Emerging flexible and wearable physical sensing platforms for healthcare and biomedical applications

Abstract

There are now numerous emerging flexible and wearable sensing technologies that can perform a myriad of physical and physiological measurements. Rapid advances in developing and implementing such sensors in the last several years have demonstrated the growing significance and potential utility of this unique class of sensing platforms. Applications include wearable consumer electronics, soft robotics, medical prosthetics, electronic skin, and health monitoring. In this review, we provide a state-of-the-art overview of the emerging flexible and wearable sensing platforms for healthcare and biomedical applications. We first introduce the selection of flexible and stretchable materials and the fabrication of sensors based on these materials. We then compare the different solid-state and liquid-state physical sensing platforms and examine the mechanical deformation-based working mechanisms of these sensors. We also highlight some of the exciting applications of flexible and wearable physical sensors in emerging healthcare and biomedical applications, in particular for artificial electronic skins, physiological health monitoring and assessment, and therapeutic and drug delivery. Finally, we conclude this review by offering some insight into the challenges and opportunities facing this field.

Keywords: Electronic skins; Flexible sensors; Health monitoring; Liquid-state devices; Microfluidics; Tactile sensing.

Figure 1

Total number of publications and citations on the topic ‘Flexible and Wearable Sensors’ in the last 5 years. Total number of (a) publications and (b) citations on the topic of ‘Flexible and Wearable Sensors’ showing a progressive increasing trend from 2011 to 2015. Source: Web of Science, January 2016.

Figure 2

Emerging flexible and wearable physical sensing platforms for healthcare and biomedical applications. Flexible physical sensors comprise two distinct building blocks, that is, the flexible template materials and the active sensing elements, which may take either solid or liquid form. The fundamental sensing mechanism of the flexible and wearable sensors is based on the mechanical deformations experienced by the sensing devices, such as pressing, stretching, bending, and twisting. Emerging applications of flexible and wearable sensors in the healthcare and biomedical fields include artificial electronic skins, physiological monitoring and assessment systems, and therapeutic and drug delivery platforms.

Figure 3

Solid-state physical sensing platforms. (a) Stretchable strain sensors based on the nanohybrid assembly of SWCNTs and PEDOT:PSS. (i) Schematic cross-section illustration of the device comprising stacked layers of PU-PEDOT:PSS, SWCNT, and PEDOT:PSS on a PDMS elastomer. (ii) Scanning electron microscope (SEM) image showing the top view of the three-layer nanohybrid strain sensor. Adapted with permission from Ref. . Copyright 2015 American Chemical Society. (b) All-carbon multimodal piezocapacitive stretchable skin sensor. (i) Schematic illustration showing the active layers of the hierarchically engineered CNT microyarn-based sensor. (ii) SEM image of the surface of the CNT microyarns (left). Scale bar, 1 μm. Inset shows the hydrophobic nature of the surface. Cross-sectional SEM image of the CNT microyarn-incorporated layered structure (right). Scale bar, 50 μm. Adapted with permission from Ref. . Copyright 2015 Wiley-VCH Verlag GmbH & Co. (c) AuNW-coated tissue paper-based flexible pressure sensor. (i) Schematic illustration showing the fabrication process of the pressure sensor. (ii) Optical image showing the flexibility of the fabricated device. (iii) SEM image showing the surface morphology of the AuNW-coated tissue paper. Scale bar 100 μm. Adapted with permission from Ref. . Copyright 2014 Macmillan Publishers Limited. (d) Stretchable and breathable skin-inspired temperature sensor. (i) Device architecture of the temperature sensor. (ii) SEM image showing the cross-section of the semipermeable PU film (left), the surface of the semipermeable PU film (center), and the microstructure of the PU layer of the semipermeable membrane. Adapted with permission from Ref. . Copyright 2015 Macmillan Publishers Limited. AuNW, Au nanowire; CNT, carbon nanotubes; PU, polyurethane; SEM, scanning electron microscope; SWCNTs, single-walled CNTs.

Figure 4

Liquid-state physical sensing platforms. (a) Ionic liquid-based electrofluidic pressure sensor. Schematic illustration depicting the device architecture of the pressure sensor and optical image showing the as-fabricated device. The top electrofluidic circuit and the bottom microfluidic channel were filled with blue and red dyes, respectively. Adapted with permission from Ref. . Copyright 2011 The Royal Society of Chemistry. (b) Hybrid soft strain sensor. Optical image showing the as-fabricated sensor with its stretchability and bendability. Adapted with permission from Ref. . Copyright 2013 IEEE. (c) Metallic liquid-based microfluidic pressure sensor. Optical image showing the as-fabricated microfluidic pressure sensor with its channel features and dimensions. Adapted with permission from Ref. . Copyright 2015 MDPI AG. (d) Iontronic microdroplet array (IMA) flexible tactile sensor. Optical image illustrating the fully fabricated IMA tactile sensor array consisting of 12×12 elements. Adapted with permission from Ref. . Copyright 2014 The Royal Society of Chemistry. (e) Microfluidics-based three-dimensional tactile force sensor. Optical image showing the actual fabricated microfluidic tactile sensing devices for three-dimensional force measurements. Scale bar 2 mm. Inset shows the device architecture of the microfluidics-based three-dimensional tactile force sensor. Adapted with permission from Ref. . Copyright 2014 The Royal Society of Chemistry. (f) Liquid-state heterojunction sensor. Optical image depicting the actual fabricated liquid-state heterojunction sensor. Scale bar 2.5 mm. Adapted with permission from Ref. . Copyright 2014 Macmillan Publishers Limited. (g) Graphene oxide (GO) nanosuspension liquid-state microfluidic tactile sensing device. Optical image showing the fully fabricated liquid-state tactile sensor with its distinctive features. Adapted with permission from Ref. . Copyright 2016 Wiley-VCH Verlag GmbH & Co.

Figure 5

Deformation-based physical sensing mechanisms. (a) AuNW-coated tissue paper-based pressure sensor. Schematic illustration depicting the pressure-induced deformation-based working mechanism of the sensor. Adapted with permission from Ref. . Copyright 2014 Macmillan Publishers Limited. (b) Micropyramid-based stretchable resistive pressure sensor. Schematic illustration showing the circuit model describing the pressure-induced deformation-based sensing principle of the device and the finite element analysis illustrating the distribution of stress on the micropyramid-based electrode upon the application of external pressure. Adapted with permission from Ref. . Copyright 2014 Wiley-VCH Verlag GmbH & Co. (c) Iontronic microdroplet array (IMA) flexible tactile sensor. Schematic illustration showing the interfacial capacitive sensing principle of the IMA flexible tactile sensor. Adapted with permission from Ref. . Copyright 2014 The Royal Society of Chemistry. (d) CNT-based elastic strain sensor. Schematic illustration showing the operating principle of the CNT fiber-based strain sensor under different strain regimes. Adapted with permission from Ref. . Copyright 2015 American Chemical Society. (e) Skin-inspired interlocked microdome array-based tactile sensor. Schematic illustration showing the normal and shear force detection capability of the interlocked microdome arrays based on the distinct surface deformation of the microdomes upon the application of different forces. Adapted with permission from Ref. . Copyright 2015 American Chemical Society.

Figure 6

Flexible and stretchable physical sensing platforms for artificial electronic skins. (a) Stretchable e-skin configured from CNT-PDMS composite film patterned with interlocked microdome arrays. (i) Schematic illustration of the structure of human skin depicting the interlocked epidermal–dermal layers and the various skin mechanoreceptors. (ii) Schematic illustration of the design of the interlocked microdome arrays and the corresponding tilted and cross-sectional SEM images of the arrays of microdomes on a composite film. Scale bars, 5 μm. (iii) Schematic illustration showing the attachment of the stress-direction-sensitive e-skin on a human arm for the directional tactile sensing and differentiation of a range of mechanical stimuli, such as normal, shear, lateral stretch, and bending forces. (iv) Schematic illustration showing the configuration of 3×3 pixel e-skin arrays sandwiched between the cross-arrays of electrodes and PDMS layers for the three-axial directional sensing of mechanical stimuli. (v) Spatial distribution and directional mappings of external finger pushes applied on the e-skin. Adapted with permission from Ref. . Copyright 2015 American Chemical Society. (b) Strain-engineered artificial e-skin sensor arrays integrated with a fingerprint-like structure. (i) Schematic illustration showing an exploded view of the device configuration with the corresponding enlarged view of the fingerprint-like structure with its four strain and one temperature sensors. (ii) Optical image showing the actual fabricated e-skin device in a 3×3 array (top) and the corresponding enlarged image of the actual fabricated fingerprint-like structure (bottom). (iii) Optical image and schematic illustrations depicting the two-dimensional force and temperature mapping capability of the 3×3 array e-skin device in response to external stimuli, such as finger touch. Scale bars, 2 cm. Adapted with permission from Ref. . Copyright 2015 American Chemical Society. (c) Smart prosthetic e-skin sensor constructed from stretchable silicone nanoribbon (SiNR) electronics. (i) Optical image illustrating the smart artificial e-skin with its stretchable SiNR electronics laminated compliantly onto a prosthetic hand. Inset shows a 20% stretched e-skin. Scale bars, 1 cm. (ii) Schematic illustration depicting the exploded view of the device architecture of the smart e-skin. (iii) Optical images of the e-skin-laminated prosthetic hand tapping a keyboard and grasping a baseball and the corresponding temporal resistance changes of the artificial e-skin in response to different external stimuli as captured and monitored by the pressure sensor. Adapted with permission from Ref. . Copyright 2014 Macmillan Publishers Limited.

Figure 7

Flexible and stretchable physical sensing platforms for physiological monitoring and assessment. (a) Soft piezoelectric compliant modulus sensor (CMS) constructed from the flexible networks of mechanical sensors and actuators based on lead zirconate titanate nanoribbons. (i) Schematic illustration showing the exploded view of the device architecture: the top view of the device is shown in the lower-left inset, whereas the cross-sectional view of the device is depicted in the black-dashed region. (ii) Optical image showing the actual fabricated device on a thin silicone substrate. Scale bar 1 cm. Insets show the gold interconnection region (upper right, scale bar, 5 mm) and the arrays of sensors and actuators (lower-right, scale bar 1 mm) with the corresponding electrical circuit diagram (upper left). (iii) Optical images of a device conformed onto a cylindrical glass (left) and a device laminated partially (center) and fully (right) on the skin. Scale bars 1 cm. (iv) SEM image showing a CMS unit consisting of an array of six sensors and seven actuators on an artificial skin (scale bar 0.5 mm) with the corresponding magnified image of the red-dashed region depicting a sensor (left) and actuator (right) pair (scale bar, 100 μm). (b) Spatiodirectional mapping capability of the rotatable CMS unit and in vivo ‘on patient’ assessment. (i) Schematic illustration showing the exploded view of the rotatable CMS unit. (ii) Diagram illustrating the spatiodirectional mapping principle of the device, where R defines the protractor radius, and w defines the distance between the protractor center and the first sensor edge in the array that describes the mapping region. (iii) Optical image showing the forearm in the absence (top) and presence (bottom) of the mounted device. (iv) Mapping data corresponding to the assessment in (iii). (v) Optical image showing the lower leg in the absence (top) and presence (bottom) of the mounted device. (vi) Mapping data corresponding to the assessment in (v). Adapted with permission from Ref. . Copyright 2015 Macmillan Publishers Limited.

Figure 8

Flexible and stretchable physical sensing platforms for thermal therapy and drug delivery. (a) Stretchable and conformal mesh heating element for articular thermotherapy application. (i) Schematic illustration showing the fabrication process of the stretchable mesh heater, which comprised a heating layer of LE Ag NW/SBS elastomer composite and two encapsulation layers of SBS elastomers pressed together at high temperature. The colorized SEM image on the right shows the good interface between the three bonded mesh layers of SBS, LE Ag NW/SBS, and SBS. Scale bar, 50 μm. (ii) Optical image showing the large-area stretchable mesh heater (left). Optical image showing a wearable and portable heating system that integrated the stretchable mesh heater and a custom-made electronic band and the application of the integrated heating system on a wrist (center). Infrared camera images showing uniform heat distribution on the wrist (right). Adapted with permission from Ref. . Copyright 2015 American Chemical Society. (b) Soft elastic electronic dura mater or e-dura neural implants. (i) Optical image illustrating the fabricated e-dura implant and the accompanying SEM images of the stretchable gold interconnects and platinum–silicone composite-coated soft electrodes. (ii) Implantation of the e-dura between the motor cortex tissues and the dura mater for 6 weeks (left) and the reconstructed spinal cord activation map in response to electrical stimulation of the left sciatic nerve based on the recorded electrospinograms (right). (iii) Spinal cord injured rats with implanted spinal e-dura over the lumbosacral sections. (iv) Recording of the bipedal locomotion of the rat under support after 3 weeks of rehabilitation in the absence and presence of electrochemical stimulation and corresponding stick diagram decompositions of the hindlimb movements and oscillations and the leg muscle activities. Adapted with permission from Ref. . Copyright 2015 American Association for the Advancement of Science. (c) Wearable tensile strain-triggered drug delivery system. (i) Schematic illustration showing the two distinct components and the working mechanism of a strain-triggered drug delivery system in which deformation of the stretchable elastomer promoted drug release from the microdepot. (ii) Schematic illustration showing the encapsulation of the drug-loaded nanoparticles within the microdepot and the passive release and partial retention of the drug-filled nanoparticles within the microdepot matrices. (iii) Conformal attachment of the wearable drug delivery system onto the index finger where drug release to the skin could be simply triggered by the finger flexion. (iv) Integration of the wearable strain-responsive drug delivery system with a microneedle array patch for the transcutaneous administration of drugs. Adapted with permission from Ref. . Copyright 2015 American Chemical Society.