Flexible substrates have become a cornerstone in the development of stretchable electronics, enabling applications ranging from wearable health monitors to conformable displays. Unlike rigid substrates, which limit device adaptability, flexible materials such as polydimethylsiloxane (PDMS), polyimide, and biodegradable polymers offer mechanical compliance, durability, and compatibility with semiconductor processing techniques. The choice of substrate material directly impacts the performance, reliability, and environmental footprint of stretchable electronic systems.

Polydimethylsiloxane (PDMS) is widely used due to its exceptional elasticity, biocompatibility, and chemical stability. With a Young’s modulus typically ranging from 0.5 to 3 MPa, PDMS can withstand strains exceeding 100% without permanent deformation. Its low surface energy, however, poses challenges for adhesion, necessitating surface treatments such as oxygen plasma or chemical functionalization to improve bonding with conductive and semiconducting layers. PDMS also exhibits high thermal stability, with degradation temperatures above 300°C, making it suitable for processes requiring moderate heat exposure. Its optical transparency in the visible spectrum further enables applications in optoelectronics.

Polyimide stands out for its high thermal endurance and mechanical robustness. With a Young’s modulus between 2 and 8 GPa, polyimide substrates are less stretchable than PDMS but offer superior dimensional stability under thermal stress, enduring temperatures up to 400°C without significant degradation. This makes polyimide ideal for applications requiring repeated thermal cycling, such as flexible printed circuit boards. Its chemical resistance to solvents and acids allows for compatibility with standard lithographic patterning techniques. However, polyimide’s relatively high stiffness compared to PDMS limits its use in highly deformable systems unless engineered with microstructured designs to enhance stretchability.

Eco-friendly biodegradable polymers, such as polylactic acid (PLA) and polycaprolactone (PCL), are gaining attention for sustainable electronics. These materials exhibit tunable mechanical properties, with Young’s moduli ranging from 0.1 to 4 GPa, depending on composition and processing conditions. While their thermal stability is lower than PDMS or polyimide—degrading at temperatures around 150–200°C—they offer the advantage of environmental degradability, reducing electronic waste. Processing biodegradable polymers requires careful control of humidity and temperature to prevent premature degradation during device fabrication. Adhesion to metal or semiconductor layers can be improved through surface roughening or the use of biocompatible adhesives.

Substrate patterning is critical for defining device geometries and enhancing functionality. Photolithography, though traditionally used on rigid substrates, can be adapted for flexible materials with modifications such as sacrificial layers or soft baking at reduced temperatures. Laser ablation offers a direct-write method for patterning polyimide and other thermoset polymers with micron-scale precision. For PDMS, micromolding and soft lithography techniques like replica molding enable the creation of microfluidic channels or stretchable interconnects. Biodegradable polymers often require solvent-based patterning or 3D printing to avoid thermal degradation.

Adhesion enhancement remains a key challenge in stretchable electronics. For PDMS, silane-based coupling agents or nanoparticle coatings can improve interfacial bonding with metals like gold or copper. Polyimide surfaces are often treated with adhesion promoters such as aminopropyltriethoxysilane (APTES) before metallization. Biodegradable polymers may require hybrid approaches, such as embedding nanostructured layers or using bio-adhesives inspired by natural systems. Peeling tests quantify adhesion strength, with values typically ranging from 0.1 to 10 N/cm depending on the material system and treatment method.

Thermal management is another consideration, as flexible substrates generally exhibit lower thermal conductivity than rigid alternatives. PDMS has a thermal conductivity of approximately 0.15 W/m·K, while polyimide ranges from 0.1 to 0.35 W/m·K. Biodegradable polymers can be even lower, around 0.1 W/m·K. These values necessitate careful design to dissipate heat in high-power applications, often through the integration of thermally conductive fillers or heat-spreading layers.

Mechanical durability under cyclic loading is essential for stretchable electronics. PDMS can endure thousands of stretching cycles at 20–30% strain with minimal degradation in electrical performance. Polyimide, though less stretchable, maintains integrity under repeated bending with radii as small as 1 mm. Biodegradable polymers show more variability, with some formulations achieving comparable cyclic stability to PDMS when plasticized or blended with elastomers.

Compatibility with semiconductor processing varies by material. PDMS is incompatible with high-temperature steps but works well with low-temperature deposition techniques like sputtering or evaporation. Polyimide can withstand short exposures to temperatures up to 350°C, enabling processes like chemical vapor deposition (CVD) of thin-film transistors. Biodegradable polymers are limited to room-temperature or low-heat processes, restricting the range of compatible semiconductors to organic or solution-processed materials.

Differentiation from rigid substrates is evident in their applications. While silicon or glass carriers excel in traditional microelectronics, flexible substrates enable conformal integration with curved or dynamic surfaces. Stretchable electronics also prioritize strain isolation designs, such as serpentine interconnects or island-bridge architectures, to mitigate mechanical stress on active components—a consideration unnecessary in rigid systems.

Future advancements may focus on hybrid substrates combining the strengths of different materials, such as PDMS-polyimide laminates for balanced stretchability and thermal stability. Innovations in biodegradable polymers could expand their use in transient electronics, where devices dissolve after a functional lifetime. Progress in nanomaterial integration, such as graphene or silver nanowires, may further enhance the electrical and mechanical performance of flexible substrates.

In summary, the selection and engineering of flexible substrates are pivotal to the success of stretchable electronics. Material properties, processing compatibility, and environmental impact must be carefully balanced to meet the demands of emerging applications. As research continues to refine these materials and techniques, the potential for stretchable electronics will expand, enabling technologies that seamlessly integrate with human and environmental systems.