Chapter 9

Sensor Skins: An Overview
Jennifer Case, Michelle Yuen, Mohammed Mohammed
and Rebecca Kramer

Abstract Sensor skins can be broadly deﬁned as distributed sensors over a surface
to provide proprioceptive, tactile, and environmental feedback. This chapter focuseson sensors and sensor networks that can achieve strains on the same order as
elastomers and human skin, which makes these sensors compatible with emerging
wearable technologies. A combination of material choices, processing limitations,and design must be considered in order to achieve multimodal, biocompatible
sensor skins capable of operating on objects and bodies with complex geometries
and dynamic functionalities. This chapter overviews the commonly used materials,
fabrication techniques, structures and designs of stretchable sensor skins, and also
highlights the current challenges and future opportunities of such sensors.









Keywords Sensor skins
Wearables
Soft robotics
Stretchable sensors
Pressure sensors Liquid metals Sensor fabrication Flexible materials Ionic
liquids Conductive ink Conductive composites Microchannels



9.1













Introduction

With growing interest in soft systems and wearable technology comes a need todevelop deformable sensing components. Traditional sensors and electrical components are often rigid and are best suited to well-deﬁned systems that have discrete
motions and conﬁned trajectories. In contrast, soft structures generally have moredegrees of freedom than rigid systems. These degrees of freedom come from thedeformability of the soft structures themselves. State information of these soft
systems may be obtained by populating the surface of the structure with sensor
skins, which are stretchable planar structures with embedded sensing components.

J. Case  M. Yuen  M. Mohammed  R. Kramer (&)
Purdue University, West Lafayette, IN, USAe-mail: rebeccakramer@purdue.edu© Springer International Publishing Switzerland 2016
J.A. Rogers et al. (eds.), Stretchable Bioelectronics for Medical Devices
and Systems, Microsystems and Nanosystems,
DOI 10.1007/978-3-319-28694-5_9

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The design of these highly deformable sensory skins has been guided by the flexible
and stretchable characteristics of elastomers and human skin.
Let us deﬁne what we mean by sensor skins and wearable systems. There are anumber of different ways that we can limit our deﬁnition of a sensor skin byincluding stipulations like stretchability, placement on a flexible host, proprioceptive feedback, etc.; however, in this chapter, we will discuss sensor skins that aremechanically compatible with human skin, meaning that they can undergo at least
20 % strain. Wearable systems will be deﬁned here as systems designed to be wornby a human, but we will focus on wearable technology that targets efﬁcient interaction with the host. Other reviews on flexible sensors that do not match this criteria
are available for further reading [1–3].
Figure 9.1 shows examples of sensor skins used in different applications.
Figure 9.1a (http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=7352295) is a sensor skin module containing three resistance-based strain gauges capable of measuring
deformation. Multiple modules can be combined into an array to measure the state of
deformation of a host. Figure 9.1b shows an example of a tactile sensor skin used in asurgical environment. This device is composed of an array of pressure sensors toprobe the environment without damaging tissues during neuroendoscopy and gives
the surgeon another tool to help operate safely on a patient [4]. Two examples ofwearable sensor skins can be seen in Fig. 9.1c, d. These sensors can detect pose of thelower limbs (Fig. 9.1c) [5] and of the hand (Fig. 9.1d) [6].
In the following sections, we discuss the materials and processing approaches ofsubstrates and conductors, the structures and designs of elements, the systems usedin sensor skins, and conclude with potential future directions of sensor skins.

Fig. 9.1 Examples of sensors skins. a Sensor skin modules composed of silicone elastomer withthree embedded liquid metal sensing strain gauges each. b Sensor skin composed of liquid metalpressure sensors for detecting tissue damage [4]. c Wearable system with liquid metal strain
sensors for detecting pose [5]; and d wearable system with ionic liquid strain sensors for detecting
hand pose [6]. http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=7352295

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175

Materials and Processing

Sensor skins are generally composed of at least two types of materials: substrates
and conductors. These materials give the skins stretchability as well as the ability tointeract in some capacity with itself or a host. This section overviews common
substrates and conductors that have been used in sensor skin fabrication and theirprocessing techniques.

9.2.1

Substrate

A sensor skin is, in essence, a substrate onto which or into which sensors areintegrated. Substrates can be deﬁned in terms of attributes such as stretchability,
breathability, toughness, tear resistance, weight, and compatibility with the host or
existing manufacturing techniques. Due to the wide range of substrates available
and the complex interactions between the substrate and the host, the substrateshould be carefully chosen so that it matches the target properties of the sensor andthe fabrication process. Furthermore, in some wearable applications there is a need
to use two substrates: the sensor substrate and the garment that is meant to be worn.
The integration of two substrates into a single device adds another layer of complexity, since the compatibility between substrate materials must be considered inaddition to the substrate–host compatibility.
In the following subsections, we discuss two common types of substrate materials: elastomers and woven fabrics.

9.2.1.1

Elastomers

Elastomers are the most common substrates in soft sensor applications. They arecapable of supporting structures and encasing functional elements. In addition,
elastomers are compatible with other types of substrates, such as woven fabrics [7–9]. There are many commercially available low-cost elastomers with a wide rangeof stretchability (from 40 % [10] up to 700 % [11]). Because elastomers are highly
stretchable, they are conformable to human skin, which can strain up to 30 %, andthus are less likely to limit natural motion than nonstretchable substrates. However,this high flexibility and stretchability comes at a cost; elastomers are subject toviscoelastic behaviors, such as the Mullins effect [12], creep, and stress relaxation[13, 14].
Commercial elastomers usually are sold as two parts (free chains and crosslinker). Polymerization starts by mixing both parts at a speciﬁc ratio, and can betriggered by heating [10] or exposure to UV light [15]. Uncured elastomer can beeasily cast in a pre-made mold, which is convenient for creating speciﬁc geometries

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for different part functions and material properties. Researchers use different
techniques to fabricate molds such as lithography [16] and 3D printing [17].
It is possible to use methods to shape the elastomer other than replica molding,such as spin coating or other coating methods to make thin ﬁlms. For example,
elastomer ﬁlms cast onto polyethylene terephthalate (PET) sheets can be integrated
into roll-to-roll machines. Elastomers can be both physically and chemically alteredthrough processes such as laser ablation [18] and plasma treatment [19].
Elastomers were originally used to coat and support the structure of conductive
solid substrates [20–23]. However, the concept of building microchannels into
elastomers [24] allowed researchers to build sensor components by ﬁlling elastomeric microchannels with functional materials, such as liquid metals [4, 16, 18,
25–37] or ionic liquids [6, 38, 39].

9.2.1.2

Woven Fabrics

Woven fabric is most recognizable as the material of which our garments arecomprised. More generally, woven fabrics are composed of two sets of ﬁbers
interlaced together. This construction gives rise to the tensile strength and tearresistance of fabrics. Properties of the fabric such as the elasticity, stiffness,chemical resistance, and thermal properties can be tuned based on the choice of theconstituent ﬁbers and the pattern with which they are woven.
The ﬁrst attempts to integrate sensors and fabrics were by means of simple
attachment, such as sewing. This concept was improved upon by the invention of
conductive ﬁbers that can be woven the same way as conventional ﬁbers and act as
sensory elements without additional components [40]. Woven fabrics using conductive threads and ﬁbers have been employed as strain, pressure, respiratory, heartrate, and electrochemical sensors, as well as gesture-input devices [41–43].
Conductive threads woven with a known spacing can act as capacitive pressure
sensors by measuring the change in capacitance between ﬁbers due to thread
shifting from applied pressure. Conductive ﬁbers can act as resistive sensors when
they are woven as single or multiple threads that gain contact with each other andreduce resistance when strain or pressure is applied to them [43, 44].
Woven skin sensors can achieve higher strains by sewing the components onto
pre-wrinkled fabrics. This technique allows the devices to be stretched beyond thestretch limit of the fabric itself [45]. Alternately, conductive coatings can be applied
to the same pre-wrinkled construct to form stretchable electronics-compatible
fabrics [46, 47].

9.2.2

Conductor

A conductor is a material that allows the flow of electrons or ions through it. In
sensor skins, conductors have two major functions: conveying information (i.e. a9 Sensor Skins: An Overview

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trace on a circuit board) and collecting information (i.e. a sensor). In many cases,
conductors can be used for both purposes. The stability of the conductor and its
conformal contact with the interface are essential to ensure efﬁcient sensor performance. In wearable applications, it is also important to consider the biocompatibility (i.e. toxicity) of a sensor.
The conductors that we highlight in this section are thin metal ﬁlms, liquid
metals, ionic liquids, conductive polymers, and conductive inks.

9.2.2.1

Thin Metal Films

Conductive materials are, in most cases, rigid. They show flexible behavior when
they are shaped as thin ﬁlms. The ﬁrst attempt to use thin metal ﬁlms in flexible
electronics was in 1967 to produce the ﬁrst flexible solar cell [48, 49]. Advanced
electronics typically consist of insulators, conductors, and semiconductors, and it ispossible to use processes like roll-to-roll to produce electronics at larger scales [50].
Metallic thin ﬁlms have been fabricated to accommodate moderate strains using
clever geometries (waves and nets) made from both highly conductive metals (suchas gold, silver, copper, and aluminum) and semiconductors (such as silicon) [51].
Thin-ﬁlm electronics enable other applications such as displays [9, 52, 53], electrodes [54, 55], LEDs [56] and wearable electronics [57–59].

Fig. 9.2 a Extremely stretchable metal ﬁlms with serpentine-design bridges [51]. b Stretchable
circuit with noncoplanar metal ﬁlms on a model ﬁnger tip [63]. c A multifunctional epidermal
electronic circuit mounted on human skin in undeformed, compressed, and stretched states [69]

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The devices fabricated using thin metal ﬁlms are flexible. However, they lackstretchability due to the very low fracture strain of most metals. Researchers haveused geometry to cause this nonstretchable material to behave elastically. For
instance, curved and wavy metal ﬁlms can undergo strains that are impossible toachieve using flat ﬁlms [21–23, 60–64]. Figure 9.2a, b shows two examples of thinﬁlm structures. This approach enables stretchable interconnects [65, 66], integrated
circuits [67], batteries [68] and epidermal electronics (Fig. 9.2c) [69, 70]. With
proper engineering design, the devices fabricated using this technique can bestrained up to 200 % [71].
Devices fabricated using metal ﬁlms or wires deform plastically with prolonged
use leading to permanent deformation that reduces the device efﬁciency or totally
disconnects the circuit [72–75]. Other conductors, such as liquid metals and ionicliquids, have been recently used as alternatives that do not suffer from thislimitation.

9.2.2.2

Liquid Metal

There is a growing interest in using liquid metals in flexible electronics as alternatives to conventional metals. The most famous liquid metals are eutectic alloys
such as eutectic gallium-indium (eGaIn) and eutectic gallium-indium-tin(Galinstan). They have high metallic conductivity ( 3:4  106 X1  m1 for
EGaIn and  3:1  106 X1  m1 for Galinstan [76]) and form a thingallium-oxide skin that allows them to form stable nonspherical structures despite
their high surface tension [77, 78]. In contrast to mercury, gallium-indium alloys are
non-toxic and therefore have potential applications in biocompatible sensors.
Gallium-indium alloys can be injected into microchannels due to their low viscosity[16, 25, 26, 32, 37, 79], and their liquid nature allows them to take the shape of the
microchannel even at very high strains (up to 700 %) without failure [11].
Researchers have developed fabrication techniques that are more automated thanmanual injection [80]. For instance, a microtip wet with the liquid metal can transfer
patterns on a substrate by direct contact [81]. The tip can be replaced by a syringe
needle that continuously extrudes the liquid metal and directly writes onto a surface[28, 78]. Liquid metal can selectively wet parts of a substrate using a predesigned
mask [82, 83], or by treating the substrate surfaces such that liquid metal
self-assembles into the desired areas [33, 84–86]. Liquid metals are not suitable for
inkjet printing due to the high surface tension, its corrosive nature to most othermetals and the presence of surface oxide. However, a dispersion of liquid metal
nanoparticles in a volatile solvent can be inkjet printed since the dispersion properties are dictated by the carrier solvent rather than the liquid metal [87]. Finally,liquid metal ﬁlms maybe subtractively patterned by selective laser ablation [88].
The various patterning techniques have enabled the fabrication of different typesof liquid metal-based sensors, such as capacitive pressure sensors [35–37], resistive
strain sensors [18, 28, 33, 87], resistive pressure sensors [4, 26, 27, 31, 32, 79, 89],

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curvature sensors [16, 25] and shear sensors [89]. Examples of other liquid
metal-based devices are antennas [90–92], soft wires [6, 39], self-healing wires[93], diodes [94], and capacitors [81].

9.2.2.3

Ionic Liquids and Solutions

Ionic liquids and salt solutions can also be used in sensor skins. Ionic liquids aremolten salts while salt solutions are salts dissolved in a solvent, typically water.
These solutions are capable of reflowing and are typically used within an elastomer
substrate, where preformed microchannels are ﬁlled with the conductive solutions by injection [6, 29, 39] or vacuum [95].
The most familiar form of a salt solution is a sodium chloride (NaCl) solution,
which has been used to make sensors [95] and sensor arrays [96]. Researchers havealso demonstrated the use of potassium chloride solution (KCl), sodium hydroxide(NaOH), and hydrogen chloride (HCl) in their sensors, but found that both NaOH
and HCl were corrosive to the sensor interface and that NaCl had a better range thanKCl [95]. An example of an ionic liquid is 1-ethyl-3-methylimidazolium ethyl
sulfate [6, 29, 38]. Ionic liquids and salt solutions have been used in fabricatingtactile sensors [29, 95, 96], strain sensors [39], curvature sensors [38], diodes [94]and wearable devices [6].
The major drawback of using salt solutions is that popular substrates, likeelastomers, are gas permeable, which means that water will slowly evaporate andleave a salt residue in the substrate. Water evaporation can be slowed down byadding glycerol to the solution [39, 97].

9.2.2.4

Conductive Inks

A conductive ink is a solvent that contains a suspension of conductive particles,
such as metallic nanoparticles, organometallic compounds, carbon nanotubes andgraphene [98–100]. Volatile organic solvents (ethanol, toluene, etc.) are commonlyused and leave behind the conductive particles on the substrate as they evaporate.
Additives are often used to keep particles suspended, increase adhesion onto thesurface, or reduce surface tension. In some cases, a means of coalescing or sintering
is necessary to bridge gaps between nanoparticles and ensure conductivity [87, 101,
102].
The flow properties of the ink are dictated by the properties of the carrier solvent,therefore conductive inks have a lower viscosity relative to many of the previously
described liquid conductors. Hence, conductive inks are compatible with inkjet
printing [87, 103–105], screen-printing (polymer thick ﬁlm) [99, 106] anddirect-writing [101, 107, 108]. Researchers invented conductive silver inks that canbe directly written on different surfaces using rollerball pens [109, 110]. As previously mentioned, liquid metal dispersion inks have also been developed, which

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bridge the gap between liquid metal conductors and conductive inks using liquid
metal nanoparticles suspended in a carried solvent [87].

9.2.2.5

Conductive Polymer Composites

Conductive polymer composites generally consist of a polymer mixed with aconductive material that is packed tightly enough in the polymer to maintainconductivity. Example conductive materials include silver nanoparticles [111–113],
graphite [112, 114], graphene [115, 116], carbon black [114, 117], carbon nanotubes [114, 116, 118, 119] and liquid metals [120]. Different polymers can beused, such as polydimethylsiloxane [114, 116, 119, 120], polyisoprene [117],
polyvinylidene fluoride [112], rubber ﬁbers [113], and a number of other polymers[115]. Conductive polymers have been used to create tactile sensors [114, 117–119]
and strain sensors [47, 117, 118, 120].
It is possible to pattern the conductive polymer using different techniques suchas extrusion [119], screen printing [111], spray deposition [118] and hot-rolling[112]. The flow properties of the polymer composite affect the patterning process;therefore, it is common to add thinners to the composite in order to reduce its
viscosity. Examples of thinners are reverse micelle solution, which also controls thehardness of the ﬁnal conductive polymer [119], and cyclohexane [114]. Researchers
have developed a novel composite material by embedding liquid metal nanoparticles in elastomer. Initially, the composite is not conductive due to the absence of aconductive path between the nanoparticles; however, applying local pressure on thecomposite breaks the boundaries between the nanoparticles and creates a conductive path within the composite [120, 121].

9.3

Structures and Designs

It is important to choose the proper materials to fabricate the sensor to ensurestability of the structure and compatibility with the host under representative
operating conditions. Therefore, the designer should be aware of different structuresand design approaches in order to fabricate a properly functioning device. Thissection highlights the common features and systems of sensor skins.

9.3.1

Features

There are several features of sensor skins that affect how sensor elements function.
These features include microchannels containing liquid conductors (which we
discussed in Sect. 9.2.2) and interfaces within the sensor skins. Examples of thelatter include interfaces between two different conductors, between substrates,9 Sensor Skins: An Overview

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between the sensor skin and external electronics, and, in the case of wearables,
between the sensor skin and the human.

9.3.1.1

Microchannels

Microchannels are deﬁned as flow passages with dimensions on the order of tens tohundreds of microns [24, 122]. In sensor skins, these microchannels can be used forsensing or as communication pathways. Chossat et al. demonstrated both of theseuses in a wearable glove, where microchannels ﬁlled with an ionic liquid serve as
the sensing component and microchannels ﬁlled with liquid metal serve as acommunication pathway [6].
Elastomeric substrates deform under the influence of pressure or strain, thereforechanging the dimensions of the embedded microchannels ﬁlled with the conductive
liquid. This is important for resistive sensors, where the resistance of the channel isguided by R ¼ qL=A, where R is the resistance, q is the resistivity, L is the lengthand A is the cross-sectional area of the sensor. This is the operational conceptbehind the variety of liquid-embedded sensors and devices described in
Sects. 9.2.2.2 and 9.2.2.3.
Replica molding, subtractive and additive manufacturing are the common
techniques used to manufacture microchannels in elastomeric matrices. Molds canbe made via 3D printing [26, 27, 89], photolithography [123], patterning ﬁlms [16]or laser engraving [39]. Microchannels can also be made by subtractively removing
material via laser ablation [18] or by adding material via direct printing [28, 119].
The choice of the mold fabrication technique depends on the required resolution of
the mold, available time and equipment. For instance, fabricating molds using
photolithography produces small features with high resolution, but it is atime-consuming process [124, 125]. 3D printing is a fully automated process;however, the feature sizes of the mold are limited by the nozzle size of the printer.
Laser ablation is also capable of achieving small feature sizes, but has less control
over channel geometry than 3D printing [126].

9.3.1.2

Interfaces

At physical interfaces within devices, the change in stiffness from a highly
deformable substrate to a rigid component or interconnect is a common cause ofdevice failure. As the device is flexed or stretched, rigid parts are unable to follow
the change in conformation of more flexible parts. For example, in Fig. 9.1a, while
the silicone elastomer substrate is capable of withstanding strains up to 150 %, theelectrical interface between the liquid metal and the copper wire limits the usablestrain to 50 %. Beyond this limit, the copper wires lose contact with the liquid metaland fail to deliver strain data out of the sensor. This often also results in permanent
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out of the channels, thus breaking the path from the sensing element to the rest of
the system.
Researchers have addressed this deﬁciency in interfacing with liquid
metal-based sensors using a variety of methods. For instance, copper wires can be
replaced by ionic liquids to detect strain signals, which are transferred to external
control circuit using liquid metal wires [6, 29]. Other approaches include using
stretchable interconnects [127, 128], stretchable wires [11] and stretchable metalﬁlms [129, 130]. Uniaxially conductive polymer composites serve as a signaltransmitter between the liquid metal circuit and the skin [131].
In addition to interfaces within the device, the interface between the human andthe device greatly affects the device performance efﬁciency. Though elastomers arebiocompatible and useful for wearable electronics, in reality, elastomer devices are
difﬁcult to secure onto the skin. Researchers have used skin adhesives to adhere
electronic devices to the skin [61, 132]. With regard to garments, sensors have been
held in place around joints via straps to create sensory suits [5]. Researchers
ensured that the interface between the sensor and the strap was robust duringmotion by creating a stiffness gradient that transitions from the relatively stiff strap
to the much softer sensor.

9.3.2

Systems

Moving beyond our discussion of the individual components and fabrication
techniques for soft sensor skins, we can start looking at examples and applications.
In this section, we focus on sensor skins for robotics and wearables as they ﬁt withthe scope of this chapter.

9.3.2.1

Sensor Skins for Robotics

Most traditional robotic systems have very ﬁne tuned position control and canoperate very quickly and efﬁciently. However, they generally lack any knowledgeof their environment, which poses a potential safety risk for robots working
alongside humans. Applying sensor skins to robots would provide environmental
information and increase their awareness with their surroundings [133]. Tactile
sensors tend to either use capacitive [30, 35–37, 111, 118, 134, 135] or resistive [4,26, 31, 32, 38, 79, 117, 136] means of measuring pressure or normal force on thesurface. Work has also been done to sense shear forces as well as normal forces on asurface [89]. While a lot of these works are on single sensors, these sensors can bearrayed into a sensor skin.
Soft robots need proprioceptive feedback through soft sensors that aremechanically compatible with the bulk of their structure. This proprioceptive
feedback can come from strain sensors [6, 18, 26, 28, 34, 39, 47, 87, 97, 117, 120]or curvature sensors [16, 25, 137]. Yuen et al. demonstrated a robotic fabric skin

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which included a strain sensor that could differentiate between bending and compressing motion [138]. Resistive strain and pressure sensors have also been combined with existing pneumatic actuators to provide data about current state [139,140]. A modular capacitive sensor skin has been developed to provide tactile
information to existing robots [141].

9.3.2.2

Wearables

Many wearable sensor skin applications are designed for proprioception on conformal interfaces. Proprioceptive devices are used to estimate the state or pose ofpart of the human body. This can be applied on a smaller scale to measure the stateof various joints on the ﬁngers [16, 25] and across the entire hand and wrist [6,142]. On a larger scale, exosuits have been developed to determine the pose of thelower body [5, 8, 143]. These devices are all composed primarily of elastomers withembedded liquid conductors injected into molded microchannels within the elastomer [144, 145]. Alternate designs rely upon direct adhesion of the strain sensor to
the skin [61, 146]. These devices leverage thin ﬁlm mechanics to measure strain due
to skin stretch during joint flexion. The principles used to gather proprioceptive
information can also be used to develop user interface devices, such as a wearable
keypad [32, 119, 147].

9.4

Frontier and Outlook

The previous sections covered much of the published research in the ﬁeld ofstretchable skin sensors. Here, we discuss a few examples of the ongoing researchand look at future opportunities.
Current research focuses on using new materials and novel fabrication strategies
to develop sensors that can do multiple sensing tasks, have higher sensitivity andbetter mechanical properties, in addition to having a long lifetime. There is a needto develop methods to integrate these devices in a larger soft-bodied system, or todesign the sensor as a built-in part of it. Such improvements will have a tremendous
influence on the future sensor skins applications. An example of an integrated
system is the exosuit that we discussed in Sect. 9.3.2.2. In the current state, exosuits
require large power supplies or power cables, which is a signiﬁcant drawback thatneeds to be addressed in order to make these devices practical.
Biocompatible soft sensors have signiﬁcant potential in surgical robots.
Integrating soft sensors in surgical tools will not only allow sensing of the force
exerted on the surface but also determine the type of tissue onto which the force isbeing applied. Furthermore, the developing ﬁeld of soft robotics holds the promiseof creating new soft surgical tools that are mechanically compatible with soft tissue.

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Conclusion

This chapter gives an overview of the current research on sensor skins. Sensor skins
are sensor-embedded substrates that have the ability to flex, bend or stretch. Sensor
skins are used to estimate large-deformation motions and changes in system states.
There is a growing interest in applying sensor skins to human skin and tissues, sincemost of the materials used are biocompatible.
Sensor skins are generally made of two components: a conductive material
which is the sensing and/or signal transmitting element, and a stretchable encasing
substrate. The wide variety of materials that have been used in skin sensors provide
a diverse foundation for researchers to develop new devices, fabrication techniquesand designs. Within the current state-of-the-art, we are able to control the shape anddimensions of the devices and therefore the resulting sensing and mechanicalproperties. However, it is important to ensure both compatibility and stability of thesensor skin with the target host of the device in order to meet performance goals.
Much of the current research in this ﬁeld is aimed at integrating multiple sensing
elements together into complex sensor skins. This poses new challenges in signal
processing and networks that do not exist at the single element level. Major challenges that researchers are working to overcome include a lack of highly scalable
manufacturing techniques for soft materials and integration of miniaturized electronics. Together, solving these challenges will signiﬁcantly improve the utility of
sensor skins outside of a laboratory environment.

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