MEMS and implantable chips

Silicon chips and microelectromechanical systems (MEMS) that can be implanted in the human body will only be possible with a clearer understanding of the interactions between single-crystal silicon and human tissues and fluids, for example, formation of mineral apatite. This understanding will ultimately allow semiconductor devices to be interfaced with living tissues and so opens the door to implantable biosensors that can test indicators of disease or symptoms and then regulate the release of a drug. For instance, an implanted glucose sensor could be coupled to an insulin release system and so help diabetes sufferers control their blood sugar without pin-prick tests or the need to inject insulin.

Paul Chu and researchers in the Plasma Laboratory at the City University of Hong Kong have used plasma immersion ion implantation to improve the surface bioactivity of silicon and investigate the mechanism of apatite formation on silicon. In the October 18 issue of Applied Physics Letters, they reveal how micro-Raman and transmission electron microscopy results demonstrate the presence of a disordered silicon surface containing silicon-hydrogen bonds after hydrogen implantation. Immersing the sample in a simulated body fluid leads to an initial reaction of these Si-H bonds with water to form a negatively charged and functionalized surface, which leads to apatite formation. While previous researchers have recognized that a negatively charged surface is one essential component of interfacing semiconductor devices with living tissues, Chu’s work provides new insights into how this might be achieved on single-crystal silicon.

Chu told Reactive Reports how this pioneering work in making single-crystal silicon bioactive is important to biomedical microdevices such as MEMS and biosensors. With acceptable surface bioactivity and biocompatibility, silicon-based microdevices can be directly implanted into human bodies to detect various biological functions such as blood sugar, fluid pressure, cartilage abrasion, and so on. In addition to the osteo-compatibility of silicon, Chu’s research group is investigating the improvement of the blood compatibility of silicon-based materials such as silicon nitride using plasma treatments and their preliminary blood platelet activation results are very favorable.

According to Chu, a clearer understanding of the interactions between silicon surfaces and body tissues, as well as the development of a means to fabricate such surfaces, will expedite direct implantation of various types of silicon-based biosensors into the human body.

Appl Phys Lett, 2004, 85, 3623-3625; Chu/paul_chu.htm

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