Flexible Hybrid Sensors for Health Monitoring: Materials and Mechanisms to Render Wearability

Abstract

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adma.201902133.Wearable electronics have revolutionized the way physiological parameters are sensed, detected, and monitored. In recent years, advances in flexible and stretchable hybrid electronics have created emergent properties that enhance the compliance of devices to our skin. With their unobtrusive attributes, skin conformable sensors enable applications toward real-time disease diagnosis and continuous healthcare monitoring. Herein, critical perspectives of flexible hybrid electronics toward the future of digital health monitoring are provided, emphasizing its role in physiological sensing. In particular, the strategies within the sensor composition to render flexibility and stretchability while maintaining excellent sensing performance are considered. Next, novel approaches to the functionalization of the sensor for physical or biochemical stimuli are extensively covered. Subsequently, wearable sensors measuring physical parameters such as strain, pressure, temperature, as well as biological changes in metabolites and electrolytes are reported. Finally, their implications toward early disease detection and monitoring are discussed, concluding with a future perspective into the challenges and opportunities in emerging wearable sensor designs for the next few years. Wearable SensorsRecent progress in flexible and stretchable electronic systems has ignited exciting applications in consumer electronics, human computer interactions, augmented reality devices, and electronic skins. [1][2][3] Among these, a significant interest lies in health monitoring, [4][5][6] where vital functions may be measured continuously. Continuous health monitoring is far more superior than conventional healthcare workflow as the conventional approach can only offer snapshots of the physiological condition Figure 3. Material substrates for flexible and stretchable electronics, highlighting the advantages of choices for each material type. a) Common polymeric materials by order of glass transition temperatures. Reproduced with permission. [29] Copyright 2015, Elsevier. b) Schematic representation of crosslinked elastomeric substrates possessing configuration entropy. Reproduced with permission. [22]

Recent Progress in Nanomaterial Enabled Chemical Sensors for Wearable Environmental Monitoring Applications

“…Usually, the substrate of a wearable sensor ascertains its overall physical characteristics that mostly depend on the composition of materials, i.e., their molecular structure and arrangement in bulk. [66,67] A wearable device demands a mechanically ductile substrate that can conform on uneven and soft surfaces. The modification in structure and arrangement at the molecular level by applying design approaches lead to empowering a substrate with improved flexibility and stretchability.…”

“…The modification in structure and arrangement at the molecular level by applying design approaches lead to empowering a substrate with improved flexibility and stretchability. [67] Polymeric substrates are the most conventional substrates utilized to fabricate the WCESs due to their flexibility, stretchability, low cost as well as roll to roll producibility. Several polymer films are used in recent WCESs including polyethylene terephthalate (PET), [23,[68][69][70][71][72][73][74][75][76][77][78][79] polyimide (PI), [80][81][82][83][84][85][86] polyurethane (PU), [22,87,88] polyethylene (PE), [89] and acrylate.…”

A review of flexible force sensors for human health monitoring

“…However, they are usually bulky, rigid, unwearable and unable to achieve multi-functionality and real-time detection. In recent years, the advancement of flexible electronic technology has promoted the development of regular and continuous healthcare monitoring [5] . As a human-interactive health monitoring device, flexible sensor (both wearable and implantable) can detect and measure various biological signals.…”

Identifying human body states by using a flexible integrated sensor

“…The identification of body states could help to remind a tired person to rest and avoid risky behavior, to develop fitness games and other entertainment programs, to assess exercise training intensity, and so on. Recent progress in flexible and wearable electronics has enhanced the possibility of continuous healthcare monitoring in daily life through the development of many kinds of wearable, conformable sensors [1][2][3][4][5][6][7][8][9][10] . These sensors mainly include flexible temperature sensors [11][12][13][14][15][16] , pressure/force sensors [17][18][19][20][21][22][23][24][25] , humidity sensors [26][27][28][29] , ultrasonic sensors 30,31 , optical sensors [32][33][34] , and biochemical sensors, but most are created to achieve noninvasive detection for a single vital indicator, such as body temperature (BT), heart rate (HR)/arterial pulse, blood pressure (BP), respiratory waves (RWs), or jugular venous pulse.…”