Flexible Hybrid Electronics: Direct Interfacing of Soft and Hard Electronics for Wearable Health Monitoring

TLDR

Interfacing soft and hard electronics is a key challenge; current multisubstrate approaches limit flexibility and cause interconnect problems. The study reports a single‑substrate interfacing approach that prints soft sensors directly onto Kapton polyimide substrates. Using an FPCB‑compatible process, the authors fabricated a wearable patch with inkjet‑printed gold ECG electrodes and a stencil‑printed nickel‑oxide thermistor. The patch delivers 1 mV peak‑to‑peak ECG at 4.7 cm spacing and a thermistor with a –5.84 % K⁻¹ temperature coefficient, enabling extension to multisensor IoT health monitoring.

Abstract

The interfacing of soft and hard electronics is a key challenge for flexible hybrid electronics. Currently, a multisubstrate approach is employed, where soft and hard devices are fabricated or assembled on separate substrates, and bonded or interfaced using connectors; this hinders the flexibility of the device and is prone to interconnect issues. Here, a single substrate interfacing approach is reported, where soft devices, i.e., sensors, are directly printed on Kapton polyimide substrates that are widely used for fabricating flexible printed circuit boards (FPCBs). Utilizing a process flow compatible with the FPCB assembly process, a wearable sensor patch is fabricated composed of inkjet‐printed gold electrocardiography (ECG) electrodes and a stencil‐printed nickel oxide thermistor. The ECG electrodes provide 1 mV p – p ECG signal at 4.7 cm electrode spacing and the thermistor is highly sensitive at normal body temperatures, and demonstrates temperature coefficient, α ≈ –5.84% K –1 and material constant, β ≈ 4330 K. This sensor platform can be extended to a more sophisticated multisensor platform where sensors fabricated using solution processable functional inks can be interfaced to hard electronics for health and performance monitoring, as well as internet of things applications.