U.S. patent application number 11/907336 was filed with the patent office on 2008-02-14 for medical fibres & fabrics.
This patent application is currently assigned to PSIMEDICA LIMITED. Invention is credited to Roger Aston, Leigh Canham.
Application Number | 20080034801 11/907336 |
Document ID | / |
Family ID | 9928130 |
Filed Date | 2008-02-14 |
United States Patent
Application |
20080034801 |
Kind Code |
A1 |
Canham; Leigh ; et
al. |
February 14, 2008 |
Medical fibres & fabrics
Abstract
A fiber or fabric comprising silicon for use as a medical fiber
or fabric. The silicon present can be biocompatible, bioactive or
resorbable material and may also be able to act as an electrical
conductor. In addition, porous silicon may be used as a slow
release means for example for drugs or fragrances, or as a
collector for example for sweat. Novel fibers, fabrics and methods
of preparation of these are also described and claimed.
Inventors: |
Canham; Leigh;
(Worcestershire, GB) ; Aston; Roger;
(Worcestershire, GB) |
Correspondence
Address: |
NIXON & VANDERHYE, PC
901 NORTH GLEBE ROAD, 11TH FLOOR
ARLINGTON
VA
22203
US
|
Assignee: |
PSIMEDICA LIMITED
Malvern
Worcestershire
GB
|
Family ID: |
9928130 |
Appl. No.: |
11/907336 |
Filed: |
October 11, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10499535 |
Jul 15, 2004 |
7294406 |
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PCT/GB02/05853 |
Dec 20, 2002 |
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11907336 |
Oct 11, 2007 |
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Current U.S.
Class: |
65/439 ;
65/444 |
Current CPC
Class: |
A61B 17/06166 20130101;
A61F 13/00063 20130101; Y10T 442/608 20150401; D02G 3/448 20130101;
Y10T 442/609 20150401; A61L 17/00 20130101; Y10T 442/3976 20150401;
A61F 2/90 20130101; A61F 13/00995 20130101; Y10T 442/3089 20150401;
Y10T 442/425 20150401; A61F 2013/00906 20130101; A61F 2013/00442
20130101; Y10T 428/2915 20150115; A61F 2013/00429 20130101; Y10T
442/614 20150401; A61F 13/00017 20130101; A61F 2013/00646 20130101;
Y10T 428/249962 20150401; A61L 27/047 20130101; D01F 1/10 20130101;
Y10T 442/697 20150401; Y10T 428/2929 20150115; D01F 9/08 20130101;
A61F 13/44 20130101; Y10T 428/2927 20150115; Y10T 442/3146
20150401; Y10T 442/613 20150401; A61L 15/08 20130101; Y10T 442/637
20150401; D10B 2401/16 20130101; A61F 2013/00187 20130101; A61F
13/00051 20130101; Y10T 442/696 20150401; Y10T 442/3065
20150401 |
Class at
Publication: |
065/439 ;
065/444 |
International
Class: |
C03C 17/02 20060101
C03C017/02; C03B 37/075 20060101 C03B037/075 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 21, 2001 |
GB |
0130608.3 |
Claims
1.-29. (canceled)
30. A method for producing a fiber comprising silicon, which method
comprises coating a microfiber with silicon or a silicon
composite.
31. A method according to claim 30 wherein the microfiber is a
biodegradable suture.
32. A method according to claim 30 wherein the microfiber is
soluble and wherein in a subsequent step, said microfiber is wholly
or partially dissolved to producing a hollow amorphous silicon or
silicon composite microfiber.
33. A method according to claim 32 wherein the said soluble
microfiber is hollow.
34. A method according to claim 30 wherein the silicon microfiber
obtained is porosified.
35.-72. (canceled)
Description
[0001] The present invention relates to fibers and fabrics
containing silicon for use as medical fibers and fabrics, as well
as to novel fibers and fabrics suitable for these applications and
others, and to methods for their production.
[0002] The production of fibers and fabrics is an ancient art.
Fibers suitable for textile use possess adequate length, fineness,
strength and flexibility for yarn formation and fabric
construction. The first fibers available for textile use were
obtained from animal and plant sources. Cotton, wool, jute, silk
and flax are today the most common natural fibers. Nylon, rayon and
polyester are common synthetic fibers.
[0003] It is now possible to manipulate ultrafine fibers into
fabrics, for example using techniques used to produce woven
metallic jewellery and Denier-grade stockings. Nylon fibers created
by spinnerets for example have diameters of about 25 microns.
[0004] The use of fabrics and other fibrous forms as biomaterials
dates back to the early Egyptians and Indians. Linen strips and
sutures were used with natural adhesives by the Egyptians to draw
the edges of wounds together. The American Indians used horsehair,
cotton and thin leather strips for similar purposes. Use of fabrics
as biomaterials was initially viewed as a new application of
conventionally woven and knitted textiles. Over the past few
decades the development of sophisticated polymer and fiber
processing technologies, nontraditional fabric forms and fibrous
products have also been employed.
[0005] These products may have a variety of medical applications,
depending upon their precise nature and form. For example, fibers
and fabrics may have general surgical applications, for examples as
sutures, threads or meshes. In the cardiovascular fields, they may
be incorporated for example in artificial heart valves. Orthopaedic
prostheses such as tendons and ligaments utilise products in the
form of fibers and fabrics and they may also have
percutaneous/cutaneous applications such as in shunts and
artificial skin.
[0006] The particular materials currently used in medical textiles
include modified natural polymers, synthetic nonabsorbable polymers
and synthetic absorbable polymers.
[0007] However, most commercial polymer textile fibers have various
additives (such as dyes, antistatic agents, delustrants,
photostabilisers) which may reduce their biocompatibility and thus
limit the options for using these in medical applications. Although
some may be biodegradable, it is difficult to ensure that fibers do
not lose their mechanical strength at too early a stage in the
tissue replacement process.
[0008] The applicants have developed new medical fibers and
textiles.
[0009] According to the present invention there is provided a fiber
or fabric comprising silicon for use as a medical fiber or
fabric.
[0010] As used herein, the term "silicon" refers to elemental
silicon material which is a semiconductor. For the avoidance of
doubt, it does not include silicon-containing chemical compounds
such as silica, silicates or silicones, although it may include
composites of semiconducting silicon combined with medical-grade
polymer, ceramic or metal phases. It may also include doped
semiconducting silicon where concentrations up to the atomic
percent level of elements like boron or phosphorus are incorporated
into the silicon lattice to raise electrical conductivity. Porous
silicon may be referred to as "pSi", crystalline silicon as "c-Si"
and amorphous silicon as "a-Si".
[0011] Silicon has several particular advantages for use in this
way. In particular, fibers and fabrics can be produced with a wide
range of desirable properties, including biocompatability,
resorbability or biodegradability or bioactive properties.
[0012] Furthermore, such fibers or fabrics may have semiconducting
properties, which may be particularly useful in the context of
certain applications, for example, in implants, protheses and the
like, where controlled levels of electric current may be applied to
stimulate incorporation into the body.
[0013] As used herein, the term "fiber" refers to a unit of matter
having length at least 100 times their diameter or width.
[0014] The term "fabric" may be defined as thin, flexible and
porous materials made by any combination of cloth, fiber or
polymer. Cloth is a thin flexible material made from yarn, and yarn
comprises, a continuous strand of fibers.
[0015] The expression "bioactive" refers to materials, which when
used in vivo, elicit a specific biological response that results in
the formation of a bond between living tissue and that
material.
[0016] "Biocompatible" as used herein refers to materials which, in
thin film form, are acceptable for use in at least some biological
applications.
[0017] As used herein, the term "resorbable" relates to material
which will dissolve at normal physiological temperatures
(37.degree. C..+-.1.degree. C.) in simulated body fluid, over a
period of time, for example of up to 8 weeks, and generally at less
than 2 weeks. Simulated body fluid in this case may comprises a
solution of reagent grade salts in deionised water so that the
ionic concentration reflect that found in human plasma, as shown in
the following Table 1, or alternatively it may comprise a simulated
synovial fluid, sweat, urine, or other body fluids. In simulated
human plasma, the mixture is buffered to physiological pH values
(7.3.+-.0.05), preferably organically, using for example
trihydroxymethylaminomethane and hydrochloric acid. TABLE-US-00001
TABLE 1 Concentration (mM) Simulated Body Ion Fluid Human Plamsa
Na.sup.+ 142.0 142.0 K.sup.+ 5.0 5.0 Mg.sup.2+ 1.5 1.5 Ca.sup.2+
2.5 2.5 HCO.sub.3-- 4.2 27.0 HPO.sub.4.sup.2- 1.0 1.0 Cr.sup.-
147.8 103.0 SO.sub.4.sup.2- 0.5 0.5
[0018] WO 97/06101 describes the formation of silicon in a form in
which is it biocompatibile, bioactive and/or resorbable. In a
particularly preferred embodiment, the silicon used in the
invention is porous silicon. Porous silicon may be classified
depending upon the nature of the porosity. Microporous silicon
contains pores having a diameter less than 20 .ANG., mesoporous
silicon contains pores having a diameter in the range of 20 .ANG.
to 500 .ANG.; and macroporous silicon contains pores having a
diameter greater than 500 .ANG.. The nature of the porosity of the
microparticles or fibers of silicon used in the invention may vary
depending upon the intended use. Factors such as the need for
biocompatability, resorbability and bioactivity need to be balanced
against the need for mechanical strength and other physical factors
discussed more fully below.
[0019] In particular the silicon used in fibers and fabrics for use
as medical fibers and fabrics is mesoporous silicon, which is
resorbable.
[0020] The silicon fiber or fabric which is the subject of the
invention may take various forms, some of which are novel and these
form further aspects of the invention.
[0021] In one embodiment, silicon containing fibers for use in the
invention are prepared by incorporating silicon microparticles, and
preferably porous silicon microparticles, to a preformed fabric.
This may comprise any of the known fabrics, but in particular is a
biocompatible fabric such as cotton, linen or a biocompatible
synthetic fabric.
[0022] Preferably the silicon microparticles are bound to the
surface of the fabric by covalent bonds. This can be achieved in
various ways. For example, hydroxy groups may be formed on the
surface of the silicon, for example by treatment with ozone in the
presence of U.V. light. These may then be reacted with surface
groups on the fabric directly, or more preferably, they may first
be functionalised with a reactive group. Examples of such
functionalisation reactions are described in WO 00/66190 and WO
00/26019.
[0023] In particular the microparticles are bound by reaction with
a compound of formula (I) X-R-Y (I) where Y is a leaving group such
as trimethoxysilane, R is a linking group, such as
C.sub.1-6alkylene and in particular C.sub.2-4alkylene such as
propylene, and X is a reactive functionality, such as halo and in
particular chloro. The reaction is suitably effected by appropriate
input of thermal energy or light.
[0024] This reaction converts the surface hydroxy groups to groups
of formula --O--R--X, where X and R are as defined above.
Subsequent reaction with for example surface hydroxy groups on the
fabric, will result in the silion microparticle becoming covalently
bound to the surface by an alkylether link. The subsequent reaction
is suitably effected in an organic solvent such as toluene or an
alcohol such as C.sub.1-4alkyl alcohols, at elevated temperatures,
for example at the reflux temperature of the solvent.
[0025] Thus a further aspect of the invention comprises a fabric
having silicon microparticles incorporated therein. In particular,
the silicon microparticles are covalently bound to the fabric.
[0026] The inclusion of silicon microparticles, and particularly
porous microparticles, will enhance the bioactivity of the fabric.
Furthermore, silicon particles may be added at a sufficient density
to bring about particle-to-particle contact which is able to
provide an electrically continuous pathway. In this way, the fabric
may acquire semi-conducting properties which may be of use in the
medical application to which it is put.
[0027] Suitable fabrics include biomedical fabrics such as cotton,
linen or synthetic polymers, which may be absorbable or
non-absorbable.
[0028] In an alternative embodiment, silicon is incorporated into a
fiber, which may then be processed into fabrics, either alone or in
combination with other types of fiber.
[0029] The silicon fibers used may comprise silicon alone, or they
may be in the form of a composite of silicon with other
materials.
[0030] The preparation of some silicon and silicon composite fibers
is known. For example, Japanese patent no. JP9296366A2 describes
the preparation of composite fibers, fabricated by either vapour
deposition of thin Si/SiOx films onto polyester fibers or spinning
of a polyester/silicon mixture.
[0031] Pure Silicon fibers of varying crystalline perfection have
also been realised by a number of techniques:
[0032] Single crystal silicon fibers and their preparation are
described for example by B. M. Epelbaum et al. in Cryst. Res.
Technol. 31, p 1077-1084 (1996). In this method a crucible
containing molten Si is connected to a graphite nozzle that acts as
the shape defining die. Due to molten silicon having a low
viscosity, high surface tension and high chemical reactivity,
pulling of single crystal fibers is difficult. Three types of
crucible die arrangement were designed and tested. Single crystal
fibers of diameter in the range 100-150 microns and lengths up to
80 mm were grown successfully. The maximum pulling speed achieved
was 1 mm/minute.
[0033] Laser-assisted chemical vapour deposition (LCVD) has been
shown to provide a higher growth rate synthesis route for a wide
variety of inorganic fibers, including silicon. In the LCVD
technique a laser beam is focussed onto a point inside a reactor to
initiate chemical vapour deposition in the direction of the laser.
By moving the substrate at the same speed as the deposition rate, a
continuous fiber is realised. For example, with the methods
described by P. C. Nordine et al in Appl. Phys. A57, p 97-100
(1993), silicon fiber growth rates up to 30 mm/minute were achieved
using silane gas pressure of 3.4 bar and Nd:YAG laser power up to
200 mW. At fiber tip temperature of 525-1412 C, poly-Si fibers of
26-93 micron diameter were realised with varying degrees of
crystallinity.
[0034] Silicon fibers have also been realised by the VLS method, as
disclosed in the early paper of R. S. Wagner and W. C. Ellis in
Appl. Phys. Lett. 4, p 89-90 (1964). Here the V represents a vapour
feed gas or gas mixture, the L represents a liquid catalyst and the
S represents a solid fiber product. In this method it is the size
of the metal catalyst droplets that primarily determine the
resulting fiber diameter. The synthesis of both crystalline silicon
microfibres and more recently, nanowires, has been demonstrated
using, for example, gold as the catalyst and silane as the vapour
phase reactant. There are also many related early reports of
silicon "whiskers", "needles" and "filaments" of relatively short
length (under 10 cms) as reviewed in Whisker Technology (John Wiley
& Sons 1970), Edited by A. P. Levitt.
[0035] The VLS method should be adaptable to mass production of
short silicon fibers for air laying or wet laying. Incorporation of
silicon onto/into pre-existing fibers/yarns should be most suitable
for weaving, knitting and embroidery of structures of many meters
length.
[0036] Silicon microwires and the preparation are described for
example by J. J. Petrovic et al. J. Mater. Res. Oct Issue 2001. In
this method, an optical FZ Si growth system was adapted to generate
microwires by the "Taylor microwire technique". The material to be
processed is melted within a glass tube and the softened glass with
the molten material mechanically drawn out into a fine wire in a
similar manner to that of drawing of optical glass fibers. The
working temperature of the glass needs to exceed the 1410.degree.
C. melting point of silicon, where the method was applied to pure
silicon. In this study Vycor glass (Corning 7913) was used which
has a softening temperature of 1530.degree. C. and a working
temperature of 1900.degree. C. A pure Si charge was loaded into
evacuated tubes that were then heated to 1900.degree. C. using
halogen lamps and mechanically pulled. Flexible 10-25 micron
diameter poly-Si microwires were synthesised by this method in
continuous lengths up to 46 cms.
[0037] J. F. Hetke et al. IEEE Trans. Biomed. Engn. 41, 314-321
(1994) describes the design, fabrication and testing of
"ultraflexible" ribbon cables for use with CNS microprobes.
Standard Si wafers were subjected to photolithography, deep boron
doping and multidielectric deposition to define the Cables that
were subsequently floated off by using a boron etch-stop and a fast
wet etch to dissolve the underlying wafer. Cables as thin as 2-3
micron and as thick as 20 micron were realised by varying the boron
diffusion temperature and time.
[0038] Multistrand cables containing 20-30 micron strands are also
described here, and these provided "enhanced flexibility in the
radial and lateral directions". An image of a 5 strand ribbon cable
tied into a knot was shown to illustrate flexibility. Such designs
had lengths of 1-5 cms and total widths ranging from 60-250 micron.
Fibers produced in this way have good flexibility as illustrated in
J. F. Hetke et al., Sensors and Actuators A21-23, 999-1002 (1990),
where a single 15 micron thick strand is shown bent through 180
degrees.
[0039] Although there are such examples of silicon fibers, fiber
arrays of a form and structure suitable for medical fabric
construction are novel, and as such form a further aspect of the
invention.
[0040] The applicants have found that fibers may also be produced
by cutting a silicon wafer using a saw with a sufficiently small
blade and pitch, for example a 75-micron blade and a 225-micron
pitch. Preferably, multiple parallel cuts are formed in a single
wafer to form a comb like structure, which allows for the
production of multiple fibers. In a particularly preferred
embodiment, two such comb-like structures are produced, and then
interweaved in a perpendicular manner. In this way, the fibers of
one comb forms a waft and the fibers of the other wafer form a weft
of a fabric like structure. Cut fibers may be subject to cleaning
for example, ultrasonic cleaning, and/or etching, for example by
anisotropic wet etch to remove saw damage and/or to shape the cross
section of the fibers.
[0041] Other methods for producing silcon or silicon composite
fibers may be used however. For example, hollow amorphous silicon
microfibres may be obtained by coating a fiber, such as a polymer,
metal, ceramic (including glass) fiber, preferably with a hollow
core, with silicon and particularly amorphous or polysilicon. This
may be achieved for example by sputtering or continuous vapour
deposition (CVD). Subsequently the initial fiber can be dissolved,
leaving a hollow amorphous or polysilicon microfiber. This may then
be porosified if desired. If the initial fiber is a biocompatible
material, for example a biodegradable suture, dissolution may not
be necessary or desirable.
[0042] In an alternative embodiment, silicon fibers are formed by
threading together silicon beads to form flexible chains on a
resilient thread or wire, which is preferably a biocompatible or
biodegradable suture. The resultant structure, which is novel and
forms a further aspect of the invention, is therefore similar in
structure to strings of beads found in jewellery. Individual
silicon beads may be of various sizes, depending upon the intended
nature of the fiber, or the fabric produced therefrom.
[0043] For example, beads may be on a macroscale, for example of
from 0.5 to 5 mm in diameter. In these cases, they may be formed by
drilling holes through appropriately sized silicon granules and
subsequently threading through the resilient thread or wire.
[0044] Where microscale beads are required, for example of from 10
microns-500microns diameter, they are suitably prepared by
photolithography and surface micromachining. For example, a silicon
membrane may be supported on a dissolvable surface, such as a
silicon oxide surface. Trenches for example of up to 500 microns
and preferably about 50 microns in depth can then be etched into
the upper surface of the membrane for example by a dry etching or
photolithographic process. A further silicon membrane may then be
deposited over the surface, so that the trench forms a central
cavity. This structure can then be etched photolithographically to
the desired depth, representing the diameter of the desired bead,
to form substantially parallel trenches on either side of the
central cavity. Further channels may be etched which channels are
substantially perpendicular to the trench to trace out the desired
bead shape. Once this has been done, and a suitable thread or
suture passed through the central cavity, the dissolvable surface
may be removed.
[0045] These production methods form yet further aspects of the
invention.
[0046] Fibers and composite fibers obtained using any of the above
methods may be suitable for use in the invention. Preferably
however, the fibers used are porous, or contain porous silicon
beads and these can be obtained by porosifying the fibers or
strings of beads produced as described above, using for example,
methods described in U.S. Pat. No. 5,348,618 and Properties of
Porous silicon (IEE 1997 Ed by L. T. Canham)
[0047] In a particular embodiment therefore, the invention provides
a method of preparing a porous silicon fiber, which method
comprises forming a silicon fiber, in particular by one of the melt
pulling, Laser-assisted chemical vapour deposition (LCVD), VLS
methods, coating of sacrificial fibrous material or micromachining
methods described above, and thereafter porosifying the silicon,
for example by anodisation or stain etching.
[0048] The applicants have found that porous silicon fibers may be
produced directly by cleaving mesoporous films on silcon wafers.
The films may be formed using conventional methods, for example by
anodisation of a silicon wafer in hydrofluoric acid (HF) for
example 40 wt % HF, and ethanol, suitably in equal volumes. Fibers
may be cleaved mechanically, for example by breaking them over a
solid edge, for example of a glass surface, for instance a glass
slide, which may be covered in filter paper. The wafer may be
prescored or scribed before the breaking is carried out.
[0049] Porous or partially porous silicon containing fibers and
fabrics are novel and as such form a further aspect of the
invention.
[0050] Substantially pure silicon fibers, for example of length
greater than 100 cm, are also novel, and these also form an aspect
of the invention. Preferably, these are porosified as discussed
above.
[0051] As used herein the expression "substantially pure" means
that the silicon is at least 98% pure, more suitably at least 99%
pure, and preferably 100% pure.
[0052] For the purposes of the invention, fibres consisting of or
comprising silicon can be used directly as sutures, if they have
sufficient flexibility and strength. The factors required to
achieve fibers having high levels of flexibility are discussed
further below.
[0053] Alternatively, they may be converted into fabrics or yarns
using one or more of any of the major processes common in textile
manufacture. These include spinning, embroidery, weaving, knitting,
braiding, fiber bonding, air-laying, wet laying, and laminating.
The possibility of applying all these various techniques in the
production of fabrics for medical use provides the opportunity to
achieve complex 2-dimensional and 3-dimensional topographies. This
may be particularly useful in certain applications, for example
when the fabric is used to assist in-bone growth, open meshes would
be preferred.
[0054] Silicon is a tough but brittle material and porous silicon
is prone to cracking. It is hence surprising that porous fibers can
be produced which are strong and flexible enough to be weaved into
intricate patterns.
[0055] Most natural fibers such as wool, cotton and flax are not
long enough to be processed into cloth without further treatment.
They are converted to usable thread by a process known as spinning,
where fibers are first laid out parallel to one another ("carded")
and twisted together into a "yarn" (as illustrated for example in
FIG. 2 hereinafter). Such processes may also be applied to fibers
comprising silicon.
[0056] Substantially pure silicon yarn is also novel, and forms yet
a further aspect of the invention.
[0057] Embroidery involves the formation of stitches on a base
cloth, which means that there is a high flexibility in design. The
base cloth may then be dissolved away after the stitching process
is terminated. This process may be particularly good for mimicking
natural fibrous arrays such as ligaments. There is also potential
in fracture fixation where load bearing threads are arranged
optimally with an open mesh for tissue in-growth.
[0058] The weaving process requires the interlacing of two separate
sets of elements to produce a fabric. The element called "warp" is
set down first, usually in a parallel arrangement; the second
element called the "weft" then interlocks with the warp to create
the stable planar structure. Weaving does not require too much
fiber flexibility. In a simple mechanical loom, the warp threads
run off a roller as wide as the finished bolt of cloth will be. The
threads run through a set of wires running vertically which can be
moved up and down. Each wire has a small eye or ring, in the middle
through which the warp yarn runs. By simple mechanical arrangements
it is possible to raise every alternate ring, making a space for
which the weft can pass. The weft is carried by a "shuttle" or jets
of air/water. When the weft has passed through the warp it is
pushed down tightly against the previous thread with a comb-like
frame. The rings carrying the warp threads are now depressed, the
shuttle turned around and the second "pass" between a set of
threads is made. The fastest industrial looms operate at around 200
passes a minute.
[0059] Woven fabrics usually display low elongation but high
breaking strength. They may have a variety of 2D and 3D topography,
depending upon the type of weave used, and typical examples are
illustrated in FIG. 1 herein after. If required, the 3D topography
of a particular fabric can be modified by for example localised
melting of the fibers during fabrication assembly. The silicon
fiber network in a composite fabric is electrically conductive and
can thereby be used to selectively heat up intersecting polymer
fibers to form a more rigid mated lattice.
[0060] Knitting is a continuous single element technique
illustrated for example in FIG. 3 hereinafter, in which a series of
loops are interlocked vertically through the repetition of knitting
stitches retained on some kind of tool or frame. The tensile
strength of knitted fabrics is usually inferior to that from
weaving but their flexibility and stretchability is greater.
[0061] Braiding is a process that utilises simple interlacing of a
single set of elements with out any type of link, loop, warp or
knot. It is differentiated from weaving by the warp serving as the
weft, and by interlacing being in a diagonal or generally oblique
pattern. Braiding is frequently called plaiting, webbing or
interlacing.
[0062] Fiber bonding is a technique commonly used in the production
of large-volume health products where fiber-to-fiber mating is
generated by heat or solvents.
[0063] Air-laying and wet-laying are techniques suitable for
forming fabrics from very short fibers. In air laying, the fibers
are fed into an air stream before being deposited on a moving belt
or perforated drum to form a soft web structure of randomly
oriented fibers. Similarly wet-laying uses a mixture of fibers and
water, which is deposited on a moving screen before being drained
to form a web, consolidated between rollers and allowed to dry.
[0064] Finally, laminating is a way of joining of one fabric to
another using an adhesive.
[0065] Alternatively, silicon and particularly amorphous or
polysilicon may be coated onto a pre-existing fabric, for example
by sputtering or continuous vapour deposition (CVD). Suitable
fabrics may comprise any of the known fabrics, but in particular is
a biocompatible fabric such as cotton, linen or a biocompatible
synthetic fabric such as polyester gauze as described above. Once
coated in this way, the resultant silicon coating may optionally be
porosified by stain etching as is known in the art, and described
above.
[0066] Any or all of these techniques can be applied in the
production of fabrics used in the present invention. Their
applicability in any particular case depends upon the nature of the
silicon fibers being used, and the requisite properties of the
final fiber or fabric product.
[0067] Factors which need to be taken into account when selecting
the type of fiber required in any particular case, and the
technique used to convert this to a fabric include stress, strain,
tensile fracture strength, malleability, and work of fracture.
[0068] Stress is simply load per unit area (units of N/m2 or Pa).
Strain is simply the amount of stretch under load per unit length
(a ratio). Different materials are stretched/compressed by
enormously varying degrees by extending/compressive forces. The
corresponding ratio stress/strain, the Young's modulus (units of
N/m2 or Pa) thus describes the "stiffness" or elastic flexibility.
It varies from 7 Pa for rubber, 1.4 kPa for most plastics, 2 MPa
for steel to about 1.2 GPa for diamond.
[0069] Tensile fracture strength is the stress needed to
break/fracture a material (N/m2 or Pa) by stretching it. It also
varies considerably about 4 MPa for ordinary concrete, 50 MPa for
plastics to 2 GPa for steel. Some values for materials used in
fabrics are 40 MPa (leather), 350 MPa (cotton and silk). For
brittle materials, fracture strength, FS, is controlled by critical
flaws and given by the Griffith equation; FS=(2VE/ c) 0.5
[0070] Where E is the Young's modulus, 2V is the fracture energy
required to form two new surfaces and c is the critical flaw
size.
[0071] Malleability refers to the extent to which a metal can be
manipulated before it breaks.
[0072] Work of fracture is the total energy needed to generate a
fracture structure (J/m 2). For ductile materials like copper and
aluminum, values range between 10 4 and 10 6 J/m 2, much higher
than the free surface energy.
[0073] The introduction of porosity makes a material more flexible
(lowered Young's modulus) but also makes it weaker (lowered
fracture strength). For brittle materials strength is limited by
critical surface flaws which initiate crack propagation and
fracture. Although textile materials generally comprise non-porous
fibers, there are examples of fabrics that contain nanometer size
pores. One such material is HPZ ceramic fiber where porosity is
20%, average.pore width is 1.4 nm. Single crystal "bulk" silicon
has a Young's modulus of 162 GPa and a fracture strength of 7 GPa.
The introduction of mesoporosity in p+ silicon has been shown to
significantly decrease Young's modulus according to the equation;
E(120.times.p 2) GPa where p is the relative density in the range
0.1 to 0.7, corresponding to 90 to 30% porosity. Values as low as 1
GPa can be achieved in high porosity material.
[0074] Micromachined silicon structures will generally have the
mechanical properties of bulk silicon prior to their
porosification. However, silicon fibers, like glass fibers, get
significantly stronger when the diameter drops below 5 micron.
[0075] For glass the important defect is usually the surface crack.
In a brittle crystal like silicon however, surface steps can act as
initiators of crack propagation by locally increasing stress. Thus
it may be preferable to reinforce the surface of the silicon fibers
used in the invention, for example by resin bonding.
[0076] Polycrystalline silicon may be used in the invention, and
therefore whenever the term "silicon" is used herein, it may
include this form. Polycrystallinity does not lower strength
provided the surface energy of the grain boundaries exceeds that of
the crystal fracture planes.
[0077] Similarly amorphous silicon may be biologically acceptable
or bioactive, and therefore whenever the term "silicon" is used
herein, it may include this form unless specified otherwise.
[0078] To achieve sufficient flexibility for use in medical fibers
and fabrics, non-porous silicon fibers are suitably less than 50
micron in diameter. Porosifying part of the fiber will improve
flexibility at a given diameter but decrease strength.
[0079] A partially porous silicon fiber will not be fully
biodegradable but could have substantially greater strength, and
thus be preferable in certain situations.
[0080] In order to avoid the surface failure mechanisms discussed
above, introducing porosity into the fiber core rather than at the
surface coating would be preferable in order to improve strength.
This could be realized by selective anodising of a p++/n- fiber or
poly-Si coating of a wholly porous fiber or partial sintering of a
wholly porous fiber.
[0081] Fabrics prepared from substantially pure silicon fibers are
novel and form a further aspect of the invention.
[0082] In yet a further aspect, the invention provides a process
for preparing a medical fabric, which process comprises weaving,
knitting, embroidering or fiber bonding substantially pure
silicon.
[0083] Fibers and fabrics constructed from silicon or silicon
composites may be semi-conducting. Thus the invention further
provides an electrically conductive silicon or silicon composite
fiber or fabric. Such fibers and fabrics are particularly useful in
medical applications since the semiconducting nature allows for
good distribution of electrical charge, where these are used in
therapy. A particular form of such a fabric is a silk based fabric
which comprises silk warp threads and low resistivity silicon
containing weft threads.
[0084] Thus in a further aspect the invention provides a method for
enhancing tissue growth, which method comprises applying to a
patient in need thereof, a semiconducting fabric comprising
silicon, and passing controlled levels of electrical current
through said fabric.
[0085] Fibers and fabrics as described above have a variety of
medical applications. For example, fabrics which have large pores
(>100 micron) for cellular infiltration can be used as scaffolds
for tissue engineering. The use of the different fabrication
techniques listed above provides for exceptional flexibility of 2D
topography.
[0086] They may also be of use in orthopaedic prostheses where the
mesoporosity of the fibers provides bioactivity whilst the
macroporosity of the textile pattern directs and allows bone
in-growth.
[0087] The electrical conductivity of the textile is also of
benefit in orthopaedic applications where osteogenesis is
controlled by application of distributed electrical charge.
Invasive bone growth stimulators that utilise a wire mesh cathode
are currently used in spinal fusion.
[0088] A stent is a mesh-like collar designed to serve as a
temporary or permanent internal scaffold to maintain or increase
the lumen of a vessel. Essential stent features include radial and
torsional flexibility, biocompatibility, visibility by X-ray and
reliable expandability. It is an example of a widely used implant
that is currently engineered from malleable but non-biodegradable
materials such as metals. A silicon or silicon containing fabric as
described above, may form the basis of these stents. Particular
preferred fabrics would comprise biodegradable forms and partially
porous forms for eluting drugs locally. These forms would be
possible using fully or partially porous silicon fibers as
described above, in the production of the stents.
[0089] A further possible application for the fabrics described
above is in flexible electrodes for neuro-interfacing. The
macroporosity of the fabric enables tissue in-growth. In addition,
the fibers used are preferable at least partially mesoporous, which
means that they offer lower impedance. In order to ensure the very
high electrical conductivity and stability which is important in
such devices, in this case, it may be preferable to use fibers
comprising a non-porous heavily doped silicon core, with a porous
silicon layer that has been electroplated with an ultrathin
conformal coating of a metal such as platinum or iridium.
[0090] The fabrics described above could also be used to produce
"wrapped" in-vivo drug delivery systems. For instance, they may be
used in the localised delivery to curved areas of an organ or for
the encapsulation of drug-eluting cells. In these cases,
biocompatibility of the composite formulation is essential.
[0091] In a variant on this application, they may be used in
"wrapped" ex-vivo drug delivery systems. In this case, it may be
preferable to use a fabric that is augmented with drug-eluting pSi
particles that dissolve in sweat. In such cases, the core fabric
need not be biodegradable and is a medical fabric only to the
extent that it does not irritate the skin.
[0092] An extension of such applications may include textiles
wherein the silicon within the fibers or fabrics is impregnated
with drugs to treat the skin for dermatological conditions, which
can be incorporated or comprised within dressings applied directly
to the skin. In addition, such fibers and fabrics can be used for
localised topical delivery of drugs used to treat conditions such
as anti-inflammatory drugs, used to treat arthritic joints for
instance.
[0093] Passive drug diffusion through the skin could be used in
particular where small molecules like silicic acid are being
diffused. Semiconducting fabric as described above might be used in
an iontophoresis-type design for transdermal delivery of large
biomolecules.
[0094] The electrically conducting properties of the fabrics
described above makes them suitable for distributed networks for
electrical stimulation. In these applications, a fabric is wrapped
around a target area and can be used to electrically stimulate an
array of sites simultaneously
[0095] Non-occluding garments, or other wearable structures such as
patches or bandages, for sweat diagnostics may also incorporate the
fabrics described above. Here the fabric used, in particular for
non-occluding garments, may be one with a relatively open lattice
as this helps maintain normal hydration levels of skin and
flexibility compared to the silicon chips currently used for this
process. It also enables vastly increased area coverage. The widely
distributed porous Si component acts as a large clean reservoir for
collecting excreted biochemical markers.
[0096] A relatively cheap garment (e.g. teeshirt plus porosified
metallurgical grade particulate polysilicon) may be employed as a
one-shot sweat collection device for analysis after strenuous
physical exercise.
[0097] Reservoir particles can be removed from the fabric of the
garment or the like, after wear and sweat incorporation by extended
sonication. Markers collected within the mesopores are then
subjected to standard analysis techniques after solvent extraction.
Biochemically stable silicon particles, which are preferably
derivitised by techniques described in WO 00/006190, are suitably
included in the garments.
[0098] The semiconductive nature of the fabric also facilitates
enhanced sweating via electrical elevation of skin temperature.
Joule heating of the silicon microfibres in particular in a bandage
or patch like structure can locally raise temperature to a
sufficient level to promote sweat production and collection. Again
this can be recovered subsequently from reservoir particles of
porous silicon. However, this may be applied more widely where
localised heating of the skin is required. In that case, the
silicon used may or may not be porous, provided the fabric is
semiconducting in nature.
[0099] A method of collecting sweat using this technique, as well
as a method of locally heating skin forms particular embodiments of
the invention.
[0100] Hollow silicon containing fibers as described above, may
suitably be employed to form fabrics for use in flexible
immuniosolation networks. In this case, foreign cells used are
housed in central channels of hollow mesoporous fibers. Suitably
the fabric has a relatively open architecture, which encourages
vascularisation around every fiber.
[0101] The porous nature (both macroporous and mesoporous) of the
silicon containing fabrics described above may be utilised in wound
repair, for example to deliver drugs such as antibiotics. Thus
these fabrics may be used in absorptive dressings. Also
polycrystalline silicon may be particularly useful in fabrics used
to facilitate wound repair.
[0102] Yet a further specific application of the silicon containing
fibers described above is in the production of X-ray opaque yarns.
Silicon is relatively opaque to X-rays and therefore could be used
instead of the current polymers, which must contain at least 50% by
weight of barium sulphate to render them sufficiently opaque to be
used as markers in surgical swabs or sponges.
[0103] Where the fibers or fabrics discussed above include porous
or non-porous silicon, this opens up the possibility that they may
be used for slow release of a variety of substances. As well as
pharmaceuticals or drugs, they may include fragrances and the like.
Particular examples of such substances are essential oils, which
may be fragrant and/or may have a therapeutic effect.
[0104] Thus the present invention is particularly advantageous
since is allows fibers and fabrics to be produced with the desired
combined biodegradability and mechanical properties for a wide
variety of application. The semiconducting nature of the silicon
used allows for the possibility of distributing electrical charge
in a medical context. Furthermore, in the case of the fabrics, the
desirable effects of dual porosity (i.e. micro or meso porosity of
the fibers themselves as produced for example by electrochemical
etching, and maccoporosity determined by selection of weave design)
can be achieved, so that for example bioactive open meshes can be
produced for bone in-growth and other complex 2D and 3D
topographies achieved depending upon the intended end use.
[0105] The invention will now be particularly described by way of
example, with reference to the accompanying drawings in which:
[0106] FIG. 1 illustrates diagrammatically the structure of woven
faorics,
[0107] FIG. 2 illustrates diagrammatically the structure of
yarns,
[0108] FIG. 3 illustrates diagrammatically the structure of knitted
fibers,
[0109] FIG. 4 shows microscope images of porous c-Si microfibres of
smallest widths (a) about 100 micron (.times.200) (b) about 25
micron and (.times.200) (c) an elastically bent 100 micron wide
fiber (.times.20),
[0110] FIG. 5 shows microscope images of (a) a partially sawn wafer
(.times.3) (b) a sawn silicon beam array (.times.32) and (c) an
etched silicon beam array (.times.100),
[0111] FIG. 6 shows a microscope image of a porous c-Si microfiber
of smallest width (a) 20 micron (b) 4 micron, and (c) a multistrand
fiber (scale and magnification shown),
[0112] FIG. 7 shows microscope images of woven c-Si structures with
(a) a 200 micron thick warp & weft and (b,c) 100 micron thick
warp and 300 micron thick weft (scale shown),
[0113] FIG. 8 shows microscope images of polyester gauze coated
with 10 micron thick layer of semiconducting silicon (scale
shown),
[0114] FIG. 9 shows SEM images of view of the surface of the
polyester gauze after coating (a,b), and the corresponding Energy
Dispersive X-ray (EDX) spectrum, (scale shown)
[0115] FIG. 10 is an enlarged view of polyester gauze fibers after
stain etching (a,b) (scale shown) and (c) the EDX spectrum of the
fibers,
[0116] FIG. 11 shows a scanning electron microscope image of a
silicon coated (a) biodegradable suture (b) aluminum wire and (c)
hollow glass fiber, (scale shown) and
[0117] FIG. 12 is an enlarged view of a coated suture and fiber
illustrating the retained flexibility of the structure (scale
shown).
EXAMPLE 1
[0118] Preparation of Extruded poly-Si Fibers and Fabrics Obtained
Therefrom
[0119] Poly Si microwires of diameter 5-25 micron diameter are
first made by the "Taylor microwire process". Here, a poly or
metallurgical grade silicon charge is melted inside a glass tube by
a local heating source and fine wire drawn out through an orifice
by mechanical pulling. The wire so produced is sheathed in silica
which is removed by HF-based treatment.
[0120] The wires are then woven, knitted or braided into
appropriate design prior to porosification in a HF-based solution.
For 5-10 micron diameter wires, full porosification is achievable
with stain-etching techniques. Anodisation of fabric made from low
resistivity poly/single crystal silicon could alternatively be
used, especially for thicker fiber networks.
EXAMPLE 2
[0121] Preparation of Reinforced Silicon Fibers
[0122] Bundles of silicon composite fibers, prepared for example as
described in Example 1, can be processed into much tougher forms
using resin bonding techniques used to convert highly brittle glass
into extremely tough fiberglass. Although the resin used in
fiberglass may itself be brittle, the interface between the silicon
and the resin (as with the glass-resin interface) will act as an
effective barrier to crack propagation across the bundle.
EXAMPLE 3
[0123] Preparation of Extruded Polymer/Si Powder Fibers
[0124] A readily extrudable polymer is first blended with porous
silicon powder to form a composite that can still be processed into
fibers at moderate temperatures. The level of silicon incorporation
is between 1-10 wt % for inducing bioactivity, 10-60 wt % for
improving yield strength and 60-90 wt % for conferring electrical
conductivity to an insulating polymer, for example.
[0125] When a biodegradable polymer such as polylactic glycolic
acid (PLGA) is used, the composite textile retains
biodegradability. When an established textile material such as
Dacron.TM. is used, the inclusion of pSi powder can be incorporated
as one additional step in a commercial sequence. Where electrically
conducting polymers such as polyaniline or polyacetylene are used,
textiles obtained are not biodegradable but could be rendered
surface-bioactive by low levels of silicon addition.
EXAMPLE 4
[0126] Preparation of Hollow Amorphous/Polysilicon Microfibres
[0127] Commercially available polymer/silica fibers of diameters in
the range 20-200 micron with a hollow core of diameter in the range
5-100 micron is first coated along its length with amorphous/poly
silicon. This can be achieved by pulling the fiber through a vacuum
sputtering/CVD chamber and with deposition at room
temperature/elevated temperature respectively. The low thermal
stability of polymers restricts their coating to amorphous films
which can be carried out at room temperature. The core of the fiber
is then removed by soaking in a suitable solvent/HF-based
solution.
[0128] Porosification can once again be by stain etching or
anodisation. Since microcracks limit fiber strength, improved
drying techniques like supercritical processing are beneficial.
EXAMPLE 5
[0129] Preparation of Micromachined Poly-Si Fibers
[0130] Ultrathin fibers can be realised by the repetitive use of
standard silicon wafers that are repetitively subjected to surface
oxidation, poly-Si deposition, micromachining and HF-induced
release. Porosification is carried out by stain etching either
before or after the HF-release step. The maximum fiber length is
defined by wafer diameter and fiber cross-section by poly-Si layer
thickness and mask design.
EXAMPLE 6
[0131] Silicon Incorporation Into Preformed Fabric
[0132] Commercially available silicon powder (metallurgical grade
or solar grade purity) is mechanically milled down to submicron
particle size and then rendered porous by stain etching in an
HF-based solution. The porous powder is then subjected to UV ozone
treatment to generate surface hydroxyl groups. Their replacement by
3-chloropropyl (CP) groups is then achieved by using
(3-chloropropyl) trimethoxysilane (CP-TMS). Covalent binding via
propyl ether linkages to commercially available fabric, such as
cotton, linen or a synthetic fiber, is then achieved by
co-incubation in boiling toluene. After sufficient reaction time
(e.g. 2 hours at 110.degree. C.) sonification can be used to remove
physisorbed pSi powder, leaving only covalently linked silcon
powder bound to the surface of the fabric.
EXAMPLE 7
[0133] Silicon Incorporation into Preformed Yarn
[0134] Non porous or porous silicon fiber prepared by the process
described in Examples 1 or 5 is wrapped around textile fibers that
have already been spun into yarn. This can provide an electrically
conductive pathway within a fiber of predominantly standard textile
material.
EXAMPLE 8
[0135] Metal Replacement by Silicon in Silk Organza
[0136] Silk organza is a finely woven silk fabric, originating from
India, which combines gold, silver or copper with silk threads into
a fabric that is anisotropically conductive. The warp consists of
parallel silk threads. Through this warp, the weft is woven with a
silk thread that has been wrapped in a metal foil helix.
[0137] In this embodiment however, low resistivity silicon fibers
are used instead of the foil wrapped fibers. The spacing between
the fibers results in a correctly orientated strip of the fabric
functioning like a ribbon cable.
EXAMPLE 9
[0138] Preparation of Porous Silicon Fiber
[0139] Porous silicon fiber was obtained by cleaving a mesoporous
film on wafers over a glass slide covered in filter paper. An 80%
porosity and 64 micron thick film was fabricated from a (100)
oriented p+ wafer (0.005.+-.-20% ohm cm) made by anodisationat 100
milliamps per square centimeter, for 20 minutes in equal volumes of
40 wt % HF and pure ethanol. Upon scribing the back of the wafer
along directions parallel or perpendicular to the "wafer flat" and
then breaking over the aligned edge of a glass slide, some short
(1-10 mms) fibers broke away from the diced chip. Examples are
shown in FIG. 4 which show optical microscope images at .times.200
magnification of fibers of rectangular cross-section (a) 95-100
micron width (b) 25-30 micron width. The fibrous structures are
fully porous (80% porosity) and brown to the eye. FIG. 4(c) is
another optical microscope image of the larger fiber bent on a
carbon pad to illustrate its flexibility.
EXAMPLE 10
[0140] Preparation of Porous Silicon Fiber
[0141] In an alternative method, porous silicon fiber was obtained
by etching and anodisation of sawn silicon comb-like structures
like that shown in FIG. 5(a). The edge of a 3 inch diameter wafer
was given a series of 45 cuts using a 75 micron wide blade with a
pitch of 225 micron. This created an array of supported single
crystal beams of approximate length 12 mms and rectangular
cross-section 150.times.380 micron.
[0142] These beams were then subjected to ultrasonic cleaning in
acetone and then an isotropic wet etch in 70% nitric acid, glacial
acetic acid and 40% aqueous HF at a 5:1:1 volume ratio. This step
removes saw damage and is used to define cross-sectional profile of
fibers to be generated in the next step. The etching solution was
continuously stirred and etch duration was 7.5 minutes for the data
shown here. FIG. 5(b,c) shows the beam array in plan-view before
and after such an etch where the width has been reduced from
145.+-.5 micron to 25.+-.5 micron. The wafer segment was then
further cleaved into a narrower comb-like structure with no bulk
silicon adjacent to the ends of the protruding beams, in
preparation for anodisation. An anodic potential of 1.0 Volt was
applied for 5 minutes in 40% aqueous HF/ethanol (1:1 by volume)
between the suspended structure and a circular platinum crucible
acting as the cathode. Only the lower half of the beams were
immersed in the electrolyte. The resulting high current densities
caused the psi films to delaminate as fibers of uniform width
defined by beam dimensions and anodisation time.
[0143] As an alternative, it would be possible to initially apply
low current densities and then a sufficiently high current density
to cause "lift-off", as is well known in the production of pSi
membranes from whole wafers.
[0144] FIG. 6(a-c) show examples of a 20 micron wide, a 4 micron
wide and the end of a multistrand fiber respectively, obtained
using this method. As a result of the anodisation conditions used,
the porosity of these wholly porous structures is in excess of
60%.
EXAMPLE 11
[0145] Preparation of Woven Silicon Structures
[0146] The sawing and etching process described above was used to
demonstrate the feasibility of weaving pure silicon fibers using
larger structures. Square sections of wafers were sawn into
comb-like structures and given etch treatments to reduce thickness
to 300, 200 and 100 microns over much longer lengths.
[0147] FIG. 7(a) shows an example of a pure single crystal Si weave
where both warp and weft fibers have 200 micron thickness. FIG.
7(b,c) shows a Si weave containing weft fibers of 300 microns and
warp fibers of 100 microns thickness. The latter have a pSi coating
derived by stain etching as described in Example 12 hereinafter,
which gives them a brown colour to the eye.
EXAMPLE 12
[0148] Preparation of Si & Porous Si Coated Fabrics, Sutures
and Threads
[0149] A range of fibrous materials were conformally coated with
sputtered amorphous silicon in a modified Blazers Bak box coater
equipped with four, 400.times.125 mm planar magnetrons arranged
concentrically around a central substrate rotation stage. Coating
was by sputtering from a 99.999% pure Si target doped with boron at
5.times.10 18/cm 3 with the target power set at 500 W, a base
pressure of 10 -8 torr, an ionisation atmosphere of 5.times.10 -3
torr of argon and a substrate temperature of 50 C. The substrates
were rotated at speeds of between 0.0025 and 0.126 mm/sec and
supported on a custom-built framework to improve uniformity and
control thickness of coatings.
[0150] A commercially available polyester gauze (denier 100, mesh
156) was coated with 200 nm, 1 micron and 10 microns of silicon.
FIG. 8(a,b) show segments of the gauze with the thickest coating
and demonstrate the retained flexibility. The coatings were found
to adhere well, even when tied into a knot as shown. FIG. 9(a,b)
shows SEM images of part of the coated gauze and FIG. 9(c) the
corresponding EDX spectrum showing a low level of oxygen in the
coating.
[0151] All exposed surfaces of the multistrand fiber network were
found to have such a Si coating. FIG. 10(a,b,c) show the
corresponding images and spectrum of the same gauze after partial
porosification of the coating by stain etching. This was achieved
by immersion of the fabric in a conventional stain etch solution
containing 1 part by volume of 70% nitric acid to 100 parts by
volume 40% HF for 60 seconds. A 90 second etch was found to cause
partial delamination of the pSi coating. The porous nature of the
coating is evident from the raised oxygen and fluorine levels in
FIG. 10(c) and the presence of some cracks in FIG. 10(b).
[0152] Other examples of silicon coated structures include a
biodegradable suture (the multifiber thread of FIG. 11(a)), a metal
(aluminum) wire (FIG. 11(b)), hollow glass fiber and tubes (FIG.
11(c)) and other single strand polymer fibers. If desired, the
glass or metal interior structures may subsequently be dissolved
away using an appropriate solvent, such as hydrofluoric acid for
glass, and hydrochloric acid for aluminum.
[0153] FIG. 12 is an optical image showing the flexibility of the
suture and polymer fibers are maintained after Si coating.
EXAMPLE 12
[0154] Illustration of the Semiconducting Nature of Medical Fabrics
Comprising Silicon
[0155] The woven pure silicon structure of FIG. 7(a) was rendered
partially mesoporous by application of anodic electrical bias in
equal volumes of 40% HF and ethanol. During and following this
process the structure remained semiconductive since porous silicon
is itself semiconducting.
[0156] The gauze coated with porosified silicon of FIG. 10 was
cathodically biased in simulated body fluid. Prior to coating, the
gauze is electrically insulating. Any current flow is thus
restricted to the surface coating. This and the higher resistivity
of the amorphous silicon compared with that of crystalline silicon
resulted in a bias of 30 volts being needed to maintain a current
flow of 1 mA through the fabric.
[0157] The conductivity of the gauze can be raised by using a more
heavily doped (10 20 B/cm 3) Si target in the sputtering process,
or by rendering the Si coating polycrystalline by laser
annealing.
EXAMPLE 13
[0158] Preparation of Flexible Si Chains of Beads
[0159] Spherical polycrystalline Si granules of diameter 1-5 mm are
commercially available in kg quantities from MEMC Inc, USA. After
mechanical sieving these are size separated to for example 1.0 mm
diameter. A 500 micron diameter hole is then drilled through each
and batches given an isotropic chemical etch to remove drill
damage. Subsequent linear alignment of holes is followed by
threading, using a lead wire attached to the medical linking fiber.
Partial porosification is achieved by stain etching or anodisation
using a conducting Pt wire to link a chain of interconnected
spheres.
EXAMPLE 14
[0160] Preparation of Flexible Si Chains of Microbeads
[0161] A silicon wafer is thermally oxidised and a 150 micron thick
Si membrane bonded to that oxide surface. A linear array of
rectangular trenches are then deep dry etched to a depth of 50
microns into that Si coating and in-filled with a spun-on resist.
This step defines what will become the hollow core of every
bead.
[0162] Following surface planarisation and cleaning, another 100
micron thick Si membrane is wafer bonded to the array.
Photolithographically defined deep dry etching right through the
Si-resist-Si structure to a depth of 250 microns is then carried
out in a linear array pattern orthogonal to the resist channel
direction. This step divides the linear Si columns into rows of
aligned particles. The resist channels are then leached out by
solvent and a suitable microwire (<50 micron diameter) threaded
through at least one chain of aligned particles running across the
wafer diameter.
[0163] When the entire array is suitably threaded the wafer is
immersed in HF to dissolve the underlying oxide and release the
particle chains. Isotropic etching can be subsequently used to
remove sharp particle edges and stain etching to porosify the
surfaces of every particle in the chain.
* * * * *