Electronic Skin Sensors

Introduction

Electronic skin sensors, also known as the wearable thin film sensors, can be directly placed on the human body to measure body parameters such as body temperature, heartbeat, sweat composition etc. Advances in materials science and electronics have enabled the development of electronic skin sensors which offer high resolution and fast response time while being flexible and stretchable. Electronic skin sensors have applications in many areas such as healthcare, sports, robotics and prosthetics, etc.

Sensors and transmitters are integrated into flexible and stretchable films, patches, bandages or tattoos which collect the data and feed them into machine-learning algorithms for monitoring vital parameters, patient abnormalities and also for tracking treatments. The doctors can monitor the efficacy of drugs as well as their patient’s safety and speed of recovery and from a remote location. This makes health care predictable, safe and efficient.

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Wound monitoring sensor:

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Interactive skin display with epidermal stimuli electrode:

The researchers at Yonsei university have developed a novel interactive skin display with epidermal stimuli electrode (ISDEE) allowing for the simultaneous sensing and display of multiple epidermal stimuli on a single device. The invention is based on a simple two-layer architecture on a topographically patterned elastomeric polymer composite with light-emitting inorganic phosphors, upon which two electrodes are placed with a certain parallel gap. The ISDEE is directly mounted on human skin, which by itself serves as a field-responsive floating electrode of the display operating under an alternating current (AC). The AC field exerted on the epidermal skin layer depends on the conductance of the skin, which can be modulated based on a variety of physiological skin factors, such as the temperature, sweat gland activity, and pressure [4].

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BodyNet sensor:

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Wearable Skin Sensor:

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Materials:

The materials used in stretchable skin sensors can be classified under two categories. One category includes intrinsically stretchable materials such as elastomers, liquid metals, and composite materials. The other category includes solid metals, semiconductors, polymers, and inorganic compounds [7].

Elastomers: Elastomers are mainly used as substrates, binders and adhesion layers. Polydimethylsiloxane (PDMS) elastomer is most commonly used.

Liquid metals: Metals such as eutectic gallium-indium (eGaIn) and gallium-indium-tin (Galinstan) are intrinsically elastic with low resistivity, low viscosity, and low toxicity. Various functional components such as pressure sensors, strain sensors, antennae, and soft wires are fabricated by injecting liquid metals into microfluidic channels.

Conductive Polymers: Intrinsically conductive polymer materials such as synthetic poly (acetylene) (PA), poly(pyrrole) (PPy), poly(thiophene) (PT), poly(aniline) (PANI), and poly-(3,4-ethylenedioxythiophene) (PEDOT) have maximum facture strain at the level of 1000%. The conductive polymer composites are composed of polymers and conductive fillers (e.g., metal nanoparticles, metal nanowires, graphite, carbon nanotubes, and graphene).

Solid Metals: Solid conductive metals are flexible when they appear as thin films. Dominant metals used in skin sensors include Au, Cu, Al, Cr, Ti and Pt, which are used as conductive interconnects, electrodes, sensors, contact pads and other circuit components.

Semiconductors: Inorganic semiconductor materials (e.g., silicon, GaAs, ZnO, InP, GaN) and organic semiconductor materials (e.g., poly(3-hexylthiophene) (P3HT), Poly(p-phenylene) vinylene, and Poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT)) are used for making various active components such as diodes, transistors, and light emitting diodes (LED).

Polymers: Polymers can be used as structural layers, electrical insulation layers, and dielectric layers in the skin sensors. Many polymers have been used to construct skin sensors, such as polyimide, poly(methyl methacrylate) (PMMA) and parylene. These polymers have high mechanical strength that makes them suitable as structural layers to support the skin sensors.

Companies working on electronic skin sensors

MC10:

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VivaLNK:

The company provides medical wearable solutions that capture and analyze human vitals and biometrics for continuous patient monitoring. Multi-Vital ECG patch, a product of VivaLNK is an ECG patch with a software development kit and weighing only 7.5 grams for continuous monitoring. The device enables easy integration with remote patient monitoring applications whether on location or in the cloud [9].

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Start-Ups:

There are a few interesting start-up companies that are working on electronic skin sensors, a few of which are listed below.

Xsensio SA: XSENSIO SA has developed next-generation wearable devices that track biochemical information at the surface of the skin, providing unprecedented real-time information about health and wellness, in a simple and non-invasive way [10].

Epicore Biosystems: Epicore Biosystems has created soft microfluidics devices that harvest and route sweat from skin pores. These novel biosensing devices monitor human physiology to optimize athletic performance and health [11].

Universities:

Interesting work in this area is progressing in universities as well.

Georgia Institute of Technology:

Scientists at Georgia Tech have developed wireless wearable device which can be worn on the body to measure the physiological signals such as electrocardiogram (ECG), heart rate, respiratory rate and motion activity. The device transmits the data to smartphone or tablet where patient data can be monitored. The ultrathin sensor can be worn for as long as two weeks, and it does not require gel to pick up signals from the skin as it directly conforms to the skin. As this device conforms to the skin it can obtain accurate signals even when a person is walking, running or climbing stairs [12].

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University of Tokyo:

Engineers at the University of Tokyo have developed an ultrathin, breathable, and stretchable electronic skin display. Electrocardiogram waveforms measured by skin sensors is displayed on skin display. It has 16 by 24 array of micro LEDs which can be deformed freely as they are made from thin and soft materials. Nanomesh electrodes present on electronic skin display pick up electrical signals from heart to monitor cardiovascular function and by using micro LEDs it displays waveform in real time. This data can also be transferred to smartphone wirelessly or can be stored in cloud [13].

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University of Houston:

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University of Texas at Austin

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Collaborations

LEO Science & Tech Hub, R&D innovation unit of LEO Pharma has partnered with Epicore Biosystems for the development of non-invasive, wearable sweat sensor for measuring prognostic biomarkers in real time by monitoring patient response as well asproviding treatment decisions [16].

Patents:

CN107778480A: Flexible electronic skin sensor and manufacturing method thereof

The invention discloses a flexible electronic skin sensor and manufacturing method, comprising of multiple pressure sensing units, each composed of two flexible and retractable ultra-thin PDMS films attached to the two ultra-thin PDMS films. It consists of two electrodes in between, and a composite pressure sensing film disposed between the two electrodes. The composite pressure sensing membrane is composed of several polyaniline hollow nanospheres and multiple polyaniline hollow nanospheres. The properties of the flexible electronic skin sensor can be used to monitor human signals such as checking for respiratory diseases, and performing speech recognition.

WO2018231444A2: Dual-mode epidermal cardiogram sensor