Patent Application
20080034801
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.
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.
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.
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.
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.
The particular materials currently used in medical textiles include modified natural polymers, synthetic nonabsorbable polymers and synthetic absorbable polymers.
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.
The applicants have developed new medical fibers and textiles.
According to the present invention there is provided a fiber or fabric comprising silicon for use as a medical fiber or fabric.
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”.
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.
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.
As used herein, the term “fiber” refers to a unit of matter having length at least 100 times their diameter or width.
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.
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.
“Biocompatible” as used herein refers to materials which, in thin film form, are acceptable for use in at least some biological applications.
As used herein, the term “resorbable” relates to material which will dissolve at normal physiological temperatures (37° C.±1° 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.
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 Å, mesoporous silicon contains pores having a diameter in the range of 20 Å to 500 Å; and macroporous silicon contains pores having a diameter greater than 500 Å. 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.
In particular the silicon used in fibers and fabrics for use as medical fibers and fabrics is mesoporous silicon, which is resorbable.
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.
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.
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.
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 C1-6alkylene and in particular C2-4alkylene 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.
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 C1-4alkyl alcohols, at elevated temperatures, for example at the reflux temperature of the solvent.
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.
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.
Suitable fabrics include biomedical fabrics such as cotton, linen or synthetic polymers, which may be absorbable or non-absorbable.
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.
The silicon fibers used may comprise silicon alone, or they may be in the form of a composite of silicon with other materials.
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.
Pure Silicon fibers of varying crystalline perfection have also been realised by a number of techniques:
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.
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.
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.
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.
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° 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° C. and a working temperature of 1900° C. A pure Si charge was loaded into evacuated tubes that were then heated to 1900° 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.
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.
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.
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.
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.
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.
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.
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.
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.
These production methods form yet further aspects of the invention.
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)
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.
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.
Porous or partially porous silicon containing fibers and fabrics are novel and as such form a further aspect of the invention.
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.
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.
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.
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.
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.
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
Substantially pure silicon yarn is also novel, and forms yet a further aspect of the invention.
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.
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.
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
Knitting is a continuous single element technique illustrated for example in
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.
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.
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.
Finally, laminating is a way of joining of one fabric to another using an adhesive.
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.
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.
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.
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.
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/ˆc)ˆ0.5
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.
Malleability refers to the extent to which a metal can be manipulated before it breaks.
Work of fracture is the total energy needed to generate a fracture structure (J/mˆ2). For ductile materials like copper and aluminum, values range between 10ˆ4 and 10ˆ6 J/mˆ2, much higher than the free surface energy.
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×pˆ2) 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.
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.
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.
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.
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.
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.
A partially porous silicon fiber will not be fully biodegradable but could have substantially greater strength, and thus be preferable in certain situations.
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.
Fabrics prepared from substantially pure silicon fibers are novel and form a further aspect of the invention.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
A method of collecting sweat using this technique, as well as a method of locally heating skin forms particular embodiments of the invention.
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.
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.
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.
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.
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.
The invention will now be particularly described by way of example, with reference to the accompanying drawings in which:
Preparation of Extruded poly-Si Fibers and Fabrics Obtained Therefrom
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.
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.
Preparation of Reinforced Silicon Fibers
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.
Preparation of Extruded Polymer/Si Powder Fibers
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.
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™ 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.
Preparation of Hollow Amorphous/Polysilicon Microfibres
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.
Porosification can once again be by stain etching or anodisation. Since microcracks limit fiber strength, improved drying techniques like supercritical processing are beneficial.
Preparation of Micromachined Poly-Si Fibers
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.
Silicon Incorporation Into Preformed Fabric
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° C.) sonification can be used to remove physisorbed pSi powder, leaving only covalently linked silcon powder bound to the surface of the fabric.
Silicon Incorporation into Preformed Yarn
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.
Metal Replacement by Silicon in Silk Organza
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.
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.
Preparation of Porous Silicon Fiber
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
Preparation of Porous Silicon Fiber
In an alternative method, porous silicon fiber was obtained by etching and anodisation of sawn silicon comb-like structures like that shown in
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.
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.
Preparation of Woven Silicon Structures
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.
Preparation of Si & Porous Si Coated Fabrics, Sutures and Threads
A range of fibrous materials were conformally coated with sputtered amorphous silicon in a modified Blazers Bak box coater equipped with four, 400×125 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 5×10ˆ18/cmˆ3 with the target power set at 500 W, a base pressure of 10ˆ−8 torr, an ionisation atmosphere of 5×10ˆ−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.
A commercially available polyester gauze (denier 100, mesh 156) was coated with 200 nm, 1 micron and 10 microns of silicon.
All exposed surfaces of the multistrand fiber network were found to have such a Si coating.
Other examples of silicon coated structures include a biodegradable suture (the multifiber thread of
Illustration of the Semiconducting Nature of Medical Fabrics Comprising Silicon
The woven pure silicon structure of
The gauze coated with porosified silicon of
The conductivity of the gauze can be raised by using a more heavily doped (10ˆ20 B/cmˆ3) Si target in the sputtering process, or by rendering the Si coating polycrystalline by laser annealing.
Preparation of Flexible Si Chains of Beads
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.
Preparation of Flexible Si Chains of Microbeads
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.
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.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0130608.3 | Dec 2001 | GB | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10499535 | Jul 2004 | US |
| Child | 11907336 | Oct 2007 | US |
