Electrical and mechanical properties of a biocompatible carbon black reinforced thermoplastic elastomer for sensing applications

The relationship between pressure-sensing capability, carbon black (CB) content, and tensile strength of a CB-reinforced poly(styrene-isobutylene-styrene) (SIBS) thermoplastic elastomer was investigated. Inexpensive pressure sensors have excellent potential in both implantable and external (stick-to-skin) biomedical applications. Due to its excellent biocompatibility and commercial use in biomedical implants, SIBS is an excellent host material for the conductive CB reinforcement. Applications include stickto- skin sensors to supplement peripheral nerve function for anesthetized patients, stroke victims, or diabetic patients suffering from peripheral neuropathy. Five specimens ranging from 0% to 25% CB content by weight were fabricated through a combination of high-shear mixing, ultrasonication, and solvent casting. Electrical properties were explored using a custom-built apparatus capable of measuring resistance as a function of applied pressure. Results indicated that the 15% CB specimen performed significantly better than other loading percentages; it produced the largest variability in resistivity within the studied pressure range, along with a 13% increase in tensile strength. A direct relationship between pressure sensitivity (resistivity range) and tensile strength appears to exist in specimens near or at the percolation threshold, whereas otherwise, an inverse relationship is present. This initial research represents a step toward understanding interactions between the competing design goals of CB content, mechanical strength, and retained biocompatibility. The results of the 15% CB specimens show particular promise, as pressure sensitivity was maximized without sacrificing tensile strength. Further, initial results showed no sign of toxicity from SIBS/CB on human stem cells after a seven-day incubation period. In conclusion, these results support the potential use of SIBS/CB composites in biomedical sensing applications without sacrificing tensile strength or biocompatibility.