PDMS with designer functionalities—Properties, modifications strategies, and applications

Rapid progress in micro- and nanotechnologies such as lab-on-a-chip (microfluidic networks, sensors, actuators, and connectors), soft lithography (replica moulding, microcontact printing and affinity contact printing), and stretchable transparent electronics has strongly benefitted from high-performance polymers like poly(dimethyl)siloxane (PDMS) that are suited for high-fidelity microsystem construction and rapid prototyping. While basic PDMS has been a unique enabling material, recent progress in tailoring PDMS to specific requirements will render this material even more valuable in the future. Basic PDMS is elastic, transparent, biocompatible, gas-permeable, and forms conformal contact with surfaces. Surface modifications of PDMS, inducing properties such as hydrophilicity, electrical conductivity, anti-fouling, energy harvesting, and energy storage (supercapacitors) are of major interest. Bulk modifications can alter PDMS properties such as elasticity, electrical and thermal conductivity. Such bulk modified PDMS composite materials can be created by embedding free molecules (e.g., dyes), nanoparticles (graphene, carbon nanotubes, and various other of organic and inorganic nature) or microparticles, or by altering the composition of the prepolymers before polymerization. Both, surface and bulk modifications of PDMS open avenues to a multitude of tuneable characteristics optimized for a diverse set of applications ranging from integrated micro- (lab-on-a-chip) to macro-systems (biomedical devices and epidermal electronics). In microfluidic systems design exploiting modified PDMS, a key aspect of this review, the unique features of these materials permit rapid, easy, and reproducible construction without the need for elaborate facilities and trained personnel as compared to other materials like silicon. This review focuses on recent progress in modification strategies to alter deliberately PDMS surface and bulk properties, especially for microfluidic, biological, flexible electronics, e-skin, and self-healing applications.