A Stretchable‐Hybrid Low‐Power Monolithic ECG Patch with Microfluidic Liquid‐Metal Interconnects and Stretchable Carbon‐Black Nanocomposite Electrodes for Wearable Heart Monitoring

Abstract

patches have been recommended by physicians for patients with heart abnormalities to correlate their activities with heart signals. [2] In order for long-duration on-skin monitoring, comfort to the wearer is an important design consideration for these patches. To enable intimate attachment to the body, the package modulus and form factor should approach that of the human skin. Besides enhancing comfort, good electrical contact for high fidelity signal acquisition that is immune to the motion of wearer and environment influences are necessary. [8] Despite significant advancement in wearable electronics, [1, major tradeoffs between form-factor, performance, and functionality remain. For ultrathin skin-like material systems fabricated by ink-printing or microcontact transfer printing, [9][10][11][12][13][14][15][20][21][22][25][26][27] the complexity and signal processing capabilities are typically limited by the weaker transistors or interconnects. Ink-printed components are also limited by lower integration density compared to rigid Silicon CMOS technologies, leading to lower functionality. The increased R-C parasitic with larger and weaker components limits the scalability toward highly-energy-efficiency under low-voltage operations. In contrast, rigid CMOS chips and printed circuit boards (PCBs) require integration with soft components to interface comfortably with the human body. The need for electrical performance with soft and robust mechanical form factor leads us to the codesign of composite materials and electronic circuits/system in a monolithic form of flexible hybrid electronics. [1,3,4,12,13,25,28] In this work, we report on a novel integration of a wearable and stretchable-hybrid SEP with monolithically integrated sensor electrodes and liquid-metal interconnects. The SEP integration involves the combination of a chip-on-board embedded in a moisture-resistant elastomer matrix with microfluidic interconnects, and soft low-resistance electrodes (Figure 1). A stretchable electrocardiogram (ECG) patch (SEP) that monolithically integrates ECG monitoring chip-on-board (COB) with polydimethylsiloxane (PDMS) and liquid-metal interconnects is presented. The 4.8 × 4.8 cm 2 SEPis conformal and robust to mechanical deformation. The use of a siliconon-insulator rigid complementary-metal-oxide-semiconductor chip allows sophisticated power management and signal processing. The chip's dense inputs/output pads are interfaced with coarser liquid-metal interconnects using a dual-sided COB design. A robust ECG signal response (≈100 mV p-p up to 1 kHz), subjected to mechanical deformation and moisture is demonstrated. The SEP allows up to 10% stretch, providing sufficient pliability to enable conformal contact to the human chest. Low profile soft carbon black-PDMS nanocomposite electrodes, robust to deformation, enable good skin contact and allow for low-noise signal acquisition that is comparable to larger commercial wet electrodes.

Flexible Sensors—From Materials to Applications

“…In addition, process compatible melting/glass transition temperatures and outgassing rates, low surface roughness, chemical stability and large area compatibility are all desirable properties for flexible substrates. Common materials used for this end are PI [13][14][15][23][24][25]34,36], PET [17,18,40,70,73,109,169], PEN [9,20,26,204,311] and PDMS [16,21,22,89,168,[312][313][314][315]. However, there are several other examples of flexible substrates suitable for sensor applications reported in literature, including PU [96,316,317], PLA [293], polysulfone (PSU) [318], polyetheretherketone (PEEK) [319], polycarbonate (PC) [79], parylene [47], polyvinyl alcohol (PVA) [12], polyarylate (PAR) [239], Ecoflex™ [320,321], Dragon Skin™ [101,[321][322]...…”

“…Some researchers have looked into positioning rigid electronics components on flexible PI substrates to create signal acquisition electronics [313,549,550]. The signal acquisition electronics were realised by positioning a rigid electropotential monitoring chip on a stretchable PDMS patch (4.8 cm × 4.8 cm) [313]. The measurements obtained from the sensor were within the chip's specification and the system endured a 5 cm bending radius.…”

“…An ideal flexible sensor system can be broadly divided into four parts: The sensor, on-site signal conditioning, power supply and signal transmission. In this context, most flexible sensors presented in literature rely on external rigid electronics for signal conditioning and readout [9,11,313,315]. While these devices present outstanding performance, their dependence on rigid circuits prevents true conformability.…”

“…Flexible sensors can be easily integrated into living tissues to provide various health-related real time metrics. Body sensors were generally attached onto skin using tapes [383,385,395,544], adhesives [313,441], and lamination [576,620]. Some researchers have also looked into adhering sensors onto skin through transfer tattoos (Figure 16b) [539,591].…”

Physical sensors for skin‐inspired electronics

“…Furthermore, electrophysiological sensors can record important information about health by visualizing human body electrical signals into diagrams, such as the electrocardiogram (ECG), electromyogram (EMG), and electroencephalogram (EEG) . Various flexible and conformable electrophysiological sensors have been developed for long‐term monitoring electrophysiological signals.…”