Utilizing the supportive structure of hydrogels, the semiconducting character of porous silicon (pSi) membranes, and the biodegradability of both, a unique biosensor for the chemical analysis of health-relevant analytes can ideally be created.
Hydrogels are water-infused, biodegradable polymer networks. Alginate-based hydrogels are particularly useful because of environmental abundance, along with their ability to interface well with human skin. These characteristics also make them an ideal medium for supporting pSi membranes and simultaneously assimilating them into a wide range of tissues.
Porous silicon (pSi), a highly porous form of the elemental semiconductor, is utilized to measure and conduct electrical signals throughout the hydrogel matrix. In diode form, these membranes exhibit measurable current values as a function of voltage, which can be used to detect bioelectrical stimuli such as the concentration of physiologically relevant ionic species (e.g. Na⁺, K⁺, and Ca²⁺).
Recent experiments center on integrating pSi membranes into various aqueous environments and hydrogels to test how variations in ion concentration affect the flow of current as a function of applied voltage. pSi membranes ~110 mm thick and 79% porosity are fabricated from the anodization of low resistivity (100) Si in methanolic HF at an applied bias of 100 mA/cm² for 30 min. Membrane pieces ~ 2 mm by 2 mm are fashioned into diodes upon the attachment of 0.25 mm diameter Cu wire using Ag epoxy and annealed in air for 10 minutes at 90 ^oC. In each ion sensing experiment, an electrochemical cell is created by placing two pSi membranes parallel each other ~3 mm apart vertically in a fixed electrolyte composition. Current is measured as a function of applied voltage (typically from 0-5 V) for systems with different NaCl concentration in the nM to mM range. As expected, the magnitude of maximum current response is proportional to ion concentration present in the electrolyte, with an order of magnitude amplification (or more) of measured current for a given voltage upon immersion of the electrodes in an alginate hydrogel matrix (relative to water alone).
This presentation will focus on initial fabrication protocols, as well establishing limits of detection for simple species such as ions present in human sweat. More refined strategies are also envisioned, including the development of methods for stabilization of sensor performance along with miniaturization of the sensing platform itself.

George Weimer

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