WO2005113467A1

WO2005113467A1 - Silicon structure

Info

Publication number: WO2005113467A1
Application number: PCT/GB2005/001910
Authority: WO
Inventors: Roghieh Saffie; Keith Gordon Barraclough; Chi Hian Lau; Nassim Torabi-Pour; Leigh Trevor Canham; Armando Loni
Original assignee: Psimedica Limited
Priority date: 2004-05-21
Filing date: 2005-05-18
Publication date: 2005-12-01
Also published as: GB0411358D0; CA2564591A1; JP2007537965A; GB2414231A; EP1747180A1

Abstract

The present invention concerns a new method of fabricating macroporous silicon, and a new macroporous silicon product. The invention also concerns a new method of combining porous silicon with a hydrophilic compound. The method of fabricating the macroporous silicon involves the consolidation of a silicon particulate product, which can then be anodised. The resulting macroporous silicon product comprises macropores that are substantially surrounded by a region of microporous and/or mesoporous silicon. The method of loading the hydrophilic compound comprises the step of consolidating a silicon particulate product and the compound.

Description

Silicon Structure

The present invention relates to a new silicon structure and a new process for fabricating a silicon structure. The silicon structure may comprise one or more hydrophilic substances.

The invention also concerns a new process of fabricating macroporous silicon, and a new macroporous silicon product. The process for the fabrication of macroporous silicon may involve the consolidation of a silicon particulate product, followed by anodisation of the consolidated product. The resulting macroporous silicon product may comprise macropores that are substantially surrounded by microporous and/or mesoporous silicon.

Porous silicon has properties that allow it to be used for a variety of medical uses. For example it is a biocompatible and resorbable material as described in WO 9706101 ; it can be used as a scaffold for the repair or replacement of damaged bone as described in WO 0195952; it can be used in dermatological compositions as described in WO 0215863; it can be used to deliver beneficial substances such as drugs as described in WO 9953898; and it can be used in a variety of diagnostic devices as described in WO 03015636. The biological properties of porous silicon are often dependent upon porosity and pore size. Porous silicon has been formed that has a porosity as low as 2%, and in excess of 90%; it may be categorised by its pore size: microporous silicon contains pores having a diameter less than 20 A, mesoporous silicon contains pores having a diameter in the range 20 A to 500 A; and macroporous silicon contains pores having a diameter greater than 500 A.

The two main methods by which porous silicon can be fabricated are: by anodisation, and by stain etching. Anodisation typically involves the immersion of a solid sample of silicon, such as a bulk crystalline silicon wafer, in hydrofluoric acid solution. An electrical contact is made with the sample of silicon, a potential difference being applied between the silicon and a second electrode also placed in the solution. The HF etches the silicon to create pores and hence porous silicon is formed. Preferably the sample is semiconducting throughout its volume, to allow a uniform potential difference to be established.

Stain etching involves the immersion of a silicon sample in a hydrofluoric acid solution containing a strong oxidising agent. No electrical contact is made with the silicon, and no potential is applied. The hydrofluoric acid etches the surface of the silicon to create pores. The technique is commonly used to etch relatively small particles of silicon, since it would be difficult to attach an electrode to each small particle.

Anodisation and stain etching typically result in the formation of silicon - hydrogen bonds at the surface of the porous silicon, making it hydrophobic. Porous silicon may be used to deliver drugs to animal or human patients, and this hydrophobic nature can make the loading of hydrophilic drug into porous silicon problematic.

One of the main disadvantages of anodisation is its relatively low throughput and hence high cost. The use of an electrochemical cell reduces the speed at which silicon can be processed, hence increasing expense. Further, the silicon used for anodisation is preferably semiconducting throughout its volume, and this typically means that relatively expensive silicon wafers are employed.

Stain etching allows the use of particulate silicon that may be obtained at a lower price than silicon wafers, and does not involve the use of a time consuming electrochemical process.

However, it is easier to control the pore size and/or porosity of porous silicon fabricated by anodisation than by stain etch techniques.

The following documents provide background information that is relevant to the present application. US 5,164,138 describes a process for manufacturing an article having particles comprising a silicon based material; the particles are bonded to one another by reaction with a liquid agent. US 4,357,443 describes a process for producing a silicon containing article comprising the step of coating the particle with boron oxide. US 4,040,848 describes a process for producing a polycrystalline silicon sintered body which comprises the step of forming a particulate mixture of silicon powder and boron. US 4,865,245 describes a method of joining together two semiconductor devices, each having a number of metallic contacts. US 6,126,894 describes a method for producing a high density sintered article from iron-silicon alloys. US 4,818,482 describes a process for producing workpieces comprising water atomising a metal alloy. US 5,711 ,866 describes a process for consolidating powders comprising the step of removing an oxide from the surface of a metal coated composite. US 6,057,469 describes a process for the preparation of a silicon powder comprising the step of grinding metallurgical grade silicon. US 4,040,848 describes a process for producing a polycrystalline sintered body. WO 01/95952 describes a fixitor, which may be used for the repair of damaged bone, comprising porous silicon. WO 03/101504 describes a method of preparing a scaffold from blocks comprising porous silicon. US 4,767,585 describes a process for producing moulded products from granular silicon. US 4,759,887 describes a process for manufacturing shaped products from silicon granules. JP 8109012 describes press moulding at high temperatures. "The Compaction of Oxidised Silicon Powder", by RG Stephen & FL Riley, Journal of European Ceramic Society 9, (1992) 301-307 describes the fabrication of silicon dioxide coated silicon particles that result in agglomeration. "Production of Polycrystalline Silicon Sheets for Photovoltaic applications by pressing and sintering of silicon powders" by A Derbouz Draoua et al describes the fabrication of wafers from silicon powder by compaction and heating at temperatures close to the melting point of silicon. "Semiconductor Wafer Bonding: Science and Technology", Wiley, New York, ISBN 0471574813, 1999, describes bonding two planar single crystal wafer surfaces.

It is an objective of the present invention to at least partly solve at least some of the above mentioned problems. It is a further objective of the present invention to provide a process that allows the low cost, rapid fabrication of porous silicon having a well defined porosity and pore size.

It is a yet further objective of the present invention to provide a method of combining a hydrophilic compound with porous silicon. It is an even further objective to provide a new method of drug loading by consolidation of a silicon particulate product and a hydrophilic drug.

According to one aspect the invention provides a process for fabricating a silicon structure, the process comprising the steps:

(a) taking a silicon particulate product comprising a multiplicity of free silicon particles; and (b) consolidating at least part of the silicon particulate product to form a multiplicity of bonded silicon particles, each bonded silicon particle being bonded to at least one of the other bonded silicon particles. The process may comprise the further step, performed between steps (a) and (b), of combining at least part of the silicon particulate product with a beneficial substance. The beneficial substance may be a hydrophilic compound.

At least one of the bonds formed between at least two of the bonded silicon particles may be a covaient Si-Si bond. At least some of the bonded silicon particles may be Si-Si covalently bonded. Step (b) may be performed in such a manner that a Si-Si covaient bond is formed between at least two of the silicon particles. Step (b) may be performed such that sufficient pressure is applied to at least part of the silicon particulate product that a multiplicity of bonded silicon particles are formed.

Steps (a) and (b) may be performed in such a manner that a silicon unitary body is formed, the silicon unitary body comprising at least some of the bonded silicon particles.

Steps (a) and (b) may be performed in such a manner that the silicon unitary body comprises at least 10 bonded silicon particles. Steps (a) and (b) may be performed in such a manner that the silicon unitary body comprises at least 100 bonded silicon particles. Steps (a) and (b) may be performed in such a manner that the silicon unitary body comprises at least 1 ,000 bonded silicon particles. Steps (a) and (b) may be performed in such a manner that the silicon unitary body comprises between 10 and 10²⁶ bonded silicon particles. Steps (a) and (b) may be performed in such a manner that the silicon unitary body comprises between 10⁴ and 10¹⁶ bonded silicon particles.

The unitary body may be porous, the pores being formed by interstices between the bonded silicon particles. This porosity may result in a relatively high surface area.

Steps (a) and (b) may be performed in such a manner that a silicon unitary body is formed having a Fracture strength between 30 MPa and 7,000 MPa.

Steps (a) and (b) may be performed in such a manner that a silicon unitary body is formed having a Fracture strength between 70 MPa and 7,000 MPa.

Steps (a) and (b) may be performed in such a manner that a silicon unitary body is formed having a Fracture strength between 40 MPa and 250 MPa.

Steps (a) and (b) may be performed in such a manner that a silicon unitary body is formed having a Fracture strength between 50 MPa and 150 MPa. Steps (a) and (b) may be performed in such a manner that a silicon unitary body is formed having an electrical resistivity, measured across its longest dimension, between 10 K Ωcm and 10^"5Ωcm. Steps (a) and (b) may be performed in such a manner that a silicon unitary body is formed having an electrical resistivity, measured across its longest dimension, between 10 K Ωcm and 200 K Ωcm. Steps (a) and (b) may be performed in such a manner that a silicon unitary body is formed having an electrical resistivity, measured across its longest dimension, between 10 K Ωcm and 60 K Ωcm.

The formation of silicon - silicon covaient bonds between the bonded silicon particles may result in the unitary body having a relatively high mechanical strength and low electrical resistivity.

In general, the electrical resistivity of the bond formed between two bonded silicon particles will be higher than that of the silicon from which either of the particles is formed. The unitary body is therefore likely to have an electrical resistivity that is significantly higher than the silicon from which each particle is formed. The greater then number of bonds, the greater the resistivity, when calculated from the resistance of the unitary body across its largest dimension.

Steps (a) and (b) may be performed in such a manner that each of the bonded silicon particles from which the unitary body is formed, are integral with each of the other bonded silicon particles from which the silicon unitary body is formed.

The process may comprise the further step (r) of chemically reducing part of the silicon particulate product. The step (r) may be performed prior to step (b). The step (r) may comprise the step of substantially removing silicon oxide from at least part of the surface of the free silicon particles. The step (r) may comprise the step of treating at least some of the free silicon particles with a reducing agent. The step (r) may comprise the step of treating at least some of the free silicon particles with a reducing agent selected from one or more of: NaOH, KOH, and HF. The step (r) may comprise the step of treating at least some of the free silicon particles with a solution of hydrofluoric acid, the solution being selected from one or more of aqueous HF solution, ethanolic HF solution, methanolic HF solution, and ethanoic HF solution. The step (r) may comprise the step of treating at least some of the free silicon particles with HF vapour. The step (r) may be performed in such a manner that Si-H bonds are formed at the surface of at least some of the free silicon particles. The step (r) may be performed in such a manner that Si-H bonds are formed at the surface of most of the free silicon particles.

The treatment of the free silicon particles with hydrofluoric acid is advantageous because it results in the formation of free silicon particles having a surface that is at least partly hydrogen terminated, and because it at least partly removes any oxygen atoms that were bonded to the surface of the free silicon particles.

It has been discovered that the presence of oxygen atoms at the surface of the free silicon particles makes it more difficult to consolidate the silicon particulate product. In other words the presence of oxygen bonded to the surface of the silicon makes it more difficult to form Si-Si covaient bonds between the bonded silicon particles. The presence of oxygen reduces the stability of the unitary body in solutions of HF, making it more likely to fragment. Surface oxide may also increase the electrical resistivity of the unitary body.

The presence of the hydrogen atoms at the surface of the free silicon particles is also advantageous, because this helps to prevent oxygen re-bonding to the silicon surface prior to consolidation.

The consolidation of a silicon particulate product comprising surface Si-H bonds may result in the formation of silane. The method may comprise the further step (h) of detecting silane gas resulting from the formation of bonded silicon particles. The formation of silane provides evidence of Si - H bond breaking and Si - Si bond formation.

Step (b) may comprise the step (p) of applying pressure to at least some of the free silicon particles.

Step (b) may comprise the steps: (ci) of placing at least some of the free silicon particles in a container; and (di) reducing the volume of the container.

Step (ci) and step (di) may be performed in such a manner that pressure is applied to at least some of the free silicon particles contained in the container. The step (b) may comprise the steps: placing at least some of the free silicon particles in a container, and applying a uniaxial pressure or isostatic pressure to the free silicon particles contained in the container.

The step (b) may comprise the steps: placing at least some of the free silicon particles in a container, and applying an isostatic pressure or isostatic pressure to the free silicon particles contained in the container.

The uniaxial pressure may be between 5,000 MPa and 50 MPa. The uniaxial pressure may be between 1 ,000 MPa and 100 MPa. The uniaxial pressure may be between 1 ,000 MPa and 200 MPa. The uniaxial pressure may be between 750 MPa and 200 MPa. The uniaxial pressure may be between 500 MPa and 10 MPa.

The isostatic pressure may be between 5,000 MPa and 50 MPa. The isostatic pressure may be between 1 ,000 MPa and 100 MPa. The isostatic pressure may be between 1 ,000 MPa and 200 MPa. The isostatic pressure may be between 750 MPa and 200 MPa. The isostatic pressure may be between 500 MPa and 10 MPa.

Step (b) may comprise the steps: (cii) of placing at least some of the free silicon particles in a volume enclosed by at least part of a mould; and (dii) reducing the enclosed volume.

Step (cii) and step (dii) may be performed in such a manner that pressure is applied to at least some of the free silicon particles contained in the mould.

The silicon particulate product may comprise semiconducting silicon. The particulate silicon product may comprise one or more of: polycrystalline silicon, amorphous silicon, bulk crystalline silicon, and metallurgical grade silicon. The silicon particulate product may comprise silicon particles prepared by chemical vapour deposition. The silicon particulate product may comprise hydrogen terminated silicon particles, each hydrogen terminated particle comprising semiconducting silicon and surface Si - H bonds. The silicon particulate product may comprise oxygen terminated silicon particles, each oxygen terminated particle comprising semiconducting silicon and surface Si - O bonds. For the purposes of this specification metallurgical grade silicon is silicon that has been produced by the reduction of silica by carbon in an arc furnace at a temperature between 1500 °C and 2500 °C, has a purity in the range 95 to 99.9%.

At least some of the free silicon particles may comprise semiconducting silicon. At least some of the free silicon particles may comprise one or more of: polycrystalline silicon, amorphous silicon, bulk crystalline silicon, and metallurgical grade silicon. At least some of the free silicon particles may comprise silicon prepared by chemical vapour deposition.

The silicon particulate product may comprise porous silicon. At least some of the free silicon particles may comprise porous silicon. Each of the free silicon particles may comprise porous silicon.

The consolidation of the silicon particulate product may result in a porous unitary body, the pores being formed from the spaces between the bonded silicon particles. However, the free silicon particles may themselves be porous prior to consolidation. For example the free silicon particles may have been porosified by stain etching.

The silicon particulate product may comprise one or more of the following elements: Y, P, Sb, In, Fe, As, La, Ca, Pd, Sr, I, Co, Ir, B, Ge, Cu, Pb, Sn, and Au. At least some of the free silicon particles may comprise one or more of the following elements: Y, P, Sb, In, Fe, As, La, Ca, Pd, Sr, I, Co, Ir, B, Ge, Cu, Pb, Sn, and Au.

Preferably the silicon particulate product may comprise one or more of the following elements: Y, B, P, and Sn. Preferably at least some of the free silicon particles may comprise one or more of the following elements: Y, B, P, and Sn.

The process may comprise the further step (e) of porosifying at least part of the silicon unitary body. The process may comprise the further step (e) of porosifying at least part of the silicon unitary body by anodising the silicon unitary body in a solution of hydrofluoric acid. The process may comprise the further step (e) of porosifying at least part of the silicon unitary body by anodising the silicon unitary body in a solution of hydrofluoric acid, the solution comprising a surfactant. The surfactant may comprise one or more of: ethanol, methanol, acetic acid, a cationic surfactant, an anionic surfactant. The addition of a surfactant to the HF acid solution may improve the wetting of the silicon unitary body by the HF solution.

The step (e) may comprise the step of allowing a solution of HF to enter the pores of the unitary body, the pores being formed by the spaces between the bonded silicon particles from which the unitary body is formed.

For porosification of the unitary body by anodisation to be effective, the unitary body must have a sufficiently high electrical conductivity, and must have sufficient structural stability when immersed in a solution of HF. The unitary body may be formed from a very large number of free silicon particles, and therefore the required stability and conductivity may only be achieved by forming a correspondingly large number of bonds between the silicon particles. The strength of the bonds formed and degree of contact between the bonded silicon particles will also affect the success of the anodisation process.

The use of a surfactant may assist the ingress of the hydrofluoric acid solution into pores located between the bonded silicon particles.

The process may comprise the further step (e) of porosifying at least part of the silicon unitary body by stain etching the silicon unitary body in a solution of hydrofluoric acid.

The step (e) may be preceded by the step of attaching at least one electrode to the silicon unitary body.

The unitary body may comprise a plurality of macropores, each pore being formed at least partly by the interstices between the bonded silicon particles. The mean size of the macropores contained in the unitary body may have a size between 500 A and 200 microns.

The unitary body may comprise a plurality of pores, each pore being formed at least partly by the interstices between the bonded silicon particles. The unitary body may comprise a multiplicity of nanoparticles, the mean size of the pores contained in the unitary body may have a size between 50 A and 1 micron. The step (e) may comprise the step of allowing a solution of hydrofluoric acid to pass into at least one of the pores of the unitary body. The step (e) may comprise the step of allowing a solution of hydrofluoric acid to pass into substantially all the pores of the unitary body. The step (e) may comprise the step of allowing a solution of hydrofluoric acid to pass into some of the pores of the unitary body.

The step (e) may be performed in such a manner that at least one of the bonded silicon particles is porosified throughout its volume. The step (e) may be performed in such a manner that at least one of the bonded silicon particles is porosified through substantially its whole volume. The step (e) may be performed in such a manner that substantially each of the bonded silicon particles is porosified through substantially its whole volume.

The fabrication of a macroporous silicon unitary body in this way, allows the anodisation of a relatively inexpensive silicon particulate product, such as metallurgical grade silicon. The silicon particulate product is consolidated to form a unitary body that has sufficient mechanical strength and size to allow the attachment of an electrode, and hence anodisation. The macroporous silicon body has a high surface area so that the yield of porous silicon is high relative to the amount of silicon used.

The step (e) may be performed in such a manner that microporous silicon and/or mesoporous silicon is formed from the silicon unitary body.

The unitary body may already be porous, as a result of pores formed from the spaces between the bonded silicon particles and/or as a result of the particulate product comprising free porous silicon particles, before step (e) is performed.

The process may comprise the further step (g), performed after step (e), of fragmenting the silicon unitary body. The step (g) may comprise the step of mechanically crushing the unitary body. The step (g) may comprise the step of ultrasonically fragmenting the unitary body. The step (g) may be performed in such a manner that a multiplicity of partially surface porous silicon particles are generated, the surface of each partially surface porous particle comprising a porous area and a non-porous area.

A method that comprises the steps (e) and (g) allows the formation of small anodised porous silicon particles, that could not be fabricated by other prior art methods. Each bonded silicon particle is bonded to at least one other bonded silicon particles, the bond or bonds may be formed by applying pressure to two or more free silicon particles.

The silicon unitary body may comprise a first silicon bonded particle and a second silicon bonded particle. The first and second bonded silicon particles may be integral with each other without being in direct contact with each other. In other words the first and second bonded silicon particles may be connected by an intermediate bonded silicon particle(s).

Step (b) may comprise the step (h) of heating the silicon particulate product. Step (b) may comprise the step of heating the silicon particulate product to a temperature between 50 °C and 500 °C. The step (b) may comprise the step of maintaining the silicon particulate product at a substantially constant temperature.

The step (b) may be performed at a temperature between -5 °C and + 5 °C for an interval of time between 1 second and 1 hour. The step (b) may comprise the step of maintaining the silicon particulate product at a temperature between -20 °C and + 20 °C for an interval of time between 0.1 seconds and 1 hour. The step (b) may be performed at a temperature between -50 °C and + 50 °C for between 1 minute and 10 hours.

The step (p) of applying a pressure to at least some of the free silicon particles may precede the step (h) of heating the silicon particulate product.

The step (b) may comprise the step of cold pressing at least part of the silicon particulate product.

Steps (a) and (b) may be performed in such a manner that the silicon unitary body has a surface area greater than or equal to 10 cm² per gram of silicon. Steps (a) and (b) may be performed in such a manner that the silicon unitary body has a surface area greater than or equal to 100 cm² per gram of silicon. Steps (a) and (b) may be performed in such a manner that the silicon unitary body has a surface area greater than or equal to 1 ,000 cm² per gram of silicon.

The surface area of a silicon unitary body formed by a cold pressing technique may be high, relative to that of a silicon unitary body formed by a hot pressing technique. This is because hot pressing can result in rearrangement of the surface silicon atoms, causing cavities and defects to be removed.

The process may further comprise the step (i) of introducing a gas to a region in which at least some of the free silicon particles are located; the gas may comprise one or more of: nitrogen, helium, argon, and hydrogen.

The process may comprise the step (v) of removing a gas from a region in which at least some of the free silicon particles are located. The process may comprise the step of removing a gas from a region in which at least some of the free silicon particles are located in such a manner that the pressure is reduced to less than 1 mm Hg.

The step (b) may be performed in an inert atmosphere or in an atmosphere comprising H₂ gas. The inert atmosphere may comprise a noble gas such as argon.

The step (b) and/or the step (h) may be performed after and/or during the step (i) and/or (v).

The process may comprise the step, performed between steps (a) and (b), of combining the silicon particulate product with a beneficial substance, steps (a) and (b) being performed in such a manner that the beneficial substance is located in the pores between the bonded silicon particles.

By, at least partly, trapping the beneficial substance within the consolidated product formed by steps (a) and (b), the release of the substance may be controlled. The process is therefore of particular value in the fabrication of pharmaceutical products comprising hydrophilic drugs, for which controlled release in physiological environments may be required. The fabrication of the bonded silicon particles from free porous silicon particles may be advantageous, since this may help to trap the beneficial substance in the pores formed by the bonded silicon particles.

The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise a multiplicity of beneficial substance molecules, each beneficial substance molecule having greater than 100 atoms. The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise a multiplicity of beneficial substance molecules, each beneficial substance molecule having greater than 1000 atoms. The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise a multiplicity of beneficial substance molecules, each beneficial substance molecule having between 100 and 5,000 atoms.

The step of combining the beneficial substance with the silicon particulate product may comprise the step of contacting at least part of the silicon particulate product with one or more of: beneficial substance vapour, beneficial substance gas, liquid beneficial substance, solid beneficial substance, and a solution of a beneficial substance.

The process may comprise the further step of fragmenting the consolidated product formed by steps (a) and (b).

For the purposes of this specification a "beneficial substance" is something, which when administered to a human or animal subject, is beneficial overall: it could be a toxin, toxic to undesirable cells/to interfere with an undesirable physiological process. For example, anti-cancer substances would be considered "beneficial", even though their aim is to kill cancer cells.

The silicon particulate product may have a mean particle size betweenl x 10^"4 and 1 x 10^"2 microns. The silicon particulate product may have a mean particle size between 1 x 10^"3 and 1 x 10^"2 microns. The silicon particulate product may have a mean particle size between 2 x 10^"3 and 1 x 10^"2 microns.

The silicon particulate product may have a mean particle size between 0.01 microns and 5 mm. The silicon particulate product may have a mean particle size betweenl micron and 500 microns. The silicon particulate product may have a mean particle size between 1 micron and 1 mm. The silicon particulate product may have a mean particle size between 1 nm and 150 microns.

At least one tenth of the free silicon particles from which the silicon particulate product is formed may each have a largest dimension between 1 x 10^"4 and 1 x 10^"2 microns. At least one tenth of the free silicon particles from which the silicon particulate product is formed may each have a largest dimension between 1 micron and 500 microns. According to a further aspect the invention provides a process for fabricating a silicon structure comprising silicon and a beneficial substance, the process comprising the steps:

(a) combining a silicon particulate product, comprising a multiplicity of silicon particles, with a beneficial substance; and

(b) consolidating at least part of the silicon particulate product and at least part of the beneficial substance to form a silicon structure comprising silicon and a beneficial substance.

The silicon structure may comprise a unitary body, the unitary body comprising at least part of the beneficial substance, and at least part of the silicon particulate product.

The method may comprise the further step of fragmenting the unitary body from which the silicon structure is at least partly formed.

The silicon particulate product may comprise one or more of porous silicon, polycrystalline silicon, bulk crystalline silicon, amorphous silicon, and metallurgical grade silicon. The silicon particulate product may comprise stain etched porous silicon and/or anodised porous silicon. The silicon particulate product may comprise silicon prepared by chemical vapour deposition.

The porous silicon may comprise one or more of: microporous silicon, macroporous silicon, and mesoporous silicon.

The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise other drugs that are difficult to introduce into the pores of porous silicon by prior art methods. The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise a multiplicity of beneficial substance molecule, each beneficial substance molecule having greater than 100 atoms. The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise a multiplicity of beneficial substance molecule, each beneficial substance molecule having greater than 1000 atoms. The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise a multiplicity of beneficial substance molecule, each beneficial substance molecule having between 100 and 5,000 atoms. The step (a) may comprise the step of contacting at least part of the silicon particulate product with one or more of: beneficial substance vapour, beneficial substance gas, liquid beneficial substance, a solution of a beneficial substance.

Step (b) may comprise the steps: (ci) of placing at least some of the silicon particles and at least some of the beneficial substance in a container; and (di) reducing the volume of the container.

Step (ci) and step (di) may be performed in such a manner that pressure is applied to at least some of the free silicon particles, and to at least some of the beneficial substance, contained in the container.

The step (b) may comprise the steps: placing the silicon particulate product and the beneficial substance into a container, and applying a uniaxial pressure to at least some of the beneficial substance, and at least some of the silicon particulate product in the container.

The uniaxial pressure may be between 5,000 MPa and 50 MPa. The uniaxial pressure may be between 1 ,000 MPa and 100 MPa. The uniaxial pressure may be between 1,000 MPa and 200 MPa. The uniaxial pressure may be between 750 MPa and 200 MPa. The uniaxial pressure may be between 500 MPa and 10 MPa.

Step (b) may comprise the steps: (cii) placing at least some of the silicon particles and at least some of the beneficial substance in a volume enclosed by at least part of a mould; and (dii) reducing the enclosed volume.

Step (cii) and step (dii) may be performed in such a manner that pressure is applied to at least some of the silicon particles contained in the mould and at least some of the beneficial substance contained in the mould.

The silicon structure may form at least part of a medical device. The step (b) may comprise the step of maintaining the silicon particulate product and the beneficial substance at a temperature between -5 °C and + 5 °C for an interval of time between 1 second and 1 hour. The step (b) may comprise the step of maintaining the silicon particulate product and the beneficial substance at a temperature between -20 °C and + 20 °C for an interval of time between 0.1 seconds and 10 hours. The step (b) may comprise the step of maintaining the silicon particulate product at a temperature between - 50 °C and + 50 °C for between 1 minute and 1 hour.

According to a further aspect the invention provides a process for fabricating a silicon structure comprising the step of sandwiching a beneficial substance between at least two silicon layers to form the structure.

The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise a multiplicity of beneficial substance molecule, each beneficial substance molecule having greater than 100 atoms. The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise a multiplicity of beneficial substance molecule, each beneficial substance molecule having greater than 1000 atoms. The beneficial substance may comprise a hydrophilic compound. The beneficial substance may comprise a multiplicity of beneficial substance molecule, each beneficial substance molecule having between 100 and 5,000 atoms.

At least one of the silicon layers may comprise a porous silicon membrane. At least one of the silicon layers may comprise a porous silicon membrane having a largest dimension between 0.5mm and 20mm. At least one of the silicon layers may be substantially planar. At least one of the silicon layers may be substantially spherical. The beneficial substance may comprise one or more layers.

The porous silicon may comprise one or more of: microporous silicon, macroporous silicon, and mesoprous silicon. The method may further comprise the step of applying a sealant substance to at least part of the surface of the silicon structure. The method may comprise the further step of applying a sealant substance to at least part of the silicon structure, in such a manner that egress of the beneficial substance, other than that resulting from erosion of the porous silicon or from diffusion through the pores of the porous silicon, is substantially prevented when the silicon structure is placed in a physiological electrolyte.

The step of sandwiching the beneficial substance may comprise the step of mechanically contacting the beneficial substance with at least part of said at least two silicon layers. The step of sandwiching the beneficial substance may comprise the step of applying pressure to both or each of the layers in such a manner that the beneficial substance contacts at least part of both or each of the layers.

By stacking alternating layers of silicon and a beneficial substance and optionally applying a sealant to the edges of the sandwich structure, a variety of configurations may be achieved. This allows greater control over the loading and release of a beneficial substance, and is of particular value with regard to loading of hydrophilic substances. Release of the beneficial substance, when the structure is immersed in a physiological environment, may occur as a result of diffusion through macropores formed by contact between the silicon layers; alternatively it may occur as a result of diffusion through, or erosion of, the silicon layer.

The method may comprise the further step of fragmenting the sandwich structure.

According to a further aspect the invention provides a product obtainable by a process as defined in any of the above aspects.

According to a further aspect, the invention provides a silicon unitary body comprising a silicon skeleton.

The silicon unitary body may further comprise macroporous silicon having a mean pore size between 500 A and 200 microns; and microporous silicon and/or mesoporous silicon. The silicon unitary body may further comprise macroporous silicon having a mean pore size between 500 A and 10 microns; and microporous silicon and/or mesoporous silicon.

The silicon unitary body may further comprise macroporous silicon having a mean pore size between 1 micron and 100 microns; and microporous silicon and/or mesoporous silicon.

The silicon unitary body may have a largest dimension between 1 mm and 5 cm. The silicon unitary body may have a largest dimension between 1 cm and 50 cm.

The at least 0.1% of the surface silicon atoms of the unitary body may each be bonded to a hydrogen atom. The at least 1% of the surface silicon atoms of the unitary body may each be bonded to a hydrogen atom. The at least 10% of the surface silicon atoms of the unitary body may each be bonded to a hydrogen atom.

The silicon unitary body may have a surface area between 10 cm² and 200 cm² per gram of silicon. The silicon unitary body may have a surface area between 50 cm² and 500 cm² per gram of silicon. The silicon unitary body may have a surface area between 10 cm² and 10,000 cm² per gram of silicon.

At least one tenth of the boned silicon particles, from which the silicon unitary body is formed, may each have a largest dimension between 0.01 microns and 500 microns.

At least one tenth of the bonded silicon particles, from which the silicon unitary body is formed, may each have a largest dimension between 1 nm and 10 microns.

The silicon unitary body may comprise bonded silicon particles having a mean particle size between 0.01 microns and 5 mm. The silicon unitary body may comprise bonded silicon particles having a mean particle size between 1 micron and 500 microns. The silicon unitary body may comprise bonded silicon particles having a mean particle size between 1 micron and 1 mm. The silicon unitary body may comprise bonded silicon particles having a mean particle size between 1 nm and 150 microns.

The silicon unitary body may comprise bonded silicon nanoparticles, having largest dimension in the range 1 to 50 nm, it may comprise micro and/or mesopores, formed by the spaces between the bonded nanoparticles, and may be resorbable in physiological environments.

The silicon unitary body may further comprise microporous silicon having a mean pore size between 1 x 10^~4 and 1 x 10^"2 microns, the micropores being formed by the spaces between the silicon particles.

The silicon unitary body may further comprise microporous silicon having a mean pore size between 1 x 10^"3 and 1 x 10^"2 microns. The silicon unitary body may further comprise microporous silicon having a mean pore size between 2 x 10³ and 1 x 10^"2 microns.

For the purposes of this specification an interconnected macropore is a macropore that is connected to at least one other macropore by one or more mesopores and/or one or more micropores.

The unitary body may comprise at least one interconnected macropore, the unitary body may comprise at least ten interconnected macropores. The unitary body may comprise at least 100 interconnected macropores. The unitary body may comprise at least 1,000 interconnected macropores.

The unitary body may comprise at least one interconnected macropore per 10 adjacent macropores. The unitary body may comprise at least one interconnected macropore per 100 adjacent macropores. The unitary body may comprise at least one interconnected macropore per 1 ,000 adjacent macropores.

At least one of the macropores may be defined by at least part of a microporous surface and/or mesoporous silicon surface. At least some of the macropores may be defined by at least part of the microporous silicon surface and/or mesoprorous silicon surface. Each of the macropores may be defined by at least part of the microporous silicon surface and/or mesoprorous silicon surface.

At least some of the macropores may be formed in at least part of the silicon skeleton, and the or part of the silicon skeleton from which the macropores are formed may comprise at least part of the microporous silicon and/or mesoporous silicon. The silicon unitary body may have an electrical resistivity, when measured across its longest dimension, between 10 K Ωcm and 10^"5Ωcm The silicon unitary body may have an electrical resistivity, when measured across its longest dimension, between 10 K Ωcm and 250 K Ωcm. The silicon unitary body may have an electrical resistivity, when measured across its longest dimension, between 10 K Ωcm and 100 K Ωcm.

The silicon unitary body may have a fracture strength between 30 MPa and 1 ,000 MPa. The silicon unitary body may have a fracture strength between 70 MPa and 7,000 MPa. The silicon unitary body may have a fracture strength between 40 MPa and 250 MPa. The silicon unitary body may have a fracture strength between 50 MPa and 150 MPa.

The silicon unitary body may comprise one or more of the following elements: Y, P, Sb, In, Fe, As, La, Ca, Pd, Sr, I, Co, Ir, B, Ge, Cu, Pb, Sn, and Au. The silicon unitary body may comprise one or more of the following isotopes: ⁹⁰Y, ³²P, ¹²⁴Sb, ¹¹⁴ln, ⁵⁹Fe, ⁷⁶As, ¹⁴⁰ La, ⁴⁷Ca, ¹⁰³Pd, ⁸⁹Sr, ¹³¹l, ¹²⁵l, ⁶⁰Co, ¹⁹²lr, ¹²B, ¹⁰B, ⁷¹Ge, ⁶⁴Cu, ²⁰³Pb and ¹⁹⁸Au.

The silicon unitary body may form at least part of a cancer treatment device comprising a radionucleotide and/or a cyotoxic agent for use in the treatment of cancer.

The silicon unitary body may form at least part of a cancer treatment device comprising a radionucleotide selected from one or more of the following radionucleotides ⁹⁰Y, ³²P, ¹²⁴Sb, ¹¹⁴ln, ⁵⁹Fe, ⁷⁶As, ¹⁴° La, ⁴⁷Ca, ¹⁰³Pd, ⁸⁹Sr, ¹³¹l, ¹²⁵l, ⁶⁰Co, ¹⁹²lr, ¹²B, ¹⁰B, ⁷¹Ge, ⁶⁴Cu, ²⁰³Pb and ¹⁹⁸ Au for use in the treatment of cancer.

The silicon unitary body may form at least part of a cancer treatment drug delivery device comprising a cytotoxic agent selected from one or more of: an alkylating agent such as chlorambucil, a cytotoxic antibody such as doxorubicin, an antimetabolite such as fluorouracil, a vinca alkaloid such as vinblastine, a hormonal regulator such as GNRH, and a platinum compound such as cis platin.

The silicon unitary body may form at least part of a drug delivery device comprising a beneficial substance. The silicon unitary body may form at least part of a drug delivery device comprising a hydrophilic beneficial substance. The silicon unitary body may form at least part of a cancer treatment device having one or more of the following radionucleotides ⁹⁰Y, ³²P, ¹²⁴Sb, ¹¹⁴ln, ⁵⁹Fe, ⁷⁶As, ¹⁴⁰ La, ⁴⁷Ca, ¹⁰³Pd, ⁸⁹Sr, ¹³¹l, ¹²⁵l, ⁶⁰Co, ¹⁹²lr, ¹²B, ⁷¹Ge, ⁶⁴Cu, ²⁰³Pb and ¹⁹⁸Au for use in the treatment of one or more of the following cancers: prostate cancer, liver cancer, pancreatic cancer, breast cancer, lung cancer, brain cancer, and testicular cancer.

The unitary body may form at least part of an orthopaedic scaffold for use in the repair or replacement of bone. The unitary body may form at least part of a tissue engineering scaffold for use in the repair or replacement of soft tissue.

The silicon unitary body may comprise semiconducting silicon. At least some of the free silicon particles may comprise one or more of: polycrystalline silicon, amorphous silicon, bulk crystalline silicon, and metallurgical grade silicon.

The silicon skeleton may comprise a multiplicity of bonded silicon particles, each bonded silicon particle being bonded to at least one of the other bonded silicon particles.

At least some of the bonded silicon particles may comprise one or more of macroporous silicon, mesoporous silicon, and microporous silicon.

The silicon unitary body may form at least part of a drug delivery implant comprising a beneficial substance and a binder substance, the binder substance having a structure and composition such that it binds at least part of the beneficial substance to at least part of the silicon skeleton.

The silicon unitary body may form at least part of a drug delivery implant comprising a beneficial substance and a fragmenting substance, the fragmenting substance having a structure and composition such that, when immersed in a physiological electrolyte, reacts with the electrolyte to release a gas.

According to a further aspect the invention provides a composite unitary body comprising a composite skeleton, the composite skeleton comprising silicon and a beneficial substance.

The silicon particulate product may comprise one or more of porous silicon, polycrystalline silicon, bulk crystalline silicon, amorphous silicon, and metallurgical grade silicon. The silicon particulate product may comprise stain etched porous silicon and/or anodised porous silicon.

The silicon particulate product may comprise one or more of: microporous silicon, macroporous silicon, and mesoporous silicon.

The beneficial substance may comprise a hydrophilic compound.

The composite unitary body may comprise a plurality of macropores. The mean size of the macropores contained in the unitary body may have a size between 50 A and 200 microns.

The composite unitary body may form part of a pharmaceutical product for the delivery of the beneficial substance to an animal or human subject. The unitary body may form part of an implant for the delivery of the beneficial substance to an animal or human subject.

The composite unitary body may form at least part of a pharmaceutical product comprising a beneficial substance and a binder substance, the binder substance having a structure and composition such that it binds at least part of the beneficial substance to at least part of the silicon.

The composite unitary body may form at least part of a pharmaceutical product comprising a beneficial substance and a fragmenting substance having a structure and composition such that, when immersed in a physiological electrolyte, reacts with the electrolyte.

According to a further aspect the invention provides a multilayer silicon structure comprising two or more silicon layers, and one or more beneficial substance layers, the beneficial substance being sandwiched between the or at least two of the silicon layers.

The multilayer structure may comprise alternating layers of beneficial substance and silicon.

The beneficial substance may comprise a hydrophilic compound. The silicon, from which both or each of the silicon layers is formed, may comprise one or more of: porous silicon, polycrystalline silicon, amorphous silicon, and bulk crystalline silicon.

At least one of the silicon layers may comprise a porous silicon membrane. The or at least one of the silicon membranes may have a largest dimension between 0.5mm and 20mm. At least one of the silicon layers may be substantially planar. At least one of the silicon layers may be substantially spherical. The beneficial substance may comprise two or more layers.

The porous silicon may comprise one or more of: microporous silicon, macroporous silicon, and mesoprous silicon.

The silicon structure may comprise a sealant substance that is in contact with at least part of said at least two silicon layers. The silicon structure may comprise a sealant substance that is in contact with at least part of said at least two silicon layers in such a manner that egress of the beneficial substance, other than that resulting from erosion of the porous silicon or from diffusion through the pores of the porous silicon, is substantially prevented when the pharmaceutical product is placed in a physiological electrolyte.

According to a further aspect the invention provides a partially surface porous silicon particulate product comprising a multiplicity of partially surface porous silicon particles, the surface of each partially surface porous particle comprising a porous area and a non- porous area.

At least one of the partially surface porous silicon particles may have at least two discrete non-porous areas. At least some of the partially porous silicon particles may each have two or more discrete non-porous areas.

At least one of the partially surface porous silicon particles may comprise a first non- porous area and a second non-porous area, the first and second non-porous area being spatially separate from each other by a porous area.

The partially surface porous silicon particulate product may comprise at least 100 partially surface porous silicon particles. The partially surface porous silicon particulate product may comprise between 100 and 10²⁶ partially surface porous silicon particles. The partially surface porous silicon particulate product may comprise between 100 and 10⁶ partially surface porous silicon particles. The partially surface porous silicon particulate product may comprise between 100 and 10³ partially surface porous silicon particles.

Substantially each partially surface porous silicon particle may have a size between 0.5 microns and 200 microns.

Between 10% and 90% of all of the partially surface porous silicon particles may have a size between 1 and 150 microns.

At least one of the partially surface porous silicon particles may comprise one or more of the following elements: Y, P, Sb, In, Fe, As, La, Ca, Pd, Sr, I, Co, Ir, B, Ge, Cu, Pb, Sn, and Au. At least one of the partially surface porous silicon particles may comprise one or more of the following isotopes: ⁹⁰Y, ³²P, ¹²⁴Sb, ¹¹⁴ln, ⁵⁹Fe, ⁷⁶As, ¹⁴⁰ La, ⁷Ca, ¹⁰³Pd, ⁸⁹Sr, ¹³¹l, ¹²⁵l, ⁶⁰Co, ¹⁹²lr, ¹²B, ¹⁰B, ⁷¹Ge, ⁶⁴Cu, ²⁰³Pb and ¹⁹⁸Au.

The partially surface porous silicon particulate product may comprise one or more of the following elements: Y, P, Sb, In, Fe, As, La, Ca, Pd, Sr, I, Co, Ir, B, Ge, Cu, Pb, Sn, and Au. The partially surface porous silicon particulate product may comprise one or more of the following isotopes: ⁹⁰Y, ³²P, ¹²⁴Sb, ¹¹⁴ln, ⁵⁹Fe, ⁷⁶As, ¹⁴⁰La, ⁴⁷Ca, ¹⁰³Pd, ⁸⁹Sr, ¹³¹l, ¹²⁵l, ⁶⁰Co, ¹⁹²lr, ¹²B, ¹⁰B, ⁷¹Ge, ⁶⁴Cu, ²⁰³Pb and ¹⁹⁸Au.

At least one of the partially surface porous silicon particles may be bonded to one or more of the other partially surface porous silicon particles from which the particulate product is formed.

At least one of the partially surface porous silicon particles may be covalently bonded to one or more of the other partially surface porous silicon particles from which the particulate product is formed.

According to a further aspect the invention provides a silicon structure, as defined in any of the above aspects, for use as a medicament. According to a further aspect the invention provides a unitary body, as defined in any of the above aspects, for use as a medicament.

According to a further aspect the invention provides a fragmented silicon unitary body, as defined in any of the above aspects, for use as a medicament.

According to a further aspect the invention provides metallurgical grade silicon for use as a medicament. The metallurgical grade silicon may comprise calcium and/or iron. The metallurgical grade silicon may comprise calcium, the molar concentration of the calcium being grater than that of any other impurity contained in the silicon. The metallurgical grade silicon may comprise iron, the molar concentration of the iron being greater than that of any other impurity contained in the silicon. The metallurgical grade silicon may comprise a toxic component selected from one or more of: arsenic, cadmium, lead, and mercury. The toxic component preferably has a concentration less than 10ppm. The metallurgical grade silicon may comprise aluminium; the aluminium may be present at a concentration less than 1 ,000 ppm.

The invention will now be described, by way of example only, with reference to the following drawings:

Figure 1 shows the variation of the release of neutral red with time, measured in days, from a silicon structure according to the present invention;

Figure 2 shows the effect of pre-loading neutral red on the rate of release from a silicon structure according to the present invention;

Figure 3 shows the variation of accumulative concentration of Interferon gamma with time, measured in days, from a silicon structure according to the present invention;

Figure 4 shows the variation of the accumulative concentration of Placental alkaline phosphate with time, measured in days, from a silicon structure according to the present invention; Figure 5 shows SEM images of a porous silicon membrane after it has been immersed in Trizma buffer for an interval of several days;

Figure 6a shows a photograph of a first cold pressing device used to fabricate a silicon unitary body according to the present invention;

Figure 6b shows a photograph of some of the components from which the figure 6a first cold pressing device is formed;

Figure 7a shows a SEM micrograph of part of a silicon unitary body according to the invention;

Figure 7b shows a SEM micrograph, of part of the same silicon unitary body shown in figure 7a, at a higher magnification;

Figure 8 shows a photograph of the components of a second cold pressing device used to fabricate a silicon unitary body according to the present invention, the second cold pressing device comprises a 5mm die 81;

Figure 9 shows a silicon unitary body fabricated using the second cold pressing device, the components of which are shown in figure 8;

Figure 10 shows a porosified surface of part of a silicon anodised unitary body according to the invention; and

Figure 11 shows the porosified surface shown in figure 10 at higher magnification.

The following description is divided into two sections. The first provides an account of the combination of silicon with a beneficial substance, particularly by consolidation of a silicon particulate product. The second contains a disclosure of silicon consolidation and anodisation of the resulting silicon unitary body.

(I) Silicon structure comprising a beneficial substance Approximately 5 mg of neutral red, which is a hydrophilic dye, was mixed with 60 mg of a silicon particulate product, and the mixture was consolidated by loading it into a clamped stainless steel press having two interlocking halves. Pressure was applied to the mixture by means of the press for 20 seconds. This method of consolidation will be referred to as Method A. Three different mixtures were prepared using particulate products comprising stain etched porous, anodised porous, and polycrystalline silicon. The three consolidated samples were then immersed in a Trizma buffer, and the release of the dye was determined by measuring the change absorbance at 573 nm. The results, presented in figure 1 , show that the use of anodised porous silicon gives the slowest rate of release, labelled AN, that of stain etched porous silicon gives an intermediate rate of release, labelled SE, and that of polycrystalline silicon gives a relatively fast release, labelled PolySi. These results are particularly relevant to drug delivery, because they show that the rate of drug release may be controlled, by varying the form of silicon used.

Figures 2 (a) and (b) show the accumulative release of neutral red from anodised porous silicon and stain etched porous silicon respectively.

The results labelled 2ai are for stain etched porous silicon and neutral red mixture that was been consolidated by method A. The results labelled 2aii are for stain etched porous silicon that was preloaded with neutral red by rotary evaporation or freeze drying before compression by method A. The results show that preloading provides better sustainable release over a 7 day dissolution period relative to the un-preloaded sample. Similar results are shown in figure 2(b); those labelled 2bi are for an anodised porous silicon and neutral red mixture silicon that has been consolidated by method A, and those labelled 2bii are for anodised porous silicon that has been preloaded with neutral red before method A consolidation.

Similar experiments were performed by replacing the neutral red with: Placental alkaline phosphate (PLAP) and Interferon gamma (γ-IFN). Figure 3 shows the accumulative release of γ-IFN using anodised porous silicon which has been pre-loaded by freeze- drying. The sample was recovered at the termination of the 4 day study, crushed, and release was again measured for the crushed sample. A further 3% of the remaining γ-IFN was released after crushing. Figure 4 shows the release of PLAP from samples prepared using method A from anodised (plot 4a) and from stain etched (plot 4b) porous silicon. Accumulative release was measured by the pNP method. Over 3 days there was approximately a 20% release of PLAP.

Similar experiments were also performed using neutral red compressed between two porous silicon membranes. Sealant was applied to the edges of the sample so that release of the neutral red was predominantly through the pores of the porous silicon, or as a result of erosion of the porous silicon. Figures 5 (a) and (b) show SEM images of the porous silicon membrane after immersion for several days in the Trizma buffer solution. The results show that the pore size of the membrane have been enlarged as a result of dissolution, which, it is believed, enhances the rate of diffusion of the dye through the membrane.

(II) Silicon structure, and anodised silicon structure

A silicon particulate product having a mean particle size between 1 and 50 microns was treated with 40 wt % (w/w) aqueous hydrofluoric acid to remove surface oxide present from the silicon product, and to create a hydrogen terminated surface. The silicon particulate product may comprise metallurgical grade silicon particles, that has been heavily p+ or n+ doped.

The hydrofluoric acid was removed from the silicon particulate product by washing with deionised water before rapid drying on filter paper in air for 15 minutes. The particles were then rapidly loaded into a stainless steel cold pressing device 1 , which is shown in Figure 6a. The drying and loading steps were carried out as quickly as possible to minimise or prevent reaction with oxygen, and to retain the hydrogen terminated particulate surface.

An estimated uniaxial pressure of between 1 ,000 and 5,000 psi may then applied by means of the cold pressing device 1 , the temperature of the silicon particulate product may be maintained at 20 C. The resulting silicon unitary body may have the form of a cylindrical consolidated macroporous silicon block having a diameter of 5 mm and a length of 46 mm. A small opening was formed in cold pressing device to allow gas produced during the pressing process to escape. Figure 6b shows components, generally indicated by 2, of the stainless cold pressing device.

Figure 7a shows a SEM micrograph of part of a silicon unitary body 3 according to the invention. The silicon body is in the form of a cylindrical unitary body. The figure 7a image shows a fracture surface 4 at which the cylinder has been broken to more clearly show the macroporous nature of the unitary body. Figure 7b shows a higher magnification SEM micrograph, of the macroporous fracture surface 4.

An electrode may be attached to the silicon unitary body, and it may then be immersed in 10-40 wt % (w/w) aqueous hydrofluoric acid with a surfactant such as ethanol, and a current density of between 1 mAcm^"2 and 10 Acm^"2, measured with respect to the external surface area of the block, the current may be passed for between 1 to 200 minutes.

The hydrofluoric acid may pass into the macroporous network of the silicon block, anodisation resulting in the formation of a porous layer on the interior surfaces of the macropores, and on the external surface of the silicon block.

Once anodisation is complete, the block may be washed, by repeated immersion in deionised water or methanol, and then air dried.

Finally, if a silicon particulate product is required, the block may be mechanically crushed to yield a multiplicity of partially surface porous silicon particles. Each partially surface porous particle having a non-porous surface area, corresponding to the region that bonded it to an adjacent silicon particle when still located in the unitary body.

A unitary silicon body according to the invention may be used as a scaffold to provide protection for, or to assist, the regrowth of damaged or diseased tissue. A unitary body having an appropriate size and shape is placed in the region in which tissue re-growth is to occur. Macropores, having a size between 10, 000 μm² and 62, 500 μm², formed in the unitary body allow the tissue to pass through the silicon scaffold. The scaffold may also comprise mesporous silicon, which may be engineered to erode once tissue growth is complete. This process is described in WO 0195952, which is herein incorporated by reference in its entirety. Five examples will now be given which describe the consolidation of a variety of silicon particulate products under a variety of conditions. Examples 4 and 5 describe the treatment of the silicon particulate product with an aqueous solution of HF prior to consolidation. Details of this HF pre-treatment, and details of the consolidation process, are both given in separate sections that follow the five examples.

Example 1

An experiment was performed on a first silicon particulate product comprising silicon kerf, having a particle size between 6 and 30 microns, which had been obtained by sawing multi-crystalline silicon ingots. The particulate product comprising the kerf was compressed uni-axially at a pressure less than 1000 MPa under vacuum at 293 K, in the 5mm diameter hardened stainless steel die 81 , shown in figure 8, using a hydraulic press.

The consolidation of the particulate product resulted in the formation of a silicon unitary body, in the form of a pellet. However, when the process was repeated at an increased pressure of 1000 MPa, it was not possible to form a single unitary body from the particulate product. This shows that it is possible to apply too much pressure to the particulate product, which can result in fracture of or damage to the unitary body upon removal from the die.

Example 2

A second silicon particulate product comprising metallurgical grade silicon having a particle size in the range 32 to 125 microns, which has been surface oxidised, was compressed uni-axially at 250 MPa, in the 5mm die 81 , to form a single silicon unitary body, in the form of a pellet. Shortly after removal from the die 81 , the unitary body was immersed a solution comprising equal volumes of ethanol and 40% w/w aqueous HF. After 5 seconds in the solution, the unitary body disintegrated.

This experiment showed that bonding between the silicon particles is possible, even in the presence of the native surface silicon oxide, however, the bonds exhibit relatively poor resistance to HF. It follows that the presence of surface oxide would be disadvantageous for the purposes of anodisation. Example 3

A silicon particulate product comprising silicon particles, comprising surface Si-H bonds, and having a size in the range 0.005 to 0.5 microns, were uni-axially compressed in the 5 mm die 81 at a pressure of 500 MPa. The resulting silicon unitary body, in the form of a pellet, was immersed in a solution comprising equal volumes of ethanol and 40% w/w aqueous HF. The silicon body was stable in the solution for 30 minutes.

This shows that the presence of surface Si-H bonds in the particulate product, allows the formation of a greater number of silicon - silicon bonds between the silicon particles when compressed, and that the bonds are more resistant to HF. This property should facilitate anodisation, and porosification, of the unitary body.

Example 4

A silicon particulate product comprising metallurgical grade silicon, having particles sizes between 32 and 125 microns, and comprising surface native silicon oxide, was treated with aqueous HF solution for ten minutes, washed in deionised water for ten minutes, before being air dried on filter paper for ten minutes. 100 mg of the resulting dried powder were transferred to the 5 mm die 81 and compressed uni-axially under vacuum at 1000 MPa. Small amounts of silane gas were detected when the compacted pellet was removed from the die 81. The density of the resulting silicon unitary body, in the form of a pellet, was 73% of solid non-porous metallurgical grade silicon. The electrical resistance of the silicon unitary body, combined with that of the electrical contacts, was 80,000 ohms. After 16 days exposure to air, the unitary body was immersed in a solution comprising equal volumes of ethanol and 40% (w/w) aqueous HF. The unitary body was stable in the solution for 20 minutes.

This result contrasts with that of example 2 and shows the effectiveness of the HF pre- treatment to replace the surface silicon oxide with Si-H bonds. The application of a uniaxial pressure causes consolidation of the silicon particulate product, the Si-H bonds are broken and replaced by Si-Si covaient bonds between adjacent silicon particles. The release of silane is a by-product of the unitary body formation, resulting from hydrolysis of the silicon - hydrogen surface. Example 5

A silicon particulate product comprising metallurgical grade silicon, having particles sizes between 32 and 125 microns, and comprising a native surface silicon oxide layer, was treated in aqueous HF solution for 10 minutes, washed in de-ionised water for 10 minutes, and air dried on filter paper for 10 minutes, before rapid transfer of the dried particulate product to the 5mm die 81. A 750 MPa uni-axial pressure was applied to consolidate a silicon particulate product, a silicon unitary body in the form of a pellet being formed, the pellet having a mass of 100 mg. The porosity of the unitary body was approximately 30%.

A platinum base was place in electrical contact, using silver paste, with the lower surface of the silicon unitary body. Approximately 1 ml droplet of electrolyte comprising equal volumes of ethanol and 40% (w/w) aqueous HF was dispensed onto the upper surface of the silicon pellet with a pipette. A thin platinum wire was then lowered into the electrolyte drop, and a 12 to 15 volt potential difference was applied, resulting in a current flow of 30mA for 20 seconds. The HF droplet gradually reduced in volume as a result of the combined effect of evaporation from the electrical heating and penetration into the pores of the pellet. SEM images, shown in figures 10 and 11, of the surface of the anodised pellet revealed that the bonded silicon particles had been porosified, mesopores being formed.

Consolidation procedure

Figure 8 shows a photograph of the components of a second cold pressing device used to fabricate a silicon unitary body according to the present invention, the second cold pressing device comprises a 5mm diameter die 81 , and one moveable plunger 82 formed both from hardened stainless steel. The die 81 is designed so that it may be evacuated. Typically 100 mg of a silicon particulate product was loaded into the 5mm die 81. The die is then placed between the platens of a ten tonne laboratory press (not shown in the figures) having a digital pressure display accurate to 0.1 tonne. A vacuum line (not shown in the figures) was connected to the die, and the die was evacuated to a pressure of approximately 10^"4 Torr. An axial pressure between 50 and 1000 MPa was then applied to the silicon particulate product for five minutes. The vacuum line was then removed, and the axial pressure was removed from the resulting silicon unitary body 91 , shown in figure 9. before its removal from the die. A silane sensor was positioned over the unitary body as it was removed.

HF Pre-treatment

The silicon particulate product was placed in a beaker containing 100 ml 40% (w/w) and 10 ml ethanol for ten minutes, the mixture being agitated occasionally. The presence of the ethanol was required to enable wetting of the silicon particles. As much of the solution as possible was then decanted, to leave the particulate product in the beaker. The beaker was then filled with 100 ml of de-ionised water and ethanol, before pouring the mixture into a drying vessel attached to a Buckner pump. The excess solution was removed, through a PTFE membrane, as a result of the pressure difference. The remaining silicon particulate product was rinsed with fresh water or ethanol, and an HF detector was used to ensure that substantially no residual HF remained. The Buckner pump was then dismantled and the PTFE membrane, on which the particulate product remained, was removed. The membrane was then placed on filter paper, so that the particulate product contacted the filter paper, and was peeled back to leave the silicon powder. Filter paper was used to remove much of the liquid, before leaving the powder in air for ten minutes to dry. The time taken between the initial decanting of the HF solution to the start of the air drying procedure was typically 10 minutes, so that the total time for the complete procedure is 30 minutes (10 minutes treatment with ethanoic HF, 10 minutes washing with water, and 10 minutes air drying).

Claims

1. A process for fabricating a silicon unitary body, the process comprising the steps:

(a) taking a silicon particulate product comprising a multiplicity of free silicon particles; and (b) applying pressure to at least part of the silicon particulate product to form a silicon unitary body;

characterised in that the method comprises the further step of (e) porosifying at least part of the silicon from which the silicon unitary body is formed.

2. A process according to claim 1 characterised in that the step (e) comprises the step of anodising the unitary body in a solution of hydrofluoric acid, the solution of hydrofluoric acid being selected from one or more of: an aqueous solution of HF, an ethanolic solution of HF, a methanolic solution of HF, and an ethanoic solution of HF.

3. A process according to claim 1 characterised in that step (e) comprises the step of stain etching at least some of the free silicon particles with hydrofluoric acid prior to step (b).

4. A process according to claim 1 characterised in that steps (a) and (b) are performed in such a manner that two or more silicon unitary bodies are formed.

5. A process according to claim 1 characterised in that the step (b) comprises the step of cold pressing the silicon particulate product.

6. A process according to claim 1 characterised in that step (b) is performed at a temperature between - 20 °C and + 50 °C.

7. A process according to claim 1 characterised in that the process comprises the further step, performed after or during step (b), of testing for the presence of silane gas.

8. A process according to claim 1 characterised in that the method further comprises the step, performed between steps (a) and (b), of combining at least some of the free silicon particles with a beneficial substance.

9. A process according to claim 1 characterised in that the beneficial substance is a hydrophilic beneficial substance.

10. A process according to claim 1 characterised in that the particulate product comprises at least 1000 free silicon particles.

11. A process according to claim 1 characterised in that the unitary body comprises at least 1000 bonded silicon particles.

12. A process according to claim 1 characterised in that the process comprises the further step of treating the silicon particulate product, prior to step (b), with a reducing agent to form surface Si-H bonds.

13. A process according to claim 12 characterised in that the reducing agent comprises HF.

14. A process according to claim 1 characterised in that the step (b) is performed such that at least one of the silicon particles is covalently bonded to one or more of the other silicon particles.

15. A silicon unitary body obtainable by a process according to any one of claims 1 to 14.

16. A process for fabricating a composite unitary body comprising silicon and a beneficial substance, the process comprising the steps: (a) combining a silicon particulate product comprising a multiplicity of porous silicon particles with a hydrophilic beneficial substance; and (b) consolidating at least part of the silicon particulate product and at least part of the hydrophilic substance to form a composite unitary body comprising at least some of silicon particulate product and at least part of the hydrophilic substance.

17. A process according to claim 16 characterised in that the process comprises the further step of fragmenting the composite unitary body.

18. A silicon unitary body comprising greater than 1000 covalently bonded silicon particles, each covalently bonded silicon particle being covalently bonded to at least one of the other covalently bonded silicon particles.

19. A silicon unitary body according to claim 18 characterised in that at least part of the surface of the unitary body comprises silicon hydride.

20. A partially surface porous silicon particulate product comprising a multiplicity of partially surface porous silicon particles, the surface of each partially surface porous particle comprising a porous area and a non-porous area.

21. A silicon particulate product according to claim 20 characterised in that the particulate product comprises at least one bonded silicon particle, the or each bonded silicon particle being covalently bonded to one or more of the other silicon particles from which the particulate product is formed.