Engineered Strain Gradients for Hybrid Integration of Rigid Electronics on Soft Biointerfaces

Soft bioelectronics is emerging as a technological solution to integrate man-made electronics with biological tissues, and enable safe and long-term communication. On the technological end, a critical challenge to overcome is related to the mechanical and electrical integration of rigid components, such as active electronics, connectors or light sources (e.g. LEDs) onto soft wiring substrates. The mechanical mismatch at the soft-rigid interface generates strain peaks, which can cause delamination of the rigid component under elongation and fracture of the interconnects, thereby representing a failure point of the entire system. One way of mitigating this effect is by introducing a mechanical strain gradient within the substrate itself and in proximity of the critical interface.<br/>Here, we exploit geometrical designs based on kirigami-like Y-shaped motifs and microtechnology of plastic films to engineer surfaces of stiff materials with programmable and distributed elasticity. Plain, 2D platforms are connected with engineered threads prepared with the same stiff material; strain fields upon uni-axial stretching are quantified.<br/>First, the influence of the motif shape, density and thickness on resulting strain is studied at the macroscopic scale using patterned PET sheets uni-axially stretched. The equivalent spring constant is computed for a range of uniform Y-shaped motifs: the results lead to a simple analytical model used to set the varying geometrical parameters of the Y-shaped motifs along a given length, in order to create a linear gradient of local strain.<br/>Next, we compare macroscopic samples with and without mechanical gradients upon tensile elongation at constant force. The abrupt strain peak at the soft-rigid interface is successfully smoothed out by the use of gradients but only for designs that support out-of-plane deformations and plastic deformation of the motifs. Changes in the electrical resistance of metallized patterns (platinum coated PET) further demonstrate the benefit of the mechanical gradient in producing lower resistance increase with strain compared to non-graded samples.<br/>Next, the graded patterns are miniaturised so that repeated motifs are scaled to the micron size and support layouts of interconnects with dimensions suitable for standard microfabricated bioelectronic devices. We report on the challenges of miniaturisation and associated encapsulation within sub-millimeter thick silicone membranes to enable robust and handleable soft bioelectronic circuits.