An Intrinsically Stretchable Multi-Biochemical Sensor for Sweat Analysis Using a Photo-Patternable Silicone Elastomer

Patterning of stretchable materials such as PDMS, Ecoflex, which is widely used in stretchable electronics, is a necessity in methods to fabricate multi-layer of stretchable integrated circuits. Nevertheless, previous patterning methods for stretchable materials, such as soft lithography and laser lithography, can leave residues or damage on the underlying layer. Fabrication of sophisticated and multi-functional stretchable biosensors requires development of an intrinsically stretchable material that can be directly patterned without any effect on the primary layer.<br/>Here, we developed Photo-patternable Ecoflex (PPE) to directly integrate intrinsically-stretchable nanocomposite. We used benzophenone as a photo-inhibitor to hinder cross-linking of Ecoflex by the radical reaction, so that Ecoflex could be patterned using photolithography. PPE was then combined with the stretchable silver (Ag) composite as an encapsulation layer by using a direct patterning process. The homogeneous interface between encapsulation and conductor contributes to efficient strain energy dissipation, so at 250% strain, stretchable electrodes with PPE encapsulation showed 50% higher electrical conductance than stretchable electrodes without PPE encapsulation. Using this PPE integrated stretchable conductor, we fabricated an intrinsically stretchable and multi-chemical sensor that can monitor the levels of pH, glucose and lactate in sweat. All sensors showed durability (i.e., ≤ 10% change at 50% strain and ≤ 15% change over 1000 cycles at 20% strain). Finally, the intrinsically stretchable device was demonstrated to continuously monitor the glucose and lactate levels in human sweat. The measured glucose level was quantified as 70 mg/dL in the fasting state and 100 mg/dL after food intake. The measured lactate level was from 5 mM to 16 mM during exercise. Both measurements were consistent with those from commercial essay kits.