Biomaterial

Details Biomaterial Biomaterial

1998-01-30

2001-11-27

Jones, Deborah (Department: 1772)

Stock material or miscellaneous articles

Composite

Of silicon containing

C428S446000, C427S058000

Reexamination Certificate

active

06322895

ABSTRACT:

The present invention relates to biomaterials.
A“biomaterial” is a non-living material used in a medical device which is intended to interact with biological systems. Such materials may be relatively “bioinert”, “biocompatible”, “bioactive” or “uresorbable”, depending on their biological response in vivo.
Bioactive materials are a class of materials each of which when in vivo elicits a specific biological response that results in the formation of a bond between living tissue and that material. Bioactive materials are also referred to as surface reactive biomaterials. Biomaterials may be defined as materials suitable for implantation into a living organism. L. L. Hench has reviewed biomaterials in a scientific paper published in Science, Volume 208, May 1980, pages 826-831. Biomaterials which are relatively inert may cause interfacial problems when implanted and so considerable research activity has been directed towards developing materials which are bioactive in order to improve the biomaterial-tissue interface.
Known bioactive materials include hydroxyapatite (HA), some glasses and some glass ceramics. Both bioactive glasses and bioactive glass ceramics form a biologically active layer of hydroxycarbonateapatite (HCA) when implanted. This layer is equivalent chemically and structurally to the mineral phase in bone and is responsible for the interfacial bonding between bone and the bioactive material. The properties of these bioactive materials are described by L. L. Hench in the Journal of the American Ceramic Society, Volume 74 Number 7, 1991, pages 1487-1510. The scientific literature on bioactive materials often uses the terms HA and HCA on an interchangeable basis. In this patent specification, the materials HA and HCA are collectively referred to as apatite.
Li et al. have reported the deposition of apatite on silica gel in the Journal of Biomedical Materials Research, Volume 28, 1994, pages 7-15. They suggest that a certain density of silanol (SiOH) groups is necessary to trigger the heterogeneous nucleation of hydroxyapatite. An apatite layer did not develop on the surface of a silica glass sample and this is attributed to the lower density of surface silanol groups compared with silica gel.
Thick films of apatite have previously been deposited on silicon single crystal wafers by placing the wafers in close proximity to a plate of apatite and wollastonite-containing glass dipped into a physiological solution at 36° C., as described by Wang et al. in the Joumal of Materials Science: Materials In Medicine, Volume 6, 1995, pages 94-104. A physiological solution, also known as a simulated body fluid (SBF), is a solution containing ion concentrations similar to those found in the human body and is widely used to mimic the behavior of the body in vitro tests of bioactivity. Wang et al. reported the growth of apatite on (111) Si wafers but reported that “hardly any” apatite could be grown on (100) Si wafers. The silicon wafer itself is not bioactive. Wang et al. state that “Si does not play any special role in the growth of (the) apatite film except that Si atoms on the substrate can bond strongly with oxygen atoms in apatite nuclei to form interfaces with low energy”. The presence of the apatite and wollastonite containing glass is required to induce the deposition of the apatite. Indeed, this so-called “biomimetic process” whereby a bioactive material is used to treat another material has been shown to induce apatite growth on a wide variety of bioinert materials, as reported by Y. Abe et al. in the Journal of Materials Science: Materials In Medicine, Volume 1, 1990, pages 233 to 238.
There is a long felt want for the ability to use silicon based integrated circuits within the human body both for diagnostic and therapeutic purposes. Silicon has been reported to exhibit a poor biocompatibility in blood, Kanda et al. in Electronics Letters, Volume 17, Number 16, 1981, pages 558 and 559, and in order to protect integrated circuits from damage in biological environments encapsulation by a suitable material is currently required. Medical applications for silicon based sensors are described in a paper by Engels et al. in the Journal of Physics E: Sci. Instrum., Volume 16, 1983, pages 987 to 994.
The present invention provides bioactive silicon characterized in that the silicon is at least partly crystalline.
Bioactive silicon provides the advantage over other bioactive materials that it is compatible with silicon based integrated circuit technology. It has the advantage over non-bioactive silicon that it exhibits a greater degree of biocompatibility. In addition, bioactive silicon may be used for forming a bond to bone or vascular tissue of a living animal. Bioactive silicon may provide a material suitable for use as a packaging material in miniaturised packaging applications.
The bioactive nature of the silicon may be demonstrated by the immersion of the material in a simulated body fluid held at a physiological temperature, such immersion producing a mineral deposit on the bioactive silicon. The mineral deposit may be apatite. The apatite deposit may be continuous over an area greater than 100 &mgr;m
2
. The bioactive silicon may be at least partially porous silicon. The porous silicon may have a porosity greater than 4% and less than 70%.
Bulk crystalline silicon can be rendered porous by partial electrochemical dissolution in hydrofluoric acid based solutions, as described in U.S. Pat. No. 5,348,618. This etching process generates a silicon structure that retains the crystallinity and the crystallographic orientation of the original bulk material. The porous silicon thus formed is a form of crystalline silicon. At low levels of porosity, for example less than 20%, the electronic properties of the porous silicon resemble those of bulk crystalline silicon.
Porous silicon may be subdivided according to 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 20 Å to 500 Å; and macroporous silicon contains pores having a diameter greater than 500 Å. The bioactive silicon may comprise porous silicon which is either microporous or mesoporous.
Silicon has never been judged a promising biomaterial, in contrast with numerous metals, ceramics and polymers, and has never been judged capable of exhibiting bioactive behavior. Indeed, no semiconductors have been reported to be bioactive. Silicon is at best reported to be relatively bioinert but generally exhibits poor biocompatibility. Despite the advances made in miniaturisation of integrated circuitry, silicon VLSI technology is still under development for invasive medical and biosensing applications, as described by K. D. Wise et al. in “VLSI in Medicine” edited by N. G. Einspruch et al., Academic Press, New York, 1989, Chapter 10 and M. Madou et al. in Appl. Biochem. Biotechn., Volume 41, 1993, pages 109-128.
The use of silicon structures for biological applications is known. International patent application PCT/US95/02752 having an International Publication Number WO 95/24472 describes a capsule having end faces formed from a perforated amorphous silicon structure, whose pores are large enough to allow desired molecular products through but which block the passage of larger immunological molecules, to provide immunological isolation of cells contained therein. No evidence as to the biocompatibility of the silicon structure is provided, and workers skilled in the field of biocompatible materials would expect that such a device would in vivo stimulate the production of fibrous tissue which would block the pores. It is known that when micromachined silicon structures are used as sensors for neural elements a layer of fibrous tissue forms between the silicon surfaces and the neural elements of interest, as reported by D. J. Edell et al. in IEEE Transactions on Biomedical Engineering, Volume 39, Number 6, 1992 page 635. Indeed the thickness and nature of any fibrous tissue layer formed is often used as one meas

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