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.
Scientists at University of Toronto have designed a super-stretchy, transparent and self-powering sensor that records the complex sensations of the human skin. It has been named as “Artificial Ionic Skin (AI skin)”. The AI skin is made of two oppositely charged sheets of stretchable substances known as hydrogels. By superimposing negative and positive ions, the researchers have created a “sensing junction” on gel’s surface. The new AI skin could open doors to skin like fitness products such as fitbits that measure multiple body parameters .
Wound monitoring sensor is a skin-inspired, open-mesh electromechanical sensor for real time wound monitoring. This wearable sensor with gold sensor cables having structure similar to that of skin, has been developed by researchers at Binghamton University. The sensor works on the principle of biosensor by measuring important biomarkers such as lactate and oxygen levels in the skin. Such sensors can provide a better understanding of disease progression, wound care, general health, fitness monitoring and more .
The device provides a precise pH reading within 15 minutes by collecting small amount of sweat from skin pores. The pH of normal healthy skin ranges between 4.4 and 5.5. It alters due to environmental factors and underlying conditions which cause skin dryness, eczema, and atopic dermatitis. The wearable microfluidic sensor received CES 2019 Innovation award, in the category of Wearable Technology Products. L’Oréal is the first company to unveil the wearable device for measurement of skin pH levels using microfluidic technology. By knowing their skin type, people can choose products which are suitable to them. My skin pH track app uses advanced algorithms and reads the pH value and local sweat loss of wearer. It calculates the rate of perspiration on the skin’s surface and evaluates skin health thereby recommending the product for balancing pH and skin care .
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 .
Scientists from Stanford University have designed a skin-worn sensor known as BodyNet sensor which measures the pulse and rate of respiration through detection of the expansion and contraction of the skin. When applied on elbows, this sensor can be used to track the movement of the body parts and knees by measuring the tightening or relaxation of the skin every time the corresponding muscle flexed. This sensor is made of a clear, stretchable, non-allergenic elastomer, with screen-printed metallic-ink sensing electronics and a flexible radio-frequency identification (RFID) antenna .
In conditions such as Hydrocephalus, where excess amount of cerebrospinal fluid (CSF) accumulates within the brain, a tube known as shunt is surgically implanted in to the brain to drain excess fluid to another part of the body. Most of the shunts fail and diagnosing shunt failure is difficult as it may require CT, MRI scan and sometimes even surgery. Researchers at Northwestern University have developed non-invasive, flexible and wearable shunt monitor which monitors the working of shunt i.e., passage of fluid through the shunt. It uses temperature and heat transfer measurements and non-invasively shows the amount of fluid flowing through shunt. This device can communicate with smartphone through bluetooth and can transmit the readings by an Android app .
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 .
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.
Mc10’s products are thin and flexible, stretchable, which can bend and twist easily with our bodies. Kintinuum, a product of MC10, is a wearable sensor conforming to the human body for maximum patient comfort. MC10 software consists of mobile interfaces, cloud storage, and uses big data analytics and machine learning tools to translate the data into language which humans can understand. By using a combination of data collected from the sensors and patient-reported data, the Kintinuum system offers a new way to quantify treatment efficacy .
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 .
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 .
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 .
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 .
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 .
Researchers at University of Houston have developed ultrathin, mechanically imperceptible, and stretchable (human-machine interface) HMI device. The device can be used as a prosthetic skin for a robotic hand or other robotic devices. It is worn on human skin to collect multiple physical data. It is a sol-gel-on-polymer-processed indium zinc oxide semiconductor nanomembrane electronics. Human-machine interface allows automatic collection of information which is relayed back to the wearer .
Researchers at university of Texas have developed a new electronic tattoo (e-tattoo), graphene-based wearable device that can be placed on the skin to measure a variety of body responses, from electrical to biomechanical signals. The device is so lightweight and stretchable that it can be placed over the heart for extended periods with little or no discomfort. By taking electrocardiograph and seismocardiograph readings simultaneously, it measures cardiac health in two ways. The smartphone app stores the data and shows heart beat in real time .
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 .
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.
CN107727723A: Ultrathin flexible glucose measuring sensor similar to skin, and preparation method thereof
The invention discloses an ultrathin flexible glucose measuring sensor similar to skin, and a preparation method thereof. The invention relates to a nano-scale counter electrode and a working electrode pattern on an ultra-thin flexible insulating support layer and an adhesive layer, forming a Prussian blue film on the pattern of the working electrode. This forms a glucose oxidase-immobilized on the Prussian blue film. Chitosan film is affixed to the skin surface. Glucose generates hydrogen peroxide under the action of glucose oxidase. The sensor has the characteristics of ultra-thin and flexible, and can be attached to any position on the surface of human skin, without affecting the movement of the human body and normal life, non-invasively and accurately measuring the concentration of glucose existing on the surface of the skin to manage diabetes.
WO2018231444A2: Dual-mode epidermal cardiogram sensor
The invention discloses a dual-mode epidermal sensor/electrode that, which when worn on a human chest, is capable of synchronously monitoring electrical activity and mechano-acoustic activity of a cardiovascular system. The dual-mode epidermal sensor/electrode consists of a pair of stretchable electrocardiogram (ECG) electrodes made out of filamentary serpentine gold nano-membranes and a stretchable seismocardiogram (SCG) sensor comprising a filamentary serpentine PVDF. The dual-mode epidermal sensor/electrode is light, thin, flexible, and requires no operational power. The sensor can be laminated conformably and unobtrusively on a human chest to provide high fidelity ECG measurements and SCG measurements, and an estimated beat-to-beat blood pressure (BP).
Electronic skin sensors segment seems to have gained considerable significance in the market and it is estimated to grow at a greater space. However, many challenges need to be overcome for a wider acceptance of electronic skin sensors. Innovations in materials, cost-effective, devices and circuit designs will help make soft biosensors even smaller, thinner, flexible and stretchable, lighter and less power-hungry. The accuracy, precision and range of measurements also need to improve. Validation, regulation and data protection will become more stringent. Close collaborations between material scientists and device engineers, data scientists and medical professionals will help in faster development. In future, electronic skin sensors have the potential to transform every aspect of medicine.