Wearable electronics are finding emerging applications in mobile health, rehabilitation, prosthetics/exoskeletons, athletic training, human-machine interaction, etc. However, our skin is soft, curvilinear and dynamic whereas wafer-based electronics are hard, planar, and rigid. As a result, state-of-the-art wearables can only be strapped or clipped on human body. The development of flexible and stretchable electronics offers a remedy for such challenge. E-tattoos represent a class of stretchable circuits, sensors, and actuators that are ultrathin, ultrasoft, skin-conformable and deformable just like a temporary tattoo. We introduce a low-cost, dry and freeform “cut-and-paste” and “cut-solder-paste” method invented by my lab to fabricate e-tattoos. This method has been proved to work for thin film metals, polymers, ceramics, as well as 2D materials. Using these method, we created the first truly imperceptible e-tattoos based on graphene, and modular and reconfigurable Bluetooth and NFC enabled wireless e-tattoos.
We present recent development on integrated flexible and stretchable photonic devices. Conventional photonic devices are fabricated on rigid semiconductor or dielectric substrates and are therefore inherently incompatible with soft biological tissues. Recently, we have developed a suite of active and passive photonic devices and systems integrated on plastic substrates which can be bent, twisted, and stretched without compromising their optical performance. Key innovations are monolithic multi-material integration and advanced micro-mechanical structures co-designed with photonic devices, which enables devices with extreme mechanical flexibility and excellent optical performance.
Tattoo-like epidermal sensors are an emerging class of truly wearable electronics owing to their thinness and softness. While most of them are based on thin metal films, silicon membrane, or nanoparticle-based printable inks, we report the first demonstration of sub-micron thick, multimodal electronic tattoo sensors that are made of graphene. The graphene electronic tattoo (GET) is designed with filamentary serpentines and fabricated by a cost- and time-effective “wet transfer, dry patterning” method. It has a total thickness of 463 ± 30 nm, an optical transparency of ~85%, and a stretchability of more than 40%. GET can be directly laminated on human skin just like a temporary tattoo and can fully conform to the microscopic morphology of the surface of skin via just van der Waals forces. The open mesh structure of GET makes it breathable and its stiffness negligible. Bare GET is able to stay attached to skin, for several hours, without fracture or delamination. With liquid bandage coverage, GET may stay functional on skin up to several days. As a dry electrode, GET-skin interface impedance is on par with medically used silver/silver-chloride (Ag/AgCl) gel electrodes, while offering superior comfort, mobility and reliability. GET has been successfully applied to measure electrocardiogram (ECG), electromyogram (EMG), electroencephalogram (EEG), skin temperature, and skin hydration. Graphene represents a new facile route for ultra-conformable multifunctional electronic tattoos, and paves the path for the introduction of other two dimensional materials for future advanced tattoo systems.
Epidermal electronic system is a class of hair thin, skin soft, stretchable sensors and electronics capable of continuous and long-term physiological sensing and clinical therapy when applied on human skin. The high cost of manpower, materials, and photolithographic facilities associated with its manufacture limit the availability of disposable epidermal electronics. We have invented a cost and time effective, completely dry, benchtop “cut-and-paste” method for the green, freeform and portable manufacture of epidermal electronics within minutes. We have applied the “cut-and-paste” method to manufacture epidermal electrodes, hydration and temperature sensors, conformable power-efficient heaters, as well as cuffless continuous blood pressure monitors out of metal thin films, two-dimensional (2D) materials, and piezoelectric polymer sheets. For demonstration purpose, we will discuss three examples of “cut-and-pasted” epidermal electronic systems in this paper. The first will be submicron thick, transparent epidermal graphene electrodes that can be directly transferred to human skin like a temporary transfer tattoo and can measure electrocardiogram (ECG) with signal-to-noise ratio and motion artifacts on par with conventional gel electrodes. The second will be a chest patch which houses both electrodes and pressure sensors for the synchronous measurements of ECG and seismocardiogram (SCG) such that beat-to-beat blood pressure can be inferred from the time interval between the R peak of the ECG and the AC peak of the SCG. The last example will be a highly conformable, low power consumption epidermal heater for thermal therapy.
Epidermal electronics is a class of noninvasive and unobstructive skin-mounted, tattoo-like sensors and electronics capable of vital sign monitoring and establishing human-machine interface. The high cost of manpower, materials, vacuum equipment, and photolithographic facilities associated with its manufacture greatly hinders the widespread use of disposable epidermal electronics. Here we report a cost and time effective, completely dry, benchtop “cut-and-paste” method for the freeform and portable manufacture of multiparametric epidermal sensor systems (ESS) within minutes. This versatile method works for all types of thin metal and polymeric sheets and is compatible with any tattoo adhesives or medical tapes. The resulting ESS are multimaterial and multifunctional and have been demonstrated to noninvasively but accurately measure electrophysiological signals, skin temperature, skin hydration, as well as respiratory rate. In addition, planar stretchable coils exploiting double-stranded serpentine design have been successfully applied as wireless, passive epidermal strain sensors.
Flexible electronics and photonics are providing revolutionary solutions for communication, energy, and health care. While some of the organic electronic and photonic materials are intrinsically deformable and low cost to manufacture, their performance and chemical stabilities are yet to match conventional inorganic semiconductors. Strategies for high performance flexible electronics and photonics must overcome challenges associated with the intrinsic stiffness and brittleness of inorganic materials. This paper discusses recent modeling and experimental advancement in the bendability and stretchability of inorganic electronics and photonics. Examples include the discovery of multiple neutral axes in multilayer structures and the comparison between freestanding and polymer-bonded serpentine ribbons.