Skin electronics from scalable fabrication of an intrinsically stretchable transistor array.

Skin-like electronics that can adhere seamlessly to human skin or within the body are highly desirable for applications such as health monitoring, medical treatment, medical implants and biological studies, and for technologies that include human-machine interfaces, soft robotics and augmented reality. Rendering such electronics soft and stretchable-like human skin-would make them more comfortable to wear, and, through increased contact area, would greatly enhance the fidelity of signals acquired from the skin. Structural engineering of rigid inorganic and organic devices has enabled circuit-level stretchability, but this requires sophisticated fabrication techniques and usually suffers from reduced densities of devices within an array. We reasoned that the desired parameters, such as higher mechanical deformability and robustness, improved skin compatibility and higher device density, could be provided by using intrinsically stretchable polymer materials instead. However, the production of intrinsically stretchable materials and devices is still largely in its infancy: such materials have been reported, but functional, intrinsically stretchable electronics have yet to be demonstrated owing to the lack of a scalable fabrication technology. Here we describe a fabrication process that enables high yield and uniformity from a variety of intrinsically stretchable electronic polymers. We demonstrate an intrinsically stretchable polymer transistor array with an unprecedented device density of 347 transistors per square centimetre. The transistors have an average charge-carrier mobility comparable to that of amorphous silicon, varying only slightly (within one order of magnitude) when subjected to 100 per cent strain for 1,000 cycles, without current-voltage hysteresis. Our transistor arrays thus constitute intrinsically stretchable skin electronics, and include an active matrix for sensory arrays, as well as analogue and digital circuit elements. Our process offers a general platform for incorporating other intrinsically stretchable polymer materials, enabling the fabrication of next-generation stretchable skin electronic devices.

Multi-scale ordering in highly stretchable polymer semiconducting films.

Stretchable semiconducting polymers have been developed as a key component to enable skin-like wearable electronics, but their electrical performance must be improved to enable more advanced functionalities. Here, we report a solution processing approach that can achieve multi-scale ordering and alignment of conjugated polymers in stretchable semiconductors to substantially improve their charge carrier mobility. Using solution shearing with a patterned microtrench coating blade, macroscale alignment of conjugated-polymer nanostructures was achieved along the charge transport direction. In conjunction, the nanoscale spatial confinement aligns chain conformation and promotes short-range π-π ordering, substantially reducing the energetic barrier for charge carrier transport. As a result, the mobilities of stretchable conjugated-polymer films have been enhanced up to threefold and maintained under a strain up to 100%. This method may also serve as the basis for large-area manufacturing of stretchable semiconducting films, as demonstrated by the roll-to-roll coating of metre-scale films.

Development of a new generation of ammonia sensors on printed polymeric hotplates.

Conducting polyaniline-based chemiresistors on printed polymeric micro-hotplates were developed, showing sensitive and selective detection of ammonia vapor in air. The devices consist of a fully inkjet-printed silver heater and interdigitated electrodes on a polyethylene naphthalate substrate, separated by a thin dielectric film. The integrated heater allowed operation at elevated temperatures, enhancing the ammonia sensing performance. The printed sensor designs were optimized over two different generations, to improve the thermal performance through careful design of the shape and dimension of the heater element. A vapor-phase deposition polymerization technique was adapted to produce polyaniline sensing layers doped with poly(4-styrenesulfonic acid). The resulting sensor had better thermal stability and sensing performance when compared with conventional polyaniline-based sensors, and this was attributed to the polymeric dopant used in this study. Improved long-term stability of the sensors was achieved by electrodeposition of gold on the silver electrodes. Response to sub-parts-per-million concentrations of ammonia even under humid conditions was observed.

Upscaled Synthesis Protocol for Phase-Pure, Colloidally Stable MXenes with Long Shelf Lives.

MXenes are electrically conductive 2D transition metal carbides/nitrides obtained by the etching of nanolaminated MAX phase compounds, followed by exfoliation to single- or few-layered nanosheets. The mainstream chemical etching processes have evolved from pure hydrofluoric acid (HF) etching into the innovative "minimally intensive layer delamination" (MILD) route. Despite their current popularity and remarkable application potential, the scalability of MILD-produced MXenes remains unproven, excluding MXenes from industrial applications. This work proposes a "next-generation MILD" (NGMILD) synthesis protocol for phase-pure, colloidally stable MXenes that withstand long periods of dry storage. NGMILD incorporates the synergistic effects of a secondary salt, a richer lithium (Li) environment, and iterative alcohol-based washing to achieve high-purity MXenes, while improving etching efficiency, intercalation, and shelf life. Moreover, NGMILD comprises a sulfuric acid (H2 SO4 ) post-treatment for the selective removal of the Li3 AlF6 impurity that commonly persists in MILD-produced MXenes. This work demonstrates the upscaled NGMILD synthesis of (50 g) phase-pure Ti3 C2 Tz MXene clays with high extraction yields (>22%) of supernatant dispersions. Finally, NGMILD-produced MXene clays dry-stored for six months under ambient conditions experience minimal degradation, while retaining excellent redispersibility. Overall, the NGMILD protocol is a leap forward toward the industrial production of MXenes and their subsequent market deployment.

Facile Fabrication of Flexible and High-Performing Thermoelectrics by Direct Laser Printing on Plastic Foil.

The emerging fields of wearables and the Internet of Things introduce the need for electronics and power sources with unconventional form factors: large area, customizable shape, and flexibility. Thermoelectric (TE) generators can power those systems by converting abundant waste heat into electricity, whereas the versatility of additive manufacturing suits heterogeneous form factors. Here, additive manufacturing of high-performing flexible TEs is proposed. Maskless and large-area patterning of Bi2Te3-based films is performed by laser powder bed fusion directly on plastic foil. Mechanical interlocking allows simultaneous patterning, sintering, and attachment of the films to the substrate without using organic binders that jeopardize the final performance. Material waste could be minimized by recycling the unexposed powder. The particular microstructure of the laser-printed material renders the-otherwise brittle-Bi2Te3 films highly flexible despite their high thickness. The films survive 500 extreme-bending cycles to a 0.76 mm radius. Power factors above 1500 µW m-1K-2 and a record-low sheet resistance for flexible TEs of 0.4 Ω sq-1 are achieved, leading to unprecedented potential for power generation. This versatile fabrication route enables innovative implementations, such as cuttable arrays adapting to specific applications in self-powered sensing, and energy harvesting from unusual scenarios like human skin and curved hot surfaces.

Scalable Electrodeposition of Liquid Metal from an Acetonitrile-Based Electrolyte for Highly Integrated Stretchable Electronics.

The advancement of highly integrated stretchable electronics requires the development of scalable sub-micrometer conductor patterning. Eutectic gallium indium (EGaIn) is an attractive conductor for stretchable electronics, as its liquid metallic character grants it high electrical conductivity upon deformation. However, its high surface tension makes its patterning with sub-micrometer resolution challenging. In this work, this limitation is overcome by way of the electrodeposition of EGaIn. A non-aqueous acetonitrile-based electrolyte that exhibits high electrochemical stability and chemical orthogonality is used. The electrodeposited material leads to low-resistance lines that remain stable upon (repeated) stretching to a 100% strain. Because electrodeposition benefits from the resolution of mature nanofabrication methods used to pattern the base metal, the proposed "bottom-up" approach achieves a record-high density integration of EGaIn regular lines of 300 nm half-pitch on an elastomer substrate by plating on a gold seed layer prepatterned by nanoimprinting. Moreover, vertical integration is enabled by filling high-aspect-ratio vias. This capability is conceptualized by the fabrication of an omnidirectionally stretchable 3D electronic circuit, and demonstrates a soft-electronic analog of the stablished damascene process used to fabricate microchip interconnects. Overall, this work proposes a simple route to address the challenge of metallization in highly integrated (3D) stretchable electronics.

Boosting the performance of printed thermoelectric materials by inducing morphological anisotropy.

Thermoelectrics can generate electrical energy from waste heat and work also as active coolers. However, their widespread use is hindered by their poor efficiency, which is aggravated by their costly and hard-to-scale fabrication process. Good thermoelectric performances require materials with high (low) electrical (thermal) conductivity. Inducing morphological anisotropy at the nanoscale holds promise to boost thermoelectric performances, in both inorganic and organic materials, by increasing the ratio electrical/thermal conductivity along a selected direction without strongly affecting the Seebeck coefficient. Recent advances in 2D/3D printed electronics are revealing new simple and inexpensive routes to fabricate thermoelectrics with the necessary morphological control to boost performance by inducing anisotropy.

Fully stretchable active-matrix organic light-emitting electrochemical cell array.

Intrinsically and fully stretchable active-matrix-driven displays are an important element to skin electronics that can be applied to many emerging fields, such as wearable electronics, consumer electronics and biomedical devices. Here, we show for the first time a fully stretchable active-matrix-driven organic light-emitting electrochemical cell array. Briefly, it is comprised of a stretchable light-emitting electrochemical cell array driven by a solution-processed, vertically integrated stretchable organic thin-film transistor active-matrix, which is enabled by the development of chemically-orthogonal and intrinsically stretchable dielectric materials. Our resulting active-matrix-driven organic light-emitting electrochemical cell array can be readily bent, twisted and stretched without affecting its device performance. When mounted on skin, the array can tolerate to repeated cycles at 30% strain. This work demonstrates the feasibility of skin-applicable displays and lays the foundation for further materials development.

An integrated self-healable electronic skin system fabricated via dynamic reconstruction of a nanostructured conducting network.

Electronic skin devices capable of monitoring physiological signals and displaying feedback information through closed-loop communication between the user and electronics are being considered for next-generation wearables and the 'Internet of Things'. Such devices need to be ultrathin to achieve seamless and conformal contact with the human body, to accommodate strains from repeated movement and to be comfortable to wear. Recently, self-healing chemistry has driven important advances in deformable and reconfigurable electronics, particularly with self-healable electrodes as the key enabler. Unlike polymer substrates with self-healable dynamic nature, the disrupted conducting network is unable to recover its stretchability after damage. Here, we report the observation of self-reconstruction of conducting nanostructures when in contact with a dynamically crosslinked polymer network. This, combined with the self-bonding property of self-healing polymer, allowed subsequent heterogeneous multi-component device integration of interconnects, sensors and light-emitting devices into a single multi-functional system. This first autonomous self-healable and stretchable multi-component electronic skin paves the way for future robust electronics.

A highly stretchable, transparent, and conductive polymer.

Previous breakthroughs in stretchable electronics stem from strain engineering and nanocomposite approaches. Routes toward intrinsically stretchable molecular materials remain scarce but, if successful, will enable simpler fabrication processes, such as direct printing and coating, mechanically robust devices, and more intimate contact with objects. We report a highly stretchable conducting polymer, realized with a range of enhancers that serve a dual function: (i) they change morphology and (ii) they act as conductivity-enhancing dopants in poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The polymer films exhibit conductivities comparable to the best reported values for PEDOT:PSS, with over 3100 S/cm under 0% strain and over 4100 S/cm under 100% strain-among the highest for reported stretchable conductors. It is highly durable under cyclic loading, with the conductivity maintained at 3600 S/cm even after 1000 cycles to 100% strain. The conductivity remained above 100 S/cm under 600% strain, with a fracture strain of 800%, which is superior to even the best silver nanowire- or carbon nanotube-based stretchable conductor films. The combination of excellent electrical and mechanical properties allowed it to serve as interconnects for field-effect transistor arrays with a device density that is five times higher than typical lithographically patterned wavy interconnects.