Current Issue

Substrates for flexible devices are usually fabricated using stretchable polymeric matrices, such as polydimethylsiloxane, poly(vinylpyrrolidone), Ecoflex, polyethylene terephthalate, and polyimide (PI) [4,11]. These materials are commonly used as dielectric layers in capacitive sensors. Moreover, intrinsically conductive polymers, including poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT: PSS) [12,25], diketopyrrolopyrrole [13], and polyaniline [14], are used as active layers in various biosensors.

Hydrogels are composed of a three-dimensional network composed of a hydrophilic polymer that provides a device interface with enhanced biocompatibility [6]. It offers effective mechanical, electrical, optical, thermal, and chemical coupling between biological tissues and the device [15]. The electrical properties of functional hydrogel interfaces can be modified by mixing conductive additives. Another unique feature that upholds hydrogel applications is its self-healing capability. However, the multifaceted advantages of hydrogels have trade-offs that must be compensated for. For instance, physical crosslinking can enable good self-healing properties but may result in low stiffness. Nonetheless, applications that require direct contact with biological tissues, such as chronic wound monitoring devices may benefit from the biocompatibility of functional hydrogels [34].

Liquid metals and ionic liquids are highly conductive materials with controllable fluid properties. Inherent material compatibility issues between substrates and sensing materials in solid-state devices can be solved by integrating liquid-state conductors such as Galinstan (GaInSn) [16,17] and ethylene glycol/sodium chloride [18]. The integration of these liquid materials with precise nanoprinting techniques has enabled the fabrication of physical sensors based on conductive liquids.

Owing to their superior physical and chemical properties and versatile features, functional nanomaterials have been widely applied to enhance the sensor performance in various ways. Advancements in the synthesis techniques for distinctive morphologies (nanoparticles, nanowires, and thin films) have made this integration feasible. Various micro/nanomaterials such as carbon nanoparticles, silver nanoparticles, carbon nanotubes (CNT), silver nanowires, gold nanowires, and graphene nanosheets have been used to enhance sensor performance factors, such as the sensitivity, response time, and linearity of sensor signals [9].

Physical sensors attached on the skin surface focus on changes in physical stimuli such as pressure, strain, and temperature. Through this information, arterial pulse rate, blood pressure, body temperature, human motion, and tactile feedback can be analyzed. [4,5,8,9]

3.1.1 Pressure & Strain

A key function of skin-interface electronics is the measurement of pressure and strain. Stretchable physical sensors have achieved significant performance quality with various sensing mechanisms, such as piezoresistive [20,21], capacitive [16, 22-23], piezoelectric [43, 46], and triboelectric measurements [44-46]. Among these types of sensors, piezoresistive and capacitive methods are favored because of the simplicity of their design and versatility in acquiring both static and dynamic data [6].

In piezoresistive pressure sensors, the resistance of the conductive active layer depends on its geometry, formation, and composition. The applied strain can be measured using an active transducer by measuring the resistance variation under loading. [8,9] Piezoresistive devices have multifunctional applications across a wide range of pressures and strains with good sensor characteristics, including high linearity, sensitivity, reliability, and fast response time. The integration of nanomaterials (graphene, carbon black, CNT, nanowires, nanoparticles, ionic liquids, and liquid metals) into the active layer and micro-engineered geometrical designs (porous, microcrack, auxetic) [4,5,8,9] have enhanced the piezoresistive sensor performance. Fig. 1a illustrates a CNT-coated porous elastomer-based flexible pressure sensor with an ultrawide pressure sensing range (10 Pa–1.2 MPa), high sensitivity (0.01 – 0.02 kPa_-1), and good linearity (R2~0.98) [20]. Next, Fig. 1b portrays a battery-free wireless pressure and temperature sensing platform that incorporates a near-field communication system-on-a-chip to monitor immobilized patients for their risk of developing ulcers [21].

Capacitive sensors measure the change in capacitance between two electrodes, as physical deformation affects the geometries and dielectric layer properties of the sensor. This simplistic mechanism offers high sensitivity and reliability, while requiring low power to measure strain and pressure. To improve sensor performance, micropatterned structures (pores, pyramids, domes, ridges, papillae, spheres, and cylinders) were implemented on the dielectric elastomer layer [4,5,24]. These designs allow enhanced sensitivity and response time, while reducing viscoelastic deformation and hysteresis effects. More importantly, through dielectric layer design parameterization, sensor features can be customized according to the necessary applications. The micro-pyramidal capacitive sensor shown in Fig. 1c demonstrates a parameterized design for tunable capacitive sensors [24]. In Fig. 1d, another application of the porous Ecoflex-CNT composite is shown comprising a capacitive-type sensor with an enhanced pressure sensitivity due to the synergy of the porous structure and percolation network of the CNTs [22]. However, one critical drawback of using flexible elastomer-based dielectric materials is the cumulative increase in the elastic modulus during compression. This phenomenon drastically reduces the sensitivity and accuracy of pressure measurements in highly compressed regions.

Fig. 1. 
Piezoresistive and capacitive physical sensors (a) Microporous carbon nanotubes (CNT) network-coated elastomer sponge based piezoresistive pressure sensor Ref. [20] Copyright (2019) American Chemical Society. (b) Battery-free wireless pressure and temperature sensing platform to monitor patients at risk for ulcers Ref. [21] Copyright (2021) Nature Research. (c) Micro-pyramidal capacitive sensor with tunable parameter Ref. [24] Copyright (2020) John Wiley and Sons. (d) Porous Ecoflex-CNT composite based capacitive pressure sensor Ref. [22] Copyright (2020) American Chemical Society.

Biomarker monitoring through the analysis of sweat, saliva, tears, interstitial fluids, and other body fluid samples provides another valuable dimension for wearable biosensors [5,7,10,11]. By applying various electrochemical analysis methods (ion-selective potentiometry, voltammetry, amperometry, bioaffinity sensing, etc.), noninvasive bioanalytical practices can be implemented [31]. The design of microfluidic structures and multisite sensor elements must be fully integrated to maintain control over the biofluid absorption and excretion. Hence, chemical biosensors require precise fabrication to ensure sensor stability. Sweat is a highly favored secreted fluid for electrochemical analysis because it is relatively hygienic, contains numerous analytes, and has easy acquisition methods [32, 33]. Fig. 2a illustrates Javey et al. ’s work [32] that incorporated hydrophilic fillers and microchannels within an enclosed wearable patch for the rapid uptake of sweat and prevention of leakage via evaporation. The microchannels are intertwined in a spiral shape and attached to electrodes that act as impedance-based sensors to measure pH, Cl-, and levodopa [32]. They also suggested a sweat and urine sample-based nutrition tracker by measuring the amount of Vitamin C and comparing it with that in blood samples [33].

Fig. 2. 
Chemical biosensors (a) Microchannels within an enclosed wearable patch for sweat uptake Ref. [32] Copyright (2021) Nature Research. (b) Soft bioaffinity sensor array for chronic wound monitoring Ref. [34] Copyright (2021) Cell Press. (c) Telemedicine platform for COVID-19 diagnosis Ref. [2] Copyright (2020) American Chemical Society. (d) Patch-based glucose monitoring wearable device Ref. [35] Copyright (2017) American Association for the Advancement of Science.

Fig. 2b shows a soft bioaffinity sensor array for chronic wound monitoring developed by Gao et al [34]. According to their study, diabetic patients face chronic problems with open wounds and the detection of inflammatory and physiological biomarkers can help treat and manage wounds [34]. Another publication suggested modes for telemedicine platforms for COVID-19 diagnosis and monitoring, as shown in Fig. 2c [2,3]. Their multisite laser-engraved graphene platform provides a portable, wireless electrochemical sensor for the rapid detection of COVID-19 in blood and saliva samples. As shown in Fig. 2d, a patch-based glucose monitoring wearable device with microneedles has been introduced for patients with diabetes. This device not only measures glucose levels but also provides feedback for drug delivery mechanisms based on the patient’s condition [35].

These studies validated the positive impacts of developing at-home personal health monitoring devices; however, critical reliability issues exist that hinder their commercial use. For instance, microfluidic channel control is susceptible to clogging problems owing to the deposition of particles dissolved in biofluids and external disruptions. Moreover, issues related to functional reliability, calibration, electrochemical selectivity, and system optimization require breakthrough solutions. Finally, the physiological data extracted from these sensors must be validated by medical experts to draw accurate conclusions.

Optoelectronic sensors use diverse optical properties to measure both biophysical and biochemical entities using photodetectors [37]. Optical sensing mechanisms mainly detect variations in phototransmittance and backscattering to process these signals into physiological information. Conventional wearable devices incorporate photoplethysmography (PPG)-based pulse oximetry to monitor the heart rate and blood oxygenation of the user, as demonstrated in Fig. 3a. [38].

Fig. 3. 
Optoelectronic biosensors (a) PPG based pulse oximetry device to monitor heart rate and blood oxygenation Ref. [38] Copyright (2017) John Wiley and Sons. (b) Piezo-transmittance based self-powered pressure sensor Ref. [40] Copyright (2020) Elsevier. (c) Auxetic metamaterial piezo-transmittance strain sensor Ref. [41] Copyright (2021) Elsevier.

Optical sensors can also be used to measure mechanical strain and pressure by using a material structure that is designed to alter optical transmittance with applied force. For example, a CNT-embedded elastomer with microcracks could detect subtle human motions using optical transmittance measurements [39]. Fig. 3b shows the application of a wearable self-powered pressure sensor that integrates a piezo-transmittance microporous elastomer with an organic solar cell [40]. The initial porous structure causes refraction of light rays, preventing the solar cell from absorbing ambient light. However, the application of pressure compresses the air gaps thereby allowing the light transmittance to increase the current output. The applied pressure was detected by monitoring the current output from the solar cell. Finally, Fig. 3c illustrates a novel implementation of auxetic metamaterials in a piezo-transmittance-based strain sensor [41]. The auxetic structure amplified the gap opening caused by strain, enhancing the sensor’s performance up to a gauge factor of 10.5.

The development of inorganic μLEDs [36], quantum dots, metamaterials, and various nanostructures has enhanced the potential of optoelectronic sensors and their feasible applications because of their fast response, sensitivity, and simplicity. However, it is important to consider the effects of undesired light sources in optoelectronic devices.