An optimized methodology to achieve irreversible bonding between PDMS and Polyimides for biomedical sensors

Polyimide (PI) and polydimethylsiloxane (PDMS) are widely used materials in biomedical sensor development. The hydrophobic property of PDMS makes it difficult to bind PDMS with other sensor materials. This paper employs chemical functionalization of the PDMS and PI surfaces via epoxy-thiol click chemistry to achieve irreversible bonding. To demonstrate the importance of strong bonding, a wireless pressure sensor is developed. The sensor masks are directly printed on the copper-coated polyimide sheets using a LaserJet printer. A wet etching technique is used to etch the sensor patterns. The plasma treatment is performed to achieve the hydroxylation on the surface of PDMS and patterned polyimide sheet. To chemically functionalize the surfaces, plasma-treated prefabricated PDMS sheets and electrode-patterned PI sheets were immersed in 2% v/v (3-glycidyloxypropyl) trimethoxysilane (GPTMS, 98%) in methanol and 2% v/v (3-mercaptopropyl) trimethoxysilane (MPTMS, 95%) in methanol, respectively. The final assembly was created after chemical treatment, and the sensor was then placed under pressure for 24 hours at room temperature. The bonding strength between the PDMS and PI is tested using a peel-off test method where adhesive and cohesive failures are observed. The sensor is tested for cyclic pressures over 1 million cycles and no bonding failure is observed. This irreversible bonding can improve sensor integrity, reliability, and stability, especially for biomedical applications.