Silicon as an Active Biomaterial
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ABSTRACT The response of a range of porous Si and poly Si films to storage in acellular simulated body fluids is summarised and its implications discussed. It is suggested that the combination of VLSI technology, micromachining and surface microstructuring achievable with silicon, could establish this prominent semiconductor as a very useful biomaterial by the next century. The 'biocompatibility' of a variety of silicon microstructures, and even bulk silicon has received surprisingly little study, but now warrants detailed in-vitro and in-vivo assessment. INTRODUCTION In biology, elemental silicon is now established as one of the trace but essential elements [1], due largely to the pioneering work of Carlisle throughout the 1970's [2,3]. Its physiological role in man has been recently scrutinised by Birchall and co-workers [4,5] who emphasise the ability of silicic acid to assist biological systems in removing aluminium, a ubiquitous ecotoxicant. Despite the beneficial properties of low levels of elemental silicon, in its bulk crystalline form it has not generally been regarded or developed as a useful biomaterial. In this paper we first give examples of where Si technology is currently commercialised and at the research stage for biomedical and biotechnology applications. An overview of our recent work on rendering Si wafer surfaces 'bioactive' [6-9] with regards to calcification is then presented, and some of the implications are considered in the final section of the paper.
BIOAPPLICATIONS OF BULK SILICON TECHNOLOGY Figure 1 illustrates some potential and already developed in-vivo applications of electronics. Commercialised products include the pacemaker and defibrillator [10], cochlear implants [11], functional electrical stimulation (FES) devices [12], and programmable drug delivery systems [13]. CMOS circuitry is generally isolated from the body by a hermetically sealed titanium package. A significant range of devices that exploit both VLSI technology and micromachining techniques are also now at the research stage. These include the 'silicon retina' [14], catheter based devices 115], glucose monitoring systems [16] and many other biomedical sensors [17-18]. In-vitro applications of Si based biotechnology span several disciplines such as clinical diagnostics, environmental monitoring and agriculture (eg disposable blood analysis chips [19], DNA chips [20], microelectrode devices for cell manipulation [21]. Once again, in most of these diverse application areas the semiconductor itself is isolated from the biological medium by either a more 'biocompatible' encapsulant or package. One motivation for the experimental work described in the next section is to explore whether the chip itself could be rendered more biocompatible. This could then enable further miniaturization of electronic implants and in general much greater flexibility in bio-electronic interfacing for both in-vitro and in-vivo applications. 579 Mat. Res. Soc. Symp. Proc. Vol. 452 ©1997 Materials Research Society
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