Material Gradients in Stretchable Substrates toward Integrated Electronic Functionality

The new technologies of flexible and stretchable electronics are widely used for their applications in the various fields of medicine and electronic devices. These systems also have the applications in the field of human body. Stretchable electronics have been used in wide variety of ways which includes Organic electronic materials (conductive polymers), Inorganic semiconductors (nanotubes and nanowires), micro-fluidic approaches and thin inorganic materials (Si, GaAs) which are embedded or patterned on soft polymers.

The approach towards a stretchable electronic substrate employs the sub-micrometer layers of inorganic materials within an electronic device because this thinning allows the stiff materials to become more flexible. However, thinning of devices causes various challenges such as losing the functionality of silicon-based electronics along with the various properties associated with embedding them into flexible materials.

For example, one of the major challenges is the drastic mismatch in mechanical properties of silicon-based electronics (Young’s modulus, E~170 GPa) and soft materials, mimicking the human body (Young’s modulus, E~100 kPa). This causes the difficulties in the properties like attachment, stretching, and functionality for wearable biomedical instruments. Silicon-based electronics are rigid with strain of less than 2%, while flexible and stretchable electronics can be bent, stretched, and twisted with strain greater than 10%.

A research team from Carnegie Mellon University (USA) designed a stretchable structure that allows easy embedment of thick silicon chips (greater than 10 μm) for its various applications. One main challenge was to interface two-material systems through the use of an intermediate gradient material. Their study is published in Advanced Materials.

The authors demonstrated quantitatively the extent of delamination from rigid devices that are embedded into stretchable substrates. For the elimination of the delamination effects between the soft and rigid material, suitable designs of stretchable systems will be required to embed standard micro fabricated electronics. The intermediate soft material with a Young’s modulus between that of the primary soft material and the silicon substrate decreases the risk of delamination of the soft material from embedded silicon chips.

By using this approach, ~140% strain before failure can be tolerated by the structure with the intermediate soft material, while similar structures without intermediate soft material fail to tolerate strain of even ~20%. Therefore, this approach reduces the delamination under high external strain levels by employing an intermediate material layer that results in the six times increase in the strain failure limit and the ability to stretch the substrate to over twice of its length before delamination occurs. It is an important approach for the next generation of stretchable (silicon) electronics on the skin that should function under high strain and will also be important in areas such as biomaterials, other flexible electronics, and biomimetics.

Further, to optimize the delamination feature at the interface, the “energy release rate” must be studied thoroughly. The energy introduced causes the initiated crack to increase which must be balanced by the amount of energy lost due to the formation of new surfaces. The crack size increases when the energy release rate equals a critical value. The risk of delamination at the interface is of great importance and this risk increases when the system is stretched and thus subjected to mechanical strain.

Therefore, due to the high stress at the interface, delamination occurs. To prevent this problem of delamination at the interface, the amount of strain and strain energy needs to be reduced to a minimum level. This task is accomplished using an approach in which the incorporated intermediate material gradient effects the change in the stiffness between the soft and rigid materials. Addition of the discrete material layer, having a stiffness value between the soft and rigid materials, helps in reducing delamination and stress around interconnects. Also, by selecting two materials with strong bonding capability, the structure can be designed to tolerate the higher strains before delamination.

Concluding, this approach towards a stretchable electronic substrate suggests the employment of multiple soft polymer layers patterned or embedded around silicon chips, mimicking the conventional electronics chips, to create a stiffness gradient that can be controlled.

Adding just one intermediate polymer layer results in six times increase in the failure strain, resulting in the substrate to be stretched to over twice of its length before delamination occurs.