KR100488643B1 - A method of accelerating or retarding the rate of deposition of a mineral deposit on silicon in a physiological electrolyte - Google Patents

Abstract

Biomaterials, such as bioactive silicon, can be produced by anodizing silicon wafers to produce wafers 10 having porous silicon regions 20. In vitro experiments have shown that when certain types of porous silicon are immersed in the simulated body fluid solution, the apatite deposits are deposited in both the porous silicon 20 and the neighboring bulk silicon regions 22. The deposition of such apatite provides an indication that suitable forms of porous silicon are bioactive and thus also biocompatible. The porous silicon form is dissolved in the simulated body fluid solution, which is indicative of resorbable biomaterial properties. In addition to porous silicon, certain types of polycrystalline silicon exhibit bioactive properties. Bioactive silicon can be used in the manufacture of biosensors for in vitro or in vivo use. Bioactivity The bioactivity of silicon can be controlled by applying a voltage.

Description

A method of accelerating or retarding the rate of deposition of a mineral deposit on silicon in a physiological electrolyte}

The present invention relates to a biomaterial.

A "biomaterial" is a non-biomaterial used in a medical device intended to interact with the biological system. Such materials may be relatively "bioinactive", "biocompatible", "bioactive" or "resorbable" depending on their in vivo biological response.

Bioactive materials are a type of material that cause specific biological reactions to form biological tissues and bonds between them in vivo. Bioactive materials are also referred to as surface reactive biomaterials. Biomaterials can be defined as materials suitable for implantation into living organisms. L. L. Hench is published in the published scientific literature [Science, Volume 208; May 1980, pp. 826-831, biomaterials were reviewed. Relatively inert biomaterials can cause interfacial problems when implanted, and there has been considerable research activity to develop bioactive materials to improve the biomaterial-tissue interface.

Known bioactive materials include hydroxyapatite (HA), some glass and some glass ceramics. Both bioactive glass and bioactive glass ceramics form a biologically active layer of hydroxycarbonateapatite (HCA) upon implantation. This layer is chemically and structurally equivalent to the mineral phase in the bone and allows for interfacial bonding between the bone and the bioactive material. The properties of these bioactive materials are described in detail in the following references: L. L. Hench; Journal of the American Ceramic Society, Volume 74 Number 7, 1991, pp. 1487-1510. The scientific literature on bioactive materials often uses the terms HA and HCA on the basis of interchangeability. In this patent specification, the materials HA and HCA are referred to collectively as apatite.

Li et al. Have reported in the literature on the deposition of apatite on silica gel (Journal of Biomedical Materials Research, Volume 28, 1994, pp. 7-15). They suggest that a certain density of silanol (SiOH) groups is essential for initiating heterologous nucleation of hydroxyapatite. The apatite layer does not develop on the surface of the silica glass sample due to the low density of surface silanol groups compared to silica gel.

Apatite thick films were deposited on silicon single crystal wafers by placing the wafers close to apatite and wollastonite-containing glass plates previously immersed in physiological solutions at 36 ° C. Wang et al., Journal of Materials Science: Materials In Medicine, Volume 6, 1995, pp. 94-104. Physiological solutions known as simulated body fluids (SBFs) are solutions containing ionic concentrations similar to those found in humans and are widely used to mimic the body's function in in vitro bioactivity testing. Wang et al. Mentioned the growth of apatite on (111) Si wafers, but reported that apatite could not be grown to "almost absent" on (100) Si wafers. The silicon wafer itself is not bioactive. "Si does not play a special role in the growth of the apatite film," said Wang et al., But Si atoms on the substrate can bond strongly with oxygen atoms in the apatite nucleus to form a low energy interface. Apatite and bolastonite containing glass need to be present to cause deposition of the apatite. In fact, this so-called "biomimetic process", which uses bioactive materials to process other materials, has been found to cause the growth of apatite on various kinds of bioinert materials as reported in the literature. Abe et al., Journal of Materials Science: Materials in Medicine, Volume 1, 1990, pp. 233 to 238].

There is a long demand for the use of silicon-based integrated circuits in the human body for diagnosis and treatment. Silicon has been reported to be poorly biocompatible in blood (Kanda et al., Electronics Letters.Vol. 17, November 16, 1981, pp. 558 to 559), protecting integrated circuits from damage in biological environments. To this end, encapsulation with a suitable material is currently required. Medical uses of silicon-based sensors are described in Engels et al., Journal of Physics E. Sci. Instrum., Volume 16, 1983, pp. 987 to 994].

The present invention provides bioactive silicon characterized in that the silicon is at least partially crystalline.

Bioactive silicon offers advantages over other bioactive materials that are compatible with silicon based integrated circuit technology. This has the advantage of being biocompatible to a higher degree than non-bioactive silicon. In addition, bioactive silicon can be used to form bonds in bone or vascular tissue of living animals. Bioactive silicon can provide a material suitable for use as a package material in miniaturized package applications.

The bioactivity of silicon can be demonstrated by immersing the material in simulated body fluids maintained at physiological temperature, such as immersion to deposit minerals on bioactive silicon. The inorganic deposit may be apatite. Apatite deposition may be present continuously over an area of at least 100 μm ² . The bioactive silicon can be at least partially porous silicon. The porosity of the porous silicon can be greater than 4% and less than 70%.

Bulk crystalline silicon can be converted to porous by partial electrochemical dissolution in hydrofluoric acid based solutions, as described in US Pat. No. 5,348,618. This etching process results in a silicon structure retaining the crystallinity and crystallographic orientation of the original bulk material. The porous silicon thus formed is in the form of crystalline silicon. At low porosity levels, for example less than 20%, the electrical properties of the porous silicon are similar to the electrical properties of bulk crystalline silicon.

Porous silicon can be subdivided according to porosity characteristics. Microporous silicon contains pores less than 20 microns in diameter; Mesoporous silicon contains pores with a diameter in the range of 20 kV to 500 kV; Macroporous silicon contains pores with a diameter greater than 500 mm 3. The bioactive silicon may comprise porous silicon which is microporous or mesoporous.

Silicon has not been shown to be a promising biomaterial in contrast to many metals, ceramics and polymers, and has not been shown to be capable of bioactive function. In fact, no semiconductor has been reported to be bioactive. Silicon has only been reported to be relatively bioinert at best and generally has poor biocompatibility. Despite advances in the miniaturization of integrated circuits, silicon VLSI technology is still in the development stage in the field of invasive medical and biosensing as described in the literature. KD Wise et al., "VLSI in Medicine ", edited by NG Einspruch et al., Academic Press, New York, 1989, Chapter 10 and M. Madou et al. in Appl. Biochem. Biotechn., Volume 41, 1993, pp. 109 to 128].

The use of silicon structures for biological use is known. International Patent Application No. PCT / US95 / 02752, International Publication No. WO 95/24472, describes capsules having end faces formed from perforated amorphous silicon structures, the pores of which are large enough to pass the desired molecular product. The passage of larger immunological molecules blocks, allowing cells contained therein to be immunologically isolated. No evidence has been provided for the biocompatibility of silicon structures, and those skilled in the art of biocompatible materials predict that such devices stimulate the production of fibrous tissue that blocks pores in vivo. As described in the literature, it is known that when micromechanical silicon structures are used as sensors for neural devices, a fibrous tissue layer is formed between the silicon surface and the targeted neural device. DJ Edell et al., IEEE Transactions on Biomedical Engineering, Volume 39, Number 6, 1992, p. 635]. In fact, the thickness and properties of the fibrous tissue layer formed are often used as a measure of biocompatibility and reflect that the thinner layers containing little cell necrosis have a higher degree of biocompatibility.

U. S. Patent No. 5,225, 374 describes the use of porous silicon as a substrate for protein-lipid membranes that generate current by interacting with a target species when exposed to the target species in an in vitro solution. Porous silicon is selected because it is oxidized to produce a hydrophilic surface and the pores act as conduits for ion-current and the structure provides a structural support for the lipid layer. Porous silicon is separated from the in vitro solution by the protein-lipid membrane so that no problem with the bioactivity or biocompatibility of the porous silicon arises.

Porous silicon has been suggested in the literature as a substrate material for in vitro biosensors. M. Thust et al., Meas. Sci. Technol. Volume 7 1996, pp. 26 to 29]. In the structure of the device described in that document, the porous silicon is thermally oxidized to form a silicon dioxide layer on the exposed silicon surface of the pores. Since the porous silicon is partially thermally oxidized, the bioactivity or biocompatibility of the silicon is not valid because it is only silicon dioxide exposed to the test solution. Porous silicon is an effective inert host for enzyme solutions.

It has been described in the literature that microperforated silicon membranes can support cellular structure. E. Richter et al., Journal of Materials Science: Materials in Medicine, Volume 7, 1996, pp. 85-97, G. Fuhr. et al., Journal of Micromechanics and Microengineering, Volume 5, Number 2, 1995, pp. 77-85]. The silicon film described in this document comprises a silicon film having a thickness of 3 μm, perforated by square pores having a width of 5 μm to 20 μm using a lithography process. Mouse fetal fibroblasts could grow on the washed membranes, but the cell adhesion was improved when the membranes were coated with polylysine. The document is silent on the bioactivity of the silicon film and no mention is made of the apatite layer formed upon exposure to cell culture medium. In fact, for the dimensions of the pores used, the structure is unlikely to exert a significant degree of bioactivity. Fuhr et al. Also acknowledge that there is still a need to find and develop cell-compatible materials with long-term stability.

A. Offenhausser et al., Journal of Vacuum Science Technology A, Volume 13, Number 5, 1995, pages 2606-2612, discloses a technique for imparting biocompatibility to silicon substrates by coating the substrate with an ultra-thin polymer membrane. It is explained. Similarly, RS Potember et al., Proc. 16th Int. Conf. IEEE Engineering in Medicine and Biology Society, Volume 2, 1994, pages 842-843, rats using synthetic peptides attached to silicon surfaces. Promoting the development of neurons is described.

In another aspect, the present invention provides a bioactive silicon structure characterized in that the silicon is at least partially crystalline.

In another aspect, the present invention provides an electronic device for operating in a living human or animal body, characterized in that it comprises bioactive silicon.

The bioactive silicon of the invention can be arranged not only as a protective coating for electronic circuits but also as a means for attaching the device to bone or other tissue.

The electronic device may be a sensor device or an intelligent drug delivery device or prosthetic device.

In another aspect, the present invention provides a method for making silicon bioactive, characterized in that at least a portion of the silicon is made porous.

In another aspect, the present invention provides a method for producing bioactive silicon, comprising depositing a polycrystalline silicon layer.

In another aspect, the present invention provides biocompatible silicon characterized in that the silicon is at least partially crystalline.

In another aspect, the present invention provides resorbable silicon.

In another aspect, the present invention provides a method of increasing or decreasing the rate of deposition of inorganic deposits on silicon in a physiological electrolyte, comprising applying an electrical bias to silicon.

The silicon may be porous silicon.

In another aspect, the present invention provides a bioactive material characterized in that the bioactivity of the material can be adjusted by applying an electrical bias to the material.

Conventional bioactive ceramics are electrically insulated and therefore their use in electrochemical applications has been ruled out. Electrical stimulation of tissue growth has already been studied, and it is typically difficult to distinguish between the direct effects of electric fields and the effects associated with altered biochemistry near implanted "bioinert" electrodes.

In another aspect, the present invention provides a composite structure comprising a bioactive silicon region and an inorganic deposit on the region, wherein the silicon region comprises silicon that is at least partially crystalline.

A possible use of the present invention is as a substrate for performing biopsy. It is desirable to be able to perform certain tests on pharmaceutical compounds without performing tests on living animals. Therefore, a significant amount of research has been done to develop in vitro assays that support cell lines on substrates and monitor the effects of pharmaceutical compounds on the cell lines. The composite structure of silicon and apatite can provide a substrate suitable for such testing.

In another aspect, the present invention provides a method of manufacturing a biosensor, comprising forming a composite structure consisting of bioactive silicon and an inorganic deposit thereon.

The invention also provides a biosensor comprising a silicon substrate and testing the pharmacological activity of the compound, wherein at least a portion of the silicon substrate consists of bioactive silicon.

For a more complete understanding of the invention, its aspects are described by way of example only with reference to the accompanying drawings.

In the case of Fig. 1, a cross section of a bioactive silicon wafer, generally indicated at 10, is shown. The silicon wafer 10 includes a porous silicon region 20 and a non-porous bulk silicon region 22. The thickness d of the porous region 20 is 13.7 µm and the average porosity is 18%. The diameter 1 of the silicon wafer 10 is 3 inches or 75 mm. The surface area per mass of material of the porous region 20 is 67 m 2 / g. This is measured using BET gas analysis techniques, as described in the following references ("Adsorption, Surface Area and Porosity", S. J. Gregg and K. S. W. Sing, 2nd edition, Academic Press, 1982).

Wafer 10 is fabricated by anodizing an arsenic heavily doped Czochralski-grown (CZ) n-type (100) silicon wafer with an initial resistance of 0.012 Ωcm. Anodization is performed in an electrochemical cell containing an electrolyte of 50 wt% aqueous HF, as described in US Pat. No. 5,348,618. The wafer is polarized using a polarization current density of 100 mA cm ^-2 for 1 minute. The wafer is placed in place in the electrochemical cell using a synthetic rubber washer around the outer surface of the wafer. As a result, the outer ring of the wafer remains unpolarized even after polarization. The unpolarized outer ring is shown in FIG. 1 as non-porous bulk silicon region 22. The width s of the unpolarized ring is 4 mm.

To measure the bioactivity of the polarized wafers, the cut wafer pieces are placed in simulated body fluid (SBF) over a period of 2 hours to 6 weeks. SBF solutions are prepared by dissolving reagent grade salts in deionized water. The solution contains an ion concentration similar to that found in human plasma. Ion concentrations of SBF solutions and ion concentrations in human plasma are shown in Table 1 below. The SBF solution is organically buffered with trihydroxymethylaminomethane and hydrochloric acid to pH 7.30 ± 0.05, equivalent to physiological pH. The porous wafer is hydrated in ambient air for at least several months before immersing it in the SBF solution. The porous silicon thus contains a silicon skeleton coated with a thin natural oxide, similar to that formed on bulk silicon as a result of storage in air.

ion Concentration (mM) Simulated body fluids Human plasma Na ⁺ 142.0 142.0 K ⁺ 5.0 5.0 Mg ²⁺ 1.5 1.5 Ca ²⁺ 2.5 2.5 HCO ₃ ^- 4.2 27.0 HPO ₄ ^2- 1.0 1.0 Cl ^- 147.8 103.0 SO ₄ ^2- 0.5 0.5

Cut sections of wafers, typically 0.4 × 50 × 20 mm ³ , are placed in 30 cm ³ volume polyethylene bottles filled with SBF solution and maintained at 37 ± 1 ° C. using a calibrated bath.

After a known period of time, the fragments are recovered from the SBF solution, washed with deionized water and then dried in ambient air prior to characterization. SBF treated fragments are examined using X-ray microanalysis (EDX) on scanning electron microscopy (SEM) and JEOL 6400F microscopy. Secondary ion mass spectroscopy was performed using a Cameca 4F instrument and infrared spectroscopy using a Biorad FTS-40 spectrometer.

After soaking in SBF solution for 2, 4 and 17 hours, apatite is deposited in negligible amounts in both porous silicon region 20 and non-porous bulk silicon region 22.

In the case of FIG. 2, the reproduction of the SEM photograph generally shown by 50 is shown. Micrograph 50 is an image of a portion of area 22 after placing wafer 10 in SBF solution for 6 days. The scale bar 52 shows the dimension of 2 micrometers. The micrograph 50 shows a continuous layer of apatite conglomerate 54 covering the surface of region 22. The apatite conglomerates are nucleated at a density high enough to produce a relatively soft film whose boundaries between conifers, such as boundary 56, are unclear. The film is present continuously over at least 100 μm ² .

In the case of FIG. 3, a representation of the SEM photograph, shown generally at 100, of the cross section of the wafer 10 in region 22 after the wafer is immersed in SBF solution for 6 days. Scale bar 102 represents a dimension of 1.0 μm. Micrograph 100 shows three distinct areas, denoted by capital letters A, B, and C. FIG. EDX analysis can confirm that region A is silicon corresponding to the original material of non-porous bulk silicon region 22. Zone B shows both silicon and oxygen peaks under EDX analysis, indicating that zone B contains silicon oxide. Region C shows calcium, phosphorus and oxygen peaks under EDX analysis, which is consistent with that comprising apatite conifers. By performing SEM and EDX analysis together, a porous silicon oxide layer (region B) was formed on the bulk silicon (region A), demonstrating that nucleation and coating could be performed using apatite (region C).

SEM analysis of the wafer 10 in the area of the porous silicon region 20 after immersion in the SBF solution for 6 days indicated that the apatite was coated at a much lower level compared to the region 22. The median porosity level of the porous silicon region 20 is high. After 10 days of immersion in the SBF solution where reliable layer corrosion of the porous silicon occurs, SEM analysis reveals large pores in region 20. Performing SEM and EDX analysis together demonstrates that, in contrast to bulk silicon region 22, apatite nucleation can occur directly on porous silicon region 20 and does not require the formation of an intermediate porous silicon oxide layer. Intentionally introducing very large (more than 100 μm in diameter) macropores may be advantageous for allowing vascular tissue to grow within the structure of porous silicon.

The formation of apatite deposits was also observed on wafers with a porosity of not more than 18% of the porous silicon. A microporous wafer having a porous silicon region having a porosity of 31% was prepared from an excessively boron doped p-type CZ silicon wafer of 0.03Ωcm by anodizing for 1 minute in 50 wt% HF at anodizing current density of 100 mAcm ^-2 . The resulting porous silicon region has a thickness of 9.4 μm and a surface area per unit mass of 250 m 2 / g. The porous silicon wafer is sufficiently aged before being immersed in SBF solution.

FIG. 4 shows a SEM photograph, generally indicated at 150, of the surface of a porous silicon layer having a porosity of 31% after immersing a fragment of a wafer in 30 cm 3 of an SBF solution for 7 days. Micrograph 150 shows a conifer, such as apatite conifer 152 on surface 154 of porous silicon.

Microporous wafers having a porous silicon region with a porosity of 48% are prepared by anodizing a lightly boron doped p-type silicon wafer having a resistance of 30 Ωcm for 5 minutes at a polarization current density of 20 mAcm ^-2 in 50 wt% HF. The thickness of the resulting porous silicon region is 6.65 mu m and the surface area per unit mass is about 800 m 2 / g. The porous wafer pieces are sufficiently aged before being immersed in 150 cm 3 polyethylene bottles filled with SBF solution.

FIG. 5A shows an SEM photograph, generally denoted 200, of the apatite deposit 202 on the unpolarized region of the wafer having 48% porosity after soaking for 4 weeks. FIG. 5B shows an SEM image, generally denoted 250, of an apatite conifer 252 deposited on a porous region with 48% porosity. Conifer 252 shows a morphology with a columnar structure characterized by apatite growth on bioactive ceramics as described in P. Li et al., Journal of Biomedical Materials Research, Volume 28, 7 To p. 15, 1994]. Apatite conifers with similar morphology are observed on the unpolarized region of the wafer. EDX cross-sectional spectra of a 48% porosity wafer after immersion in an SBF solution, selected across the unpolarized region, indicate that the conifer contains calcium, phosphorus and oxygen consistent with the apatite. Apart from the conifer, an interface layer of only 150 nm in thickness, mainly comprising silicon and oxygen, is observed. Fourier transform infrared spectroscopy confirmed the presence of apatite in both porous and non-porous regions. Both broadband due to the PO bending vibration mode of PO ₄ tetrahedra at around 600 cm ⁻¹ and the vibration mode of the carbonate group around 1400 cm ⁻¹ were observed.

Some forms of porous silicon are known to be photoluminescent. Observation of red or orange photoluminescence from porous silicon generally indicates the presence of quantum wires or quantum dots of the silicon material. Prior to immersion in the SBF solution, a fully aged 48% porosity wafer exhibits photoluminescence, which indicates that the porous silicon region retains high concentrations of quantum wires or dots despite being hydrated by exposure to ambient air. Luminescence is preserved during or after immersion in SBF solution. This indicates that apatite can be deposited on porous silicon to preserve luminescence. Preservation of luminescence after growth of the apatite layer may be a useful property in the development of electro-optical biosensors.

A 1 μm thick porous region with a porosity of 70% and a generally mesoporous luminescent porous silicon wafer with a surface area of 640 m 2 / g per unit mass are placed in an SBF solution. After about one day, the wafer is no longer luminescent when dissolved in SBF solution to completely remove the porous region. Apatite deposits are not observed both in the porous silicon region or in the non-porous region. The mesoporous silicon is more efficiently wetted by the SBF solution and therefore the dissolution rate is thought to be higher in the mesoporous silicon than the microporous silicon. The mesoporous silicon thus exhibits resorbable biomaterial properties. Bioactive silicon structures with limited area of mesoporous silicon can be fabricated to serve as a source of soluble silicon. This can produce a locally saturated silicon solution to enhance deposition of apatite.

Large porous silicon wafers having a porous area of 4% porosity and 38 μm in thickness behave like bulk silicon wafers that are unpolarized to show no growth of apatite deposits when immersed in SBF solution for 4 weeks. In addition, no apatite growth was observed in the porous silicon region having a porosity of 80% and a thickness of 50 µm and retaining luminescence even after immersion in the SBF solution for two weeks.

As another control, a non-porous silicon wafer cut piece having dimensions similar to the porous silicon wafer piece is placed in 30 cm 3 of SBF solution. Extremely low density μ size deposits of less than 5000 / cm 2 are observed after soaking in SBF solution for 5 weeks. These deposits may be present on the surface defects of the silicon wafer. Thus, non-porous bulk silicon is not bioactive because the growth rate of apatite deposits is too slow to form a bond with living tissue.

Thus, these experiments show that, with proper control of pore size and porosity, silicon structures can substantially cover the entire bioactivity spectrum. Bulk and pure macroporous silicon are relatively bioinert, medium porosity silicon with high porosity is resorbable and microporous silicon with medium porosity is bioactive.

It is known that changes in the chemical composition of biomaterials can also affect whether they are bioinert, resorbable or bioactive. The experiments were carried out on porous silicon wafers that were not intentionally doped with specific elements, except for impurity doping to control the semiconductor properties of silicon.

It is believed that elution of calcium from bioactive glasses containing SiO ₂ , Na ₂ O, CaO and P ₂ O ₅ can reliably assist apatite growth by enhancing local supersaturation. Immerse calcium into a just etched microporous silicon layer of 1.2 μm in thickness and 55% porosity formed on a lightly doped p-type (30Ωcm) CZ silicon wafer anodized at 20 mA / cm ² in 40% aqueous HF for 1 minute. . Calcium impregnation is carried out via mild oxidation by storing in 125 cm ³ of pure ethanol in a solution containing 5 g of CaCl. ₂ H ₂ O for 16 hours. Impregnation of porous silicon with calcium, sodium or phosphorus, or a combination of these species can enhance the formation of apatite on silicon.

The presence of the silicon oxide layer under the apatite deposit in the non-porous region adjacent to the porous silicon region of the anodized wafer after immersion in the SBF solution may be an important factor for the bioactivity of porous silicon in the dissolution of silicon from the porous silicon region. To indicate that it can. Dissolution of silicon can form a local supersaturated solution, which deposits a porous silicon oxide layer. Apatite is then deposited on the porous silicon oxide. This suggests that various types of non-porous crystalline, polycrystalline or amorphous silicon based structures, which are more soluble than normal crystalline bulk silicon and contain impregnated calcium in SBF solutions, may be bioactive. In order to reliably assist in apatite growth, preservation of the crystallinity of silicon is not necessary, but requires a much higher degree of calcium impregnation than previously published calcium doped silicon.

Calcium is generally recognized as a dopant that is not preferred for silicon and as a result there has been little research on calcium doped silicon. Sigmund, Journal of the Electrochemical Society, Volume 129, 1982, pp. 2809 to 2812, reports that the maximum equilibrium solubility of calcium in monocrystalline silicon is 6.0 x 10 ¹⁸ cm ^-3 . At this concentration, calcium does not seem to have a significant effect on apatite growth. Supersaturation levels of calcium in excess of 10 ²¹ cm ^-3 (2at%) are required. Very high concentrations as above can be achieved in the following way:

(a) solution doping of porous silicon as described above;

(b) ion implantation of porous silicon or bulk silicon with calcium ions; or

(c) Heat treatment after lamination deposition of calcium or calcium compounds.

In the case of FIG. 6, the schematic diagram of the general-purpose sensor shown generally 300 for medical use containing bioactive silicon is shown. Sensor 300 includes two silicon wafer pieces 302 and 304. Fragment 302 includes a CMOS circuit 306 and a sensitizing element 308 connected to the circuit 306. The sensitizing element 308 may be an oxygen sensor such as, for example, a Clark cell. The CMOS circuit is powered by a small battery (not shown) and generates signals for external monitors using standard telemetry data transmission.

Wafer piece 304 is a minimechanized top cover for piece 302. The fragment 304 has two main pupils 310 and 312 mechanized into the fragment 304. The pupil 310 is shaped like a dome. When combining the fragments 302 and 304 together, the pupil 310 is on top of the CMOS circuit 306. Pupil 312 has a circular cross section and extends through fragment 304 to allow sensitizing element 308 to monitor around the sensor. Pupil 312 is covered with a permeable membrane 314. In addition to main pupils 310 and 312, small pupils such as pupil 316 are distributed on top surface 322 of fragment 304. The shape of the small pupil is frusto-conical in that the diameter of the cross section increases into the fragment. The presence of small pupils allows the growth of vascular tissue or bone for biological fixation. The pupils 310, 312 and 316 are formed by standard etching techniques such as ion-beam grinding and reactive ion etching through photoresist masks. At least a portion of the outer surfaces of the fragments 302 and 304 are polarized to form porous silicon regions to promote deposition of apatite and bonding of the sensor to tissue. In FIG. 6, the porous silicon is represented by a ring 330 on the upper side of the fragment 304 and a groove 332 in the other side. Although the outer surfaces of the fragments 302 and 304 are entirely covered with porous silicon in FIG. 6, the surface 322 and bottom surface 334 of the fragment 302 may be sufficient to include the porous silicon. Such a device is simpler to manufacture. Fragments 302 and 304 are joined together using techniques developed for silicon on insulators. Anodization techniques have been described for the production of porous silicon, and dye etching is also known for the production of porous silicon. Such techniques may be advantageous for creating porous silicon surfaces on complex shaped structures.

In addition to sensors, bioactive silicon can be used in electronic prosthetic devices, for example, bills. Other electronic devices that may include bioactive silicon may refer to intelligent drug delivery systems.

In addition to sensors for incorporation into humans and other animals, bioactive porous silicon can be used in the manufacture of biosensors for in vitro testing. The composite structure of porous silicon having an apatite layer on the porous silicon can improve cellular suitability compared to biosensor devices of the prior art. Biosensors are potentially very important in the field of in vitro pharmaceutical testing. For automated pharmaceutical testing, the biopsy device may comprise a silicon wafer having a matrix arrangement of porous silicon regions. The cells can then be preferentially placed in the porous silicon region, which facilitates automated cell analysis after exposure to the drug product. The luminescent properties of porous silicon can be used to enable optical cell analysis techniques. Those skilled in the biosensor field can use their experience to determine whether cell culture is appropriate and how to monitor cell behavior.

While the results of in vitro experiments are described, they are not described in vivo. However, in vitro experiments are designed to mimic the environment in humans. From the results of in vitro experiments, it can be concluded that silicon wafers with reliably deposited apatite in SBF solutions also exhibit bioactive behavior in vivo.

The formation of an apatite membrane on silicon or porous silicon surfaces in vitro indicates that the bioactive silicon can be silicon in biocompatible form to a certain extent. The term "biocompatibility" refers to that the material is not necessarily biologically acceptable for all uses, but that the material may be biologically acceptable for a particular use. Some practitioners in the field of biocompatibility recognize "tissue suitability" as a more appropriate term to describe the definition of biocompatibility. The apatite layer can serve as a protective barrier to reduce the physiological effects of silicon.

As mentioned above, mesoporous silicon exhibits resorbable biomaterial properties. From the above-mentioned references by Hench, Journal of the American Ceramic Society, resorbable biomaterials are materials designed to be gradually degraded and replaced with natural host tissue over a period of time. The properties of mesoporous silicon in simulated body fluids indicate that mesoporous mesoporous silicon of suitable porosity may be a resorbable biomaterial. As discussed above, the porous region 20 of the bioactive silicon wafer 10 of FIG. 1 has a high intermediate porosity level. This indicates that the porosity of the intermediate porous silicon can be controlled to control whether the porous silicon region is bioactive or resorbable. By adjusting the porosity, the rate at which the porous silicon region is absorbed can be controlled.

Dissolution of porous silicon in SBF solution provides an indication of resorbable biomaterial properties, but the behavior of porous silicon regions in vivo can be influenced by non-reproducible factors in SBF solution. If living cells grow on the surface of the porous silicon, these cells can interact with the porous silicon. Thus, experiments performed in SBF solutions do not provide clear indications of the suitability of special types of porous silicon for resorbable material applications. Experiments must be performed in vivo to determine whether a particular desired physiological response is achieved.

Other experiments have shown that application of a bias current in the SBF solution can enhance or inhibit the formation of an apatite layer on porous silicon.

In the case of FIG. 7, a schematic diagram of an electrochemical cell 400 for loading galvanostatic on the entire silicon wafer 402 is shown. The wafer 402 was polarized for 1 minute in 40 wt% aqueous HF at 100 mAcm ^-2 for 1 minute before loading to the battery 400, having a porosity of about 20%, a thickness of 11 mu m, and a BET surface area of about 70 m 2 / g. A heavily doped n-type (100) oriented silicon wafer having a resistance of 0.012 Ωcm, which forms a phosphorous bioactive porous silicon layer. After anodization, the wafer is spin dried until its weight is stable and immediately loaded into the battery 400.

The wafer 402 is inserted into a PTFE cassette 404 and secured using a woven PTFE ring 406 that is secured to the cassette 404 to compress the PTFE coated O-rings 408 and 410. In the cassette 404, the silicon wafer is pressed against the metal back plate 412. The back plate 412 provides electrical contact with the back side of the silicon wafer, with 36 cm 2 of the area of the porous front side of the silicon wafer exposed in the cassette. The cassette 404 is placed in a polycarbonate tank 414 in a water bath containing 2 liters of SBF solution organically buffered at pH 7.3 ± 0.05 and maintained at 37 ± 1 ° C. Spiral platinum counterelectrode 416 is also inserted into the SBF solution. A dc constant current power source 418 is used to maintain a constant current between the wafer 402 and the reverse electrode 416. Wafer 402 may be under cathode or anode bias control. The power source 418 provides a constant current of ³⁶ mA, which is about 1 mA cm ^-2 or if the current flows mainly through the silicon skeleton, or evenly across the entire silicon-SBF interface through the pore network of porous silicon. When dispersed, it corresponds to the current density of a silicon wafer of about 1 μAcm ^-2 . The current is maintained for 3 hours. After removal from the battery 400, the wafer 402 is washed in deionized water and spin dried.

After 3 hours of exposure to SBF, the porous silicon wafer surface is irradiated with a JEOL 6400F scanning electron microscope (SEM) at an elevated potential of 6 kV. Along with the porous control wafer that did not accept bias current, the anode biased porous wafer had no evidence of surface deposits on porous silicon. However, the cathode biased wafers were completely covered with conifers, which merged to form a continuous layer. PlanView EDX analysis indicates that the overlap layer is primarily minerals containing calcium and phosphorus, and other SBF constituents such as carbon, magnesium, sodium and chlorine are approximated to EDX detection limits (ie <1 atomic%). . Plan view EDX analysis of the unbiased wafer and the anode biased wafer shows only the presence of silicon and oxygen.

Cross-sectional SEM and EDX analysis showed that the calcium and phosphorus rich minerals produced under the cathode bias were limited to the top of the porous silicon layer and were relatively thin, with a thickness of about 0.2 μm. Calcium and phosphorus levels in porous silicon are below the EDX detection limit for all samples. An anode loaded porous silicon layer was found to significantly accumulate oxygen in the upper 0.5 mu m of the layer.

Secondary ion mass spectroscopy (SIMS) is used to compare the depth and depth of layer calcification after three fractionation treatments with the depth distribution of other specific elements. The microporous silicon just etched has been found to contain, for example, very low levels of calcium and sodium (which are present in SBF) but significant levels of fluorine (not present in SBF).

FIG. 8 is a SIMS plot illustrating different levels of calcification resulting from electrical bias processing. In FIG. 8, the SIMS plot from the negatively biased wafer is represented by line 450, the SIMS plot from the unbiased wafer is represented by line 452, and the SIMS plot from the positively biased wafer is represented by line 454. Deposition mainly occurs near the surface of the porous silicon, but in all cases the calcium level is above the ground level through the 11 μm thick layer. Line 450 indicates that the degree of calcification with cathodic biasing is lowered and the degree of calcification with anode biasing is lower than the unbiased wafer. SIMS measurements also indicate the presence of SBF constituents through the porous silicon layer and indicate that fluorine is significantly migrated and lost as a result of cathode biasing, with some degree of retention in the overlapping layer.

It has been established that in vitro and in vivo tissues preferentially respond over an extremely limited range of input power, current and voltage in electrostimulation experiments. These ranges are sensitive to a number of factors, including the nature of the stimulation electrode. The biasing experiment described above indicates that the kinetic of the porous silicon calcification process can be raised in vitro and thus possibly in vivo by applying a cathode bias. They also suggest that when dissimilar silicon structures, such as porous and bulk silicon, are immersed together in physiological electrolytes, the cell erosion process can improve calcification at any negative electrode site created.

The potential application is varied for bias adjustment of mineral deposition. It is known that the insertion of an electrode into a living organism can form a fibrous layer around the electrode, the thickness of which is an indicator of the biocompatibility of the electrode. The rapid formation of stable mineral deposits around the microelectrode in vivo provides a strong advantage for the electrical stimulation of tissue growth or the stimulation of muscles in a paraplegic patient. Localization control of inorganic deposition, which can arrange localization regions so that inorganic deposits do not form, can be used in the field of biosensing devices, both in vivo and in vitro. An enhanced inorganic deposition process may be advantageous for the coating of silicon based integrated circuits before implanting the silicon based integrated circuits in the body.

Although the above technique for the electrical control of inorganic deposition is related to the deposition on porous silicon, inorganic deposition is also observed when applying a cathode bias to an unpolarized wafer in an SBF solution.

In another embodiment, certain types of polycrystalline silicon (polysilicon) can also lead to the deposition of calcium phosphate from SBF solutions and are thus found to be bioactive.

To produce bioactive polycrystalline silicon, a 100 mm diameter p-type CZ silicon wafer with a resistivity range of 5 to 10 microns was coated on the front and back surfaces with a wet thermal oxide 0.5 탆 thick followed by various microstructures. It is coated with a 1 μm thick polysilicon layer. The oxide layer is grown in Thermco TMX9000 diffusion furnace and the polysilicon layer is grown in Thermco TMX9000 low pressure chemical vapor deposition hot wall furnace. In the case of thermal oxide growth, the tube of the furnace is kept at a uniform temperature of 1000 ° C. and wet thermal oxides are grown using steam oxidation for 110 minutes. Subsequent deposition of the polysilicon layer involves pyrolysis of SiH ₄ at a pressure in the range from 250 to 300 mtorr using an electric furnace tube maintained at a temperature in the range from 570 to 620 ° C.

It has been established that the microstructure of a polysilicon layer is sensitive to numerous deposition factors such as temperature, pressure, gas flow rate, and substrate type. See Chapter 2, "Polycrystalline Silicon for Integrated Circuit. Applications ", T. Kamins, published by Kluwer Acad. Publ. 1988]. Polysilicon layers of widely varying microstructures and shapes are obtained using different deposition temperatures of 570 ° C, 580 ° C, 590 ° C, 600 ° C, 610 ° C, and 620 ° C. Cross-sectional transmission electron microscopy revealed that the layers deposited at 570 ° C. were substantially amorphous near their surface, while the layers deposited at 600 ° C. and 620 ° C. were polycrystalline throughout their depth. Particle size varies considerably with deposition temperature and varies dramatically with depth of field given.

The cut pieces of the wafer with typical dimensions of 0.5 × 50 × 20 mm ³ are then placed in separate 30 cm 3 polyethylene bottles filled with SBF solution as described above and the temperature of SBF is maintained at 37 ° C. ± 1 ° C. Observation of different polysilicon layers changes the level of stability in the SBF solution as measured by cross-sectional SEM imaging. After 64 hours in SBF solution, the polysilicon layer deposited at 620 ° C. thinned to about 60% of its original thickness, while the thickness of the layer deposited at 570 ° C. was substantially unchanged after 160 hours in SBF solution.

Inorganic deposits are observed to nucleate and proliferate across certain polysilicon layers. These deposits are observed using Plan View SEM. After soaking in SBF solution for 2 weeks, inorganic deposits are observed on the polysilicon layer deposited at 600 ° C and 620 ° C but not on the layer deposited at 570 ° C. These observations indicate that for porous silicon, there is a reactive window, depending on the microstructure, for optimal bioactivity. The maximum density of inorganic deposits is observed for the polysilicon layer deposited at 600 ° C. Consistent with the deposition of polysilicon on both front and back, significant levels of inorganic deposits were observed on both the front and back of the silicon wafer.

EDAX analysis of the deposits shows the presence of calcium, phosphorus and oxygen, consistent with some forms of nucleated apatite. The form of the deposit, however, differs from that of the conifers described above in relation to porous silicon, which appears to be more angled. The reason for this is not known but may reflect a slightly different local pH at the nucleation site on the polysilicon. The literature (see P. Li et al., Journal of Applied Biomaterials, Volume 4, 1993, p. 221) reports that, on growth on silica gel, the apatite morphology observed at pH 7.3 differs significantly from that observed at pH 7.2. It is.

The potential use for bioactive polysilicon is much wider than that for bioactive porous silicon. Various substrates that cannot be coated with monocrystalline silicon can be coated with polysilicon. Surgical implants can be coated with a layer of polysilicon to improve adhesion to bone. Polysilicon is also highly compatible with VSLI technology, which offers the prospect of making complex electronic circuits biocompatible. Polysilicon can be surface micromachined to produce a variety of devices and package devices.

One possible bioactive silicon package design has already been described with reference to FIG. 6. In the case of bioactive polysilicon, more compact biochips can be produced. 9 is a schematic diagram of a biosensor device 500 including bioactive polysilicon. The device 500 includes a bulk silicon wafer 510 on which a CMOS circuit 512 and a sensor element 514 are assembled. Sensor element 514 is electrically connected to circuit 512. The circuit 512 is protected by a barrier layer 516 made of, for example, silicon oxide and silicon nitride. Except for the window 518 for the sensor element 514, the device 500 is entirely covered with a bioactive polysilicon layer 520. Barrier layer 516 is required because polysilicon itself is not a good protective layer for silicon based circuits due to diffusion through the grain boundary. Thus, barrier layer 516 is present between circuit 512 and polysilicon layer 520.

Similar to the results when using porous silicon, the bioactivity of polycrystalline silicon can be improved by doping it with calcium, sodium or phosphorus or a combination of these species.

Bioactive polysilicon may be a suitable substrate for biopsy devices. L. Bousse et al., IEEE Engineering in Medicine and Biology, 1994, pp. 396-401, describe biosensors for trapping cells in micromechanical pupils on silicon chips to perform in vitro measurements. . Such a device may advantageously comprise a composite structure of polysilicon having an apatite layer thereon, wherein the cells themselves are preferentially located in the apatite region.

1 is a cross-sectional view of a bioactive silicon wafer.

FIG. 2 is a scanning electron micrograph (SEM) image of the apatite deposit on the bulk silicon region adjacent to the porous region of the wafer of FIG. 1.

3 is a SEM photograph of a cross section of the silicon region of FIG. 2.

FIG. 4 is a SEM photograph showing apatite spherulite deposited on a porous silicon region having a porosity of 31%.

FIG. 5A is an SEM image of the unpolarized region of a silicon wafer anodized to have a porosity of 48% after immersion in a simulated body fluid solution.

FIG. 5B is a SEM photograph of the polarization region of the wafer of FIG. 5A.

6 is a schematic diagram of a biosensor comprising bioactive silicon.

7 is a schematic diagram of an electrochemical cell for electrically regulating bioactivity.

FIG. 8 shows the calcium concentration profile in the porous silicon wafer after processing in the battery of FIG. 7.

9 is a schematic diagram of a biosensor device comprising the bioactive polycrystalline silicon of the present invention.

Claims (4)

A method of increasing or decreasing the rate of deposition of inorganic deposits on bioactive silicon in a physiological electrolyte, wherein the bioactive silicon is immersed in the simulated body fluid maintained at physiological temperature in the absence of electrical bias, And microporous silicon having a diameter of less than 20 microns in diameter having a structure that causes the deposition of inorganic deposits on the silicon, the method comprising applying an electrical bias to the silicon. The method of claim 1, wherein the method comprises applying a cathode bias to the microporous silicon. The method of claim 1, wherein the method comprises applying an anode bias to the microporous silicon. The method of claim 1 wherein the current density produced by applying said electrical bias to silicon is 0-1 mA cm ^-2 .