Apatite Nucleation on Low Porosity Silicon in Acellular Simulated Body Fluids
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1.
INTRODUCTION
It has recently been suggested that Si structures might be rendered 'bioactive', and thereby considerably more biologically acceptable for specific applications, than previously imagined (1). This could have exciting implications for Si-based biomedical engineering and biosensor technologies. These commonly exploit VLSI circuitry and micromachining techniques, but suffer from problems with the interfacing of the device to its biological environment. The novel concept of utilising Si itself as a bioactive miniaturisable packaging material (1) might provide a solution to some of these problems. We focus here on different aspects of the first in-vitro studies wherein a form of hydroxyapatite (HA) growth was observed to develop on low porosity silicon films. Section 2 briefly describes how Si can be rendered porous, the widely variable types of porous Si, and the dominant current
interest in the material. The in-vitro solution utilised is then described in section 3, together with characterisation data revealing HA nucleation and growth (section 4). Finally section 5 discusses why further study could provide new insight into the phenomenon of bioactivity, and could lead to the realisation of a range of bioactive semiconductor structures.
2.
POROUS SILICON
Highly variable levels of porosity can be introduced into semiconducting silicon through anodization in hydrofluoric acid based electrolytes. High porosity (k70%) structures can emit visible luminescence efficiently (2) and there has been an explosion of interest in their properties (3), aimed at extending the functionality of Si technology into optoelectronics. Through control of the anodization process and silicon substrate type, the semiconductor can be made primarily macroporous (pore width > 50 nm), mesoporous (2-50 nm wide pores) or even wholly microporous (all pores < 2 nm wide). Porous Si of high specific surface area, is considerably more chemically reactive than bulk Si, a property which is a disadvantage for many optoelectronic applications, but which could be directly responsible for the in-vitro behaviour described in the next section. Table 1 lists some of the porous Si films used in the first in-vitro studies (1) covering a very wide range of both porosity and BET surface area derived from nitrogen adsorption analysis (4). In every case the material had been stored in ambient air for 189 Mat. Res. Soc. Symp. Proc. Vol. 414 0 1996 Materials Research Society
a sufficient period so that thin native oxide covered its Si skeleton. The 'aged' structures of 48% and 75% porosity were visibly luminescent, the others not.
Table 1 Porus Si Structures Studied Wafer ID No
74P
NB76
Wafer type and resistivity (0cm)
N (0.3) 10
Anodization current density (mA cm"2) Anodization time (min)
3WU P"
OXB P+
(30)
(0.03)
100
100
20
100
5
1
1
5
1
20%
50 wt%
50 wt%
50wt%
20%
ethanoic BF
aqueous HF
aqueous IHF
aqueous HI-F
ethanoic HF
Layer porosity (%)
4
18
31
48
75
Layer thickness (pm)
40
13.7
9.4
6.65
4.3
BET sur
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