and t4). In some cases, the entire measurement
cycle is repeated periodically with the time t.sub.cyc as the
period such that each measurement is separated by the time
t.sub.cyc. However, the system 500 can be operated without
repeating in a regular periodic fashion, such as where the time
separation between subsequent measurements is dynamically adjusted
or where measurements are performed according to available power
(e.g., in an energy storage device powering the system to generate
radiation to be harvested to initiate the measurement mode). In
some examples, the system can be operated with a measurement mode
duty cycle of less than 10%. For example, the duration of the
measurement mode, t.sub.meas, divided by the duration of the full
measurement cycle, t.sub.cyc, can be less than 10%. In some
examples, the measurement mode duty cycle may be approximately 1%,
or approximately 2%, or some other fraction depending on desired
system performance.
[0117] FIG. 8A is a functional block diagram of an example
electrochemical sensor system 800 including a measurement
electronics power supply 810 and a standby bias power supply 820.
By contrast with the system 500, rather than a dual mode power
supply, the system 800 includes a measurement power supply 810 that
operates by harvesting energy from incident radio frequency
radiation and generating a DC supply voltage to turn on the
measurement and communication electronics 524 and thereby cause the
system 800 to obtain an amperometric current measurement through
the working electrode 502 and communicate the sensor result through
the antenna 522. The measurement power supply 810 may be a power
supply that is dedicated to providing power to the measurement and
control electronics 524. The measurement power supply 810 can
generally be similar to the energy harvesting power supply system
described in connection with FIG. 3 and may include one or more
rectifiers, energy storage devices, and/or voltage
regulators/conditioners configured to harvest energy in radio
frequency electrical signals on leads of the antenna 522 caused by
incident radiation and output a DC supply voltage to power the
measurement and communication electronics 524.
[0118] In some embodiments, the measurement power supply 810 does
not include an output for applying a bias voltage across the sensor
electrodes 502, 504. However, the measurement and control
electronics 524, which receive power from the measurement power
supply 810, may apply a voltage across the sensor electrodes 502,
504 while obtaining an amperometric current measurement (e.g.,
similar to the operation of a potentiostat).
[0119] The standby bias power supply 820 generates the bias voltage
Vbias and applies the bias voltage across the sensor electrodes
502, 504 during the standby mode to pre-charge the sensor
electrodes 502, 504 and allow the amperometric current to stabilize
at a steady state value prior to obtaining a measurement (e.g.,
with the measurement and communication electronics 524). In some
examples, the standby bias power supply 820 can be an energy
harvesting system that captures, rather than stores, energy in
order to generate the bias voltage Vbias that is applied across the
sensor electrodes 502, 504. In some examples, the standby bias
power supply 820 can receive power from an auxiliary power source
separate from the radio frequency energy harvesting antenna 522.
For example, the standby bias power supply 820 can use a
photovoltaic cell that generates a voltage in response to receiving
incident light radiation. In some embodiments, the standby bias
power supply 820 can also be powered from another energy harvesting
source, such as an inertial motion energy harvesting system.
Additionally or alternatively, the standby bias power supply 820
can be powered from incident radiation received at the antenna 522
(or another antenna dedicated to the standby bias power supply
820).
[0120] In some examples, the measurement and communication
electronics 524 apply a voltage across the sensor electrodes 502,
504 that is different from the standby bias voltage Vbias generated
by the standby bias power supply 820. For example, the measurement
and communication electronics 524 may apply a more accurate voltage
across the sensor electrodes 502, 504 than the Vbias output of the
standby bias power supply 820. Thus, while the voltage across the
electrodes in the standby mode (e.g., Vbias) is generally
approximately equal to the voltage across the electrodes 502, 504
while in the measurement mode (e.g., Vmeas), there may be a
difference of approximately 20%. Generally, the voltage applied
during the standby bias mode is selected to be sufficient to allow
the amperometric current to reach a stable value, even if not as
precise as the sensor voltage that is applied by the measurement
and communication electronics 524 during the active measurement
mode (e.g., Vmeas).
[0121] FIG. 8B is a functional block diagram of an example
embodiment where the standby bias power supply 820 includes a
photovoltaic cell 822. As shown in FIG. 8B, a photovoltaic cell 822
provides a voltage output to the standby bias power supply 820.
Moreover, the photovoltaic cell can be included in the standby bias
power supply 820. In some embodiments, the standby bias power
supply 820 may comprise a photovoltaic cell (e.g., the photovoltaic
cell 822) that outputs a voltage Vbias across two terminals in
response to incident light radiation. The two terminals of the
photovoltaic cell 822 can then be connected to the two sensor
electrodes 502, 504 (e.g., via the nodes 510, 512) to apply the
bias voltage across the sensor electrodes and thereby allow the
electrochemical sensor to achieve stabilization. The photovoltaic
cell 822 can be, for example, a solar cell or a combination of such
solar cells. The photovoltaic cell can be activated in response to
the receipt of light at a range of different wavelengths, such as
visible light, ultraviolet light, near infrared light, etc.
Although, a particular photovoltaic cell may be configured to be
activated at a selected range of wavelengths as desired. In an
application where the electrochemical sensor is included in an
eye-mountable device (e.g., embedded in a transparent polymeric
material configured to be contact-mounted to an eye surface) the
photovoltaic cell 822 can be embedded in the eye-mountable device
and can receive incident light radiation that is transmitted
through the eye-mountable device.
[0122] In some examples, the standby bias power supply 820 can
operate to apply a bias voltage Vbias to the sensor electrodes 502,
504 substantially continuously and thereby keep the system 800
substantially continuously pre-charged in a state where the
amperometric current is stabilized. Thus, the standby bias power
supply 820 may be operated substantially independent of the
incident radiation received at the antenna 522. For example, the
standby bias power supply 820 can apply the bias voltage to the
sensor electrodes 502, 504 whenever the standby bias power supply
820 receives a power input (e.g., incident light radiation,
inertial motion, etc.). Thus, the system 800 can be used to obtain
a time series of amperometric current measurements by
intermittently powering the measurement and communication
electronics 524 to measure the amperometric current and communicate
the results. Where the standby bias power supply 820 is operated
substantially continuously, the system 800 may intermittently
receive radio frequency radiation (at the antenna) to initiate an
active measurement mode. Energy from the received radio frequency
radiation can be harvested by the measurement power supply 810 to
generate a DC supply voltage to power the measurement and
communication electronics 524, and the measurement and
communication electronics 524 can measure the amperometric current
through the working electrode 502 and communicate the measurement
result through the antenna 522 (e.g., by modulating the antenna
impedance to adjust the backscatter radiation).
[0123] FIG. 8C is a flowchart of an example process 830 for
operating the example electrochemical sensor of FIG. 8A with a
startup bias mode prior to obtaining a measurement. Energy is
harvested from incident radiation to generate a stabilization
voltage (or bias voltage) using the standby bias power supply 820
(832). The standby bias power supply 820 can harvest energy from
incident light radiation, from an inertial energy harvesting
system, etc. The stabilization voltage (or bias voltage) is applied
between the working electrode 502 and the reference electrode 504
(834). The stabilization voltage is a voltage sufficient to cause
the analyte in the sensor region 501 to electrochemically react at
the working electrode and thereby generate an amperometric current.
A measurement signal indicating the initiation of an active
measurement mode is received (836). For example, the measurement
signal can be radio frequency radiation with sufficient energy to
operate the measurement power supply 810 to generate a DC supply
voltage by harvesting the energy in the received radiation. The
measurement signal can additionally or alternatively include a
message embedded in the received radiation that instructs the
sensor system 800 (e.g., via associated receiver electronics) to
initiate the active measurement mode.
[0124] Incident radio frequency radiation is harvested by the
measurement power supply 810 to generate a DC supply voltage
sufficient to turn on the measurement and communication electronics
524 (838). For example, the measurement power supply 810 can output
a voltage Von that causes the measurement and communication
electronics 524 to transition from a standby mode to an active
measurement mode. The turn on voltage (e.g., Von) is applied to the
measurement and communication electronics to activate the
measurement electronics (840). The measurement and communication
electronics 524 are then operated to obtain a sensor reading and
communicate the results (842). For example, the measurement and
communication electronics can measure the amperometric current
through the working electrode 502 and communicate the results
through the antenna 522. In some examples, the measurement and
communication electronics 524 can be turned off while in the
standby mode and turn on upon receiving the DC supply from the
measurement power supply 810. Generally, the measurement and
communication electronics 524 consume less power in the standby
mode (or idle mode) than in the active measurement mode. Thus, the
process 830 allows for obtaining a time series of amperometric
current measurements without continuously powering the measurement
and communication electronics 524.
[0125] FIGS. 9A-9E illustrate sensor voltage, sensor current,
electronics supply voltages, incident radiation, and power
consumption for an example repeated measurement cycle using the
example electrochemical system 800 shown in FIG. 8A. FIG. 9A shows
the voltage applied across the sensor electrodes over time. FIG. 9B
shows the sensor current over time. FIG. 9C shows the supply
voltage provided to the measurement and communication electronics
524 over time. FIG. 9D shows the power of the incident radio
frequency radiation over time that is needed to power the system by
harvesting energy from the radiation. FIG. 9E shows the power
consumption of the system over time when operated in the
intermittent measurement scheme. Various time points in the
operation scheme are labeled in FIG. 9E, but apply to all of the
timing diagrams of FIGS. 9A-9E.
[0126] At time t6 the bias voltage Vbias is generated by the
standby bias power supply 820 (e.g., by energy harvested with a
photovoltaic cell and/or inertial energy harvesting system) and
applied across the sensor electrodes 502, 504. The sensor current
stabilizes at the current i.sub.stab over a period of time given by
t.sub.stab. After a time period t.sub.startup, which is greater
than t.sub.stab, the system 800 can transition to the active
measurement mode at time t7. For example, at time t7, the radio
frequency radiation received at the antenna 522 can increase to a
high power level and can optionally include an embedded message
instructing the system 800 to transition to the active measurement
mode. For example, the high power level radio frequency radiation
can be emitted from an external reader configured to operate the
electrochemical sensor system 800. The measurement mode power
supply 810 can harvest energy from the incident radio frequency
radiation to generate a turn on voltage (e.g., the voltage Von) and
apply the turn on voltage to the measurement and communication
electronics 524. During the period from t7 to t8, with duration
t.sub.meas, the measurement and communication electronics 524 can
measure the amperometric current through the working electrode 502
and communicate the sensor result through the antenna 522.
[0127] Following the duration t.sub.meas, the incident radio
frequency radiation can return to a low power level and the
measurement power supply 810 can cease supplying the DC power
supply to the measurement and communication electronics 524, so as
to cause the system to return to the standby mode. Following a
duration t.sub.standby, the active measurement mode is activated
again at time t9, and the system 800 obtains and communicates
another amperometric current measurement. The bias voltage Vbias
can be substantially continuously applied to the sensor electrodes
to keep the sensor constantly pre-charged and ready to obtain and
communicate a measurement. The active measurement mode can be
repeated intermittently to obtain a time series of measurements. In
some examples, the active measurement mode is repeated periodically
with a period t.sub.cyc=t.sub.meas+t.sub.standby. Alternatively,
the measurement mode can be repeated with an irregular period, and
can optionally be repeated with a period that is dynamically
adjusted (e.g., based on the rate of change of subsequent
amperometric current readings). In some examples, the system can be
operated with a measurement mode duty cycle of less than 10%. For
example, the duration of the measurement mode, t.sub.meas, divided
by the duration of the full measurement cycle, t.sub.cyc, can be
less than 10%. In some examples, the measurement mode duty cycle
may be approximately 1%, or approximately 2%, or some other
fraction depending on desired system performance.
[0128] In some examples, either of the electrochemical sensor
systems (e.g., the system 500 and/or the system 800) can be
operated to verify that the sensor has achieved stabilization
during each active measurement mode. For example, rather than
performing a single amperometric current reading during each active
measurement mode (e.g., during the period t.sub.meas), the system
can be obtain two (or more) measurements and communicate both
results. The external reader can then compare the two sensor
results to determine whether the amperometric current was at a
stable value during the measurement. For example, where two (or
more) amperometric current readings are near the same value, the
external reader may conclude that the system was at a stable
current value, and therefore the current readings are reliable
indicators of the analyte concentration. On the other hand, where
two (or more) amperometric current readings evidence non-stable
trend in the current (e.g., a downward trend approaching a stable
value) then the external reader may conclude the system was not at
a stable current value, and therefore the current readings are not
reliable indicators of the analyte concentration. In such an
example, the external reader may optionally signal the
electrochemical sensor to immediately obtain an additional
measurement (e.g., by immediately initiating the standby bias mode)
rather than wait for a pre-determined period of time typically
taken between subsequent measurements.
[0129] FIG. 10A is a block diagram of a system 1000 with an
ophthalmic electrochemical sensor 1030 operated by an external
reader 1010 to obtain a series of amperometric current measurements
over time. The ophthalmic electrochemical sensor 1030 is included
in an eye-mountable device configured to be contact-mounted over a
corneal surface of an eye 10. The ophthalmic electrochemical sensor
1030 can be operated to be transitioned into an active measurement
mode in response to receiving a measurement signal from the
external reader 1010.
[0130] The external reader 1010 includes a processing system 1012
and a memory 1014. The processing system 1012 can be a computing
system that executes software stored in the memory 1014 to cause
the system 1000 to operate as described herein to obtain a time
series of measurements (e.g., by intermittently transmitting a
measurement signal to cause the ophthalmic electrochemical sensor
1030 to obtain a measurement and communicate the results as shown
in connection with FIGS. 7 and 9). The external reader 1010 can
also include an antenna (not shown) for transmitting radio
frequency radiation 1020 to be harvested by the ophthalmic
electrochemical sensor 1030. The external reader 1010 can also
receive indications of sensor results 1022 transmitted back to the
reader by backscatter radiation. For example, the antenna impedance
of the ophthalmic electrochemical sensor 1030 can be modulated in
accordance with the sensor result such that the backscatter
radiation 1022 indicates the sensor results. The external reader
1010 can also use the memory 1014 to store indications of
amperometric current measurements communicated by the ophthalmic
electrochemical sensor 1030. The external reader 1010 can thus be
operated to intermittently power the ophthalmic electrochemical
sensor 1030 so as to obtain a time series of amperometric current
measurements.
[0131] FIG. 10B is a block diagram of the ophthalmic
electrochemical sensor 1030 described in connection with FIG. 10A.
The ophthalmic electrochemical sensor 1030 can be configured to
operate similar to the system 500 and/or the system 800 described
in connection with FIGS. 5-9 above. Thus, the ophthalmic
electrochemical sensor 1030 can include energy harvesting systems
for harvesting energy from incident radiation (and/or other
sources) to generate bias voltage to apply across sensor electrodes
during a standby mode. The ophthalmic electrochemical sensor can
also be configured to generate power from incident radiation to
power measurement and communication electronics in response to
receiving a measurement signal indicating initiation of an active
measurement mode.
[0132] The ophthalmic electrochemical sensor 1030 can include
stabilization electronics 1032, measurement electronics 1034, an
antenna 1036, and sensor electrodes 1038. The stabilization
electronics 1032 can be configured to apply a stabilization voltage
(e.g., the bias voltage Vbias) between the sensor electrodes 1038
while the ophthalmic electrochemical sensor 1030 is operating in
the standby mode (or stabilization mode). Thus, the stabilization
electronics 1032 may include the dual mode power supply 520 or an
auxiliary power supply such as the standby bias power supply 820
described above, for example. The measurement electronics 1034 are
configured to measure the amperometric current through the working
electrode of the sensor electrodes 1038 and communicate the
measured amperometric current through the antenna 1036. The
measurement electronics 1034 can also be configured to harvest
energy from incident radio frequency radiation via the antenna 1036
and use the harvested energy to power the measurement and
communication of the amperometric current. Thus, the measurement
electronics may include the measurement and communication
electronics 524, the measurement power supply 810, and/or the dual
mode power supply 520 described above.
[0133] FIG. 10C is a flowchart of an example process 1040 for
operating the ophthalmic electrochemical sensor system shown in
FIG. 10A. A stabilization signal is transmitted from the external
reader 1010 to the ophthalmic electrochemical sensor 1030 (1042).
The stabilization signal can include radio frequency radiation with
a low power level that is sufficient to cause the stabilization
electronics 1032 in the ophthalmic electrochemical sensor 1030 to
apply a stabilization voltage (e.g., the voltage Vbias) between the
sensor electrodes 1038 and thereby cause an analyte of interest to
electrochemically react at the working electrode, which reactions
generate an amperometric current. The stabilization signal
transmitted in block 1042 can thus cause the ophthalmic
electrochemical sensor to reach a stable amperometric current
value. In some examples, the stabilization signal may be
substantially continuously transmitted for a duration sufficient to
allow the electrochemical sensor to reach a steady state (e.g.,
such that the amperometric current is at a stable value).
[0134] A measurement signal is then wirelessly transmitted from the
external reader 1010 to the ophthalmic electrochemical sensor 1030
(1044). The measurement signal can include radio frequency
radiation with a high power level that is sufficient to cause the
measurement electronics 1034 in the ophthalmic electrochemical
sensor 1030 to measure an amperometric current through the working
electrode and use the antenna to communicate the measured
amperometric current. In some examples, the measurement signal is
transmitted immediately following the end of transmission of the
stabilization signal to cause the ophthalmic electrochemical sensor
to transition immediately from the stabilization mode (or standby
mode) to the measurement mode. An indication of the measured
amperometric current is received back at the external reader
(1046). The measured amperometric current may be indicated by
modulating the impedance of the antenna 1036 in the ophthalmic
electrochemical sensor 1030 such that the modulation in the antenna
impedance can be detected by the external reader 1010 and mapped to
an associated amperometric current reading. For example, the
impedance modulation may be detected via the backscatter radiation
1022 from the ophthalmic electrochemical sensor.
[0135] FIG. 11 depicts a computer-readable medium configured
according to an example embodiment. In example embodiments, the
example system can include one or more processors, one or more
forms of memory, one or more input devices/interfaces, one or more
output devices/interfaces, and machine-readable instructions that
when executed by the one or more processors cause the system to
carry out the various functions, tasks, capabilities, etc.,
described above.
[0136] As noted above, in some embodiments, the disclosed
techniques can be implemented by computer program instructions
encoded on a non-transitory computer-readable storage media in a
machine-readable format, or on other non-transitory media or
articles of manufacture (e.g., the instructions 184 stored on the
memory storage 182 of the external reader 180 of the system 100).
FIG. 11 is a schematic illustrating a conceptual partial view of an
example computer program product that includes a computer program
for executing a computer process on a computing device, arranged
according to at least some embodiments presented herein.
[0137] In one embodiment, the example computer program product 1100
is provided using a signal bearing medium 1102. The signal bearing
medium 1102 may include one or more programming instructions 1104
that, when executed by one or more processors may provide
functionality or portions of the functionality described above with
respect to FIGS. 1-10. In some examples, the signal bearing medium
1102 can be a non-transitory computer-readable medium 1106, such
as, but not limited to, a hard disk drive, a Compact Disc (CD), a
Digital Video Disk (DVD), a digital tape, memory, etc. In some
implementations, the signal bearing medium 1102 can be a computer
recordable medium 1108, such as, but not limited to, memory,
read/write (R/W) CDs, R/W DVDs, etc. In some implementations, the
signal bearing medium 1102 can be a communications medium 1110,
such as, but not limited to, a digital and/or an analog
communication medium (e.g., a fiber optic cable, a waveguide, a
wired communications link, a wireless communication link, etc.).
Thus, for example, the signal bearing medium 1102 can be conveyed
by a wireless form of the communications medium 1110.
[0138] The one or more programming instructions 1104 can be, for
example, computer executable and/or logic implemented instructions.
In some examples, a computing device such as the processor-equipped
external reader 180 of FIG. 1 is configured to provide various
operations, functions, or actions in response to the programming
instructions 1104 conveyed to the computing device by one or more
of the computer readable medium 1106, the computer recordable
medium 1108, and/or the communications medium 1110.
[0139] The non-transitory computer readable medium 1106 can also be
distributed among multiple data storage elements, which could be
remotely located from each other. The computing device that
executes some or all of the stored instructions could be an
external reader, such as the reader 180 illustrated in FIG. 1, or
another mobile computing platform, such as a smartphone, tablet
device, personal computer, etc. Alternatively, the computing device
that executes some or all of the stored instructions could be
remotely located computer system, such as a server.
[0140] While various aspects and embodiments have been disclosed
herein, other aspects and embodiments will be apparent to those
skilled in the art. The various aspects and embodiments disclosed
herein are for purposes of illustration and are not intended to be
limiting, with the true scope being indicated by the following
claims.
* * * * *