Stretchable Tattoo-Like Heater with On-Site Temperature Feedback Control

Smart Tattoo Sensors 2.0: A Ten-Year Progress Report through a Narrative Review

The increased interest in sensing tattoos reflects a shift in wearable technology, emphasizing their flexible, skin-adherent nature. These devices, driven by advancements in nanotechnology and materials science, offer highly sensitive and customizable sensors. The growing body of research in this area indicates a rising curiosity in their design and applications, with potential uses ranging from vital sign monitoring to biomarker detection. Sensing tattoos present a promising avenue in wearable healthcare technology, attracting attention from researchers, clinicians, and technology enthusiasts. The objective of this study is to analyze the development, application, and integration of the sensing tattoos in the health domain. A review was conducted on PubMed and Scopus, applying a standard checklist and a qualification process. The outcome reported 37 studies. Sensing tattoos hold transformative potential in health monitoring and physiological sensing, driven by their focus on affordability, user-friendly design, and versatile sensorization solutions. Despite their promise, ongoing refinement is essential, addressing limitations in adhesion, signal quality, biocompatibility, and regulatory complexities. Identified opportunities, including non-invasive health monitoring, multiplexed detection, and cost-effective fabrication methods, open avenues for personalized healthcare applications. However, bridging gaps in medical device standards, cybersecurity, and regulatory compliance is imperative for seamless integration. A key theme calls for a holistic, user-centric approach, emphasizing interdisciplinary collaboration. Balancing innovation with practicality, prioritizing ethics, and fostering collaboration are crucial for the evolution of these technologies. The dynamic state of the field is evident, with active exploration of new frontiers. This overview also provides a roadmap, urging scholars, industry players, and regulators to collectively contribute to the responsible integration of sensing tattoos into daily life.

E-Tattoos: Toward Functional but Imperceptible Interfacing with Human Skin

The human body continuously emits physiological and psychological information from head to toe. Wearable electronics capable of noninvasively and accurately digitizing this information without compromising user comfort or mobility have the potential to revolutionize telemedicine, mobile health, and both human-machine or human-metaverse interactions. However, state-of-the-art wearable electronics face limitations regarding wearability and functionality due to the mechanical incompatibility between conventional rigid, planar electronics and soft, curvy human skin surfaces. E-Tattoos, a unique type of wearable electronics, are defined by their ultrathin and skin-soft characteristics, which enable noninvasive and comfortable lamination on human skin surfaces without causing obstruction or even mechanical perception. This review article offers an exhaustive exploration of e-tattoos, accounting for their materials, structures, manufacturing processes, properties, functionalities, applications, and remaining challenges. We begin by summarizing the properties of human skin and their effects on signal transmission across the e-tattoo-skin interface. Following this is a discussion of the materials, structural designs, manufacturing, and skin attachment processes of e-tattoos. We classify e-tattoo functionalities into electrical, mechanical, optical, thermal, and chemical sensing, as well as wound healing and other treatments. After discussing energy harvesting and storage capabilities, we outline strategies for the system integration of wireless e-tattoos. In the end, we offer personal perspectives on the remaining challenges and future opportunities in the field.

Pattern design of a liquid metal-based wearable heater for constant heat generation under biaxial strain

As the wearable heater is increasingly popular due to its versatile applications, there is a growing need to improve the tensile stability of the wearable heater. However, maintaining the stability and precise control of heating in resistive heaters for wearable electronics remains challenging due to multiaxial dynamic deformation with human motion. Here, we propose a pattern study for a circuit control system without complex structure or deep learning of the liquid metal (LM)-based wearable heater. The LM direct ink writing (DIW) method was used to fabricate the wearable heaters in various designs. Through the study about the pattern, the significance of input power per unit area for steady average temperature with tension was proven, and the directionality of the pattern was shown to be a factor that makes feedback control difficult due to the difference in resistance change according to strain direction. For this issue, a wearable heater with the same minimal resistance change regardless of the tension direction was developed using Peano curves and sinuous pattern structure. Lastly, by attaching to a human body model, the wearable heater with the circuit control system shows stable heating (52.64°C, with a standard deviation of 0.91°C) in actual motion.

Advanced thermal sensing techniques for characterizing the physical properties of skin

Measurements of the thermal properties of the skin can serve as the basis for a noninvasive, quantitative characterization of dermatological health and physiological status. Applications range from the detection of subtle spatiotemporal changes in skin temperature associated with thermoregulatory processes, to the evaluation of depth-dependent compositional properties and hydration levels, to the assessment of various features of microvascular/macrovascular blood flow. Examples of recent advances for performing such measurements include thin, skin-interfaced systems that enable continuous, real-time monitoring of the intrinsic thermal properties of the skin beyond its superficial layers, with a path to reliable, inexpensive instruments that offer potential for widespread use as diagnostic tools in clinical settings or in the home. This paper reviews the foundational aspects of the latest thermal sensing techniques with applicability to the skin, summarizes the various devices that exploit these concepts, and provides an overview of specific areas of application in the context of skin health. A concluding section presents an outlook on the challenges and prospects for research in this field.

An Analytic Model of Transient Heat Conduction for Bi-Layered Flexible Electronic Heaters by Symplectic Superposition

In a flexible electronic heater (FEH), periodic metal wires are often encapsulated into the soft elastic substrate as heat sources. It is of great significance to develop analytic models on transient heat conduction of such an FEH in order to provide a rapid analysis and preliminary designs based on a rapid parameter analysis. In this study, an analytic model of transient heat conduction for bi-layered FEHs is proposed, which is solved by a novel symplectic superposition method (SSM). In the Laplace transform domain, the Hamiltonian system-based governing equation for transient heat conduction is introduced, and the mathematical techniques incorporating the separation of variables and symplectic eigen expansion are manipulated to yield the temperature solutions of two subproblems, which is followed by superposition for the temperature solution of the general problem. The Laplace inversion gives the eventual temperature solution in the time domain. Comprehensive time-dependent temperatures by the SSM are presented in tables and figures for benchmark use, which agree well with their counterparts by the finite element method. A parameter analysis on the influence of the thermal conductivity ratio is also studied. The exceptional merit of the SSM is on a direct rigorous derivation without any assumption/predetermination of solution forms, and thus, the method may be extended to more heat conduction problems of FEHs with more complex structures.

Fabrication, characterization and applications of graphene electronic tattoos

Numerous fields of science and technology, including healthcare, robotics and bioelectronics, have begun to switch their research direction from developing 'high-end, high-cost' tools towards 'high-end, low-cost' solutions. Graphene electronic tattoos (GETs), whose fabrication protocol is discussed in this work, are ideal building blocks of future wearable technology due to their outstanding electromechanical properties. The GETs are composed of high-quality, large-scale graphene that is transferred onto tattoo paper, resulting in an electronic device that is applied onto skin like a temporary tattoo. Here, we provide a comprehensive GET fabrication protocol, starting from graphene growth and ending with integration onto human skin. The methodology presented is unique since it utilizes high-quality electronic-grade graphene, while the processing is done by using low-cost and off-the-shelf methods, such as a mechanical cutter plotter. The GETs can be either used in combination with advanced scientific equipment to perform precision experiments, or with low-cost electrophysiology boards, to conduct similar operations from home. In this protocol, we showcase how GETs can be applied onto the human body and how they can be used to obtain a variety of biopotentials, including electroencephalogram (brain waves), electrocardiogram (heart activity), electromyogram (muscle activity), as well as monitoring of body temperature and hydration. With graphene available from commercial sources, the whole protocol consumes ~3 h of labor and does not require highly trained personnel. The protocol described in this work can be readily replicated in simple laboratories, including high school facilities.

Sustainably Powered, Multifunctional Flexible Feedback Implant by the Bifacial Design and Si Photovoltaics

Advanced design and integration of functioning devices with secured power is of interest for many applications that require complicated, sophisticated, or multifunctional processes in confined environments such as in human bodies. Here, strategies for design and realization are introduced for multifunctional feedback implants with the bifacial design and silicon (Si) photovoltaics in flexible forms. The approaches provide efficient design spaces for flexible Si photovoltaics facing up for sustainable powering and multiple electronic components for feedback functions facing down for sensing, processing, and stimulating in human bodies. The computational and experimental results including in vivo assessments ensure feasibility of the approaches by demonstrating feedback multifunctions, power-harvesting in milliwatts, and mechanical compatibility for operations in live tissues. This work should useful for wide range of applications that require sustainable power and advanced multifunctions.

Integration of Transparent Supercapacitors and Electrodes Using Nanostructured Metallic Glass Films for Wirelessly Rechargeable, Skin Heat Patches

Here we demonstrate an unconventional fabrication of highly transparent supercapacitors and electrodes using random networks of nanostructured metallic glass nanotroughs for their integrations as wirelessly rechargeable and invisible, skin heat patches. Transparent supercapacitors with fine conductive patterns were printed using an electrohydrodynamic jet-printing. Also, transparent and stretchable electrodes, for wireless antennas, heaters and interconnects, were formed using random network based on nanostructured CuZr nanotroughs and Ag nanowires with superb optoelectronic properties (sheet resistance of 3.0 Ω/sq at transmittance of 91.1%). Their full integrations, as an invisible heat patch on skin, enabled the wireless recharge of supercapacitors and the functions of heaters for thermal therapy of skin tissue. The demonstration of this transparent thermotherapy patch to control the blood perfusion level and hydration rate of skin suggests a promising strategy toward next-generation wearable electronics.

On-Demand Printing of Wearable Thermotherapy Pad

Thermotherapy is an effective method for pain relief, recovery from injury, and general healthcare. The ordinary heat pad used for thermotherapy at home is not usually tailored to the individual but supplied in a few different pre-fixed sizes and shapes for mass marketing. A customized wearable heat pad often requires expert support. Herein, an instant, custom-fit, and on-demand heat pad for thermotherapy is demonstrated. The heater is directly printed using silver nanoparticle ink on an off-the-shelf medical grade tape by inkjet technology. By coating the tape with silica nanoparticles as ink-absorbing layer and chloride ions as chemical sintering agent, stable heater patterns are printed without the need for subsequent high temperature sintering process. A 3D scanner is used to acquire body information, and a customized heater is produced using the information. The printed heat pad is attached to the shoulder and the effect of thermotherapy is verified objectively through electroencephalography and subjectively through survey. This printed heat pad produced by simple and low-cost fabrication provides wearable medical devices for personal thermotherapy.

Recent Developments in Printing Flexible and Wearable Sensing Electronics for Healthcare Applications

Wearable biosensors attract significant interest for their capabilities in real-time monitoring of wearers' health status, as well as the surrounding environment. Sensor patches are embedded onto the human epidermis accompanied by data readout and signal conditioning circuits with wireless communication modules for transmitting data to the computing devices. Wearable sensors designed for recognition of various biomarkers in human epidermis fluids, such as glucose, lactate, pH, cholesterol, etc., as well as physiological indicators, i.e., pulse rate, temperature, breath rate, respiration, alcohol, activity monitoring, etc., have potential applications both in medical diagnostics and fitness monitoring. The rapid developments in solution-based nanomaterials offered a promising perspective to the field of wearable sensors by enabling their cost-efficient manufacturing through printing on a wide range of flexible polymeric substrates. This review highlights the latest key developments made in the field of wearable sensors involving advanced nanomaterials, manufacturing processes, substrates, sensor type, sensing mechanism, and readout circuits, and ends with challenges in the future scope of the field. Sensors are categorized as biological and fluidic, mounted directly on the human body, or physiological, integrated onto wearable substrates/gadgets separately for monitoring of human-body-related analytes, as well as external stimuli. Special focus is given to printable materials and sensors, which are key enablers for wearable electronics.

Wearable transparent thermal sensors and heaters based on metal-plated fibers and nanowires

Electrospun metal-plated nanofibers and supersonically sprayed nanowires were used to fabricate hybrid films exhibiting a superior low sheet resistance of 0.18 Ω sq-1, a transparency of 91.1%, and a figure-of-merit of 2.315 Ω-1. The films are suitable to serve as thermal sensors and heaters. Such hybrid transparent conducting films are highly flexible and thus wearable. They can be used as body-temperature monitors and heaters. The employed hybrid approach improved the sheet resistance diminishing it to a minimum, while maintaining transparency. In addition, the low sheet resistance of the films facilitates their powering with a low-voltage battery and thus, portability. The thermal sensing and heating capabilities were demonstrated for such films with various sheet resistances and degrees of transparency. The temperature sensing was achieved by the resistance change of the film; the resistance value was converted back to temperature. The sensing performance increased with the improvement in the sheet resistance. The temperature coefficient of resistivity was TCR = 0.0783 K-1. The uniform distribution of the metal-plated nanofibers and nanowires resulted in a uniform Joule heating contributing to an efficient convection heat transfer from the heaters to the surrounding, demonstrated by an improved convective heat transfer coefficient.

The Use of a Water Soluble Flexible Substrate to Embed Electronics in Additively Manufactured Objects: From Tattoo to Water Transfer Printed Electronics

The integration of electronics into the process flow of the additive manufacturing of 3D objects is demonstrated using water soluble films as a temporary flexible substrate. Three process variants are detailed to evaluate their capabilities to meet the additive manufacturing requirements. One of them, called water transfer printing, shows the best ability to fabricate electronics onto 3D additively manufactured objects. Moreover, a curved capacitive touchpad hidden by color films is successfully transferred onto the 3D objects, showing a potential application of this technology to fabricate fully additively manufactured discrete or even hidden electronic devices.