Direct interfacing of nanosensors onto biomaterials could revolutionize areas ranging from health quality monitoring to adaptive threat detection. Due to its exceptional electrical properties, nanosensors based on graphene have been shown to exhibit extremely sensitive analyte detection. Further, the high interfacial adhesion exhibited by graphene renders it ideal for interfacing onto curvilinear and rugged surfaces. Here we introduce a new approach to directly interfacing graphene nanosensors onto biomaterials. Specifically, we demonstrate that graphene can be printed onto water-soluble silk. This in turn permits intimate biotransfer of graphene nanosensors onto biomaterials including tooth enamel and skin, like the attachment of a temporary tattoo. The result is a fully biointerfaced sensing platform, which can be tuned to detect specific target analytes. For example, via bifunctional self-assembly of antimicrobial peptides onto graphene, we demonstrate bioselective detection of bacteria at single-cell levels. The incorporation of a resonant circuit consisting of interdigitated electrodes and an inductive coil eliminates the need for onboard power and external connections. Combining these elements yields two-tiered interfacing of peptide-graphene nanosensors with biomaterials. In particular, we demonstrate integration onto a tooth for remote monitoring of breath and detection of bacteria in saliva at single cell levels. Overall, this strategy of hierarchically interfacing biomolecules with nanosensors and biomaterials represents a versatile approach for ubiquitous detection of biochemical targets.

The emerging field of conformable electronics places new physical requirements on electronic components. Integration directly into or onto soft materials such as textiles or biological tissues is of increasing interest for applications spanning medical, safety, security, infrastructure, and communication industries among many others. The unique requirement imposed in this field is that the electronics must be highly flexible in order to survive the mechanical deformation of the malleable host material.
Here we report the thinnest and most flexible organic thin film transistors and demonstrate an active-matrix-based touch panel with unprecedented mechanical resilience, that can conform to virtually any surface. At 2 µm total thickness, our devices endure extreme bending into radii smaller than 20 µm without failure or performance degradation. When transferred to elastomer substrates the thin film transistors become ultra-compliant, withstanding mechanical stretching and relaxation cycles to more than 200 % tensile strain repeatedly.
Constructed on only 1.2 µm thick poly(ethylene 2,6-)naphthalate (PEN) substrates, the thin-film transistors exhibit excellent carrier mobilities of 3 cm2/Vs and are operated at only 3V under ambient conditions. A hybrid gate dielectric comprising ultra-dense anodic aluminum oxide and a phosphonic acid self assembled monolayer enables extreme mechanical resilience and low-voltage operation. Large-area tactile sensors with 12x12 sensing nodes are fabricated, where all 144 devices are fully functional.
The suggested new form of ultrathin electronics provides an active matrix integrated backplane, that can be readily coupled with various sensors and actuators, enabling ultra-conformable and imperceptible electronic circuit wraps as electronic skin, for bio-medical monitoring and treatment systems or smart packaging.

The development of stretchable electronics has the potential to facilitate important new fields, such as electronic skin, conformal displays, and biometrics monitoring. Current stretchable active matrices are based on rigid device islands connected by stretchable interconnects. While these methods can provide good performance, there is a trade-off between stretchability and device density. Moreover, some of the targeted applications require large area coverage, and would therefore benefit from high-throughput, low-cost fabrication methods. We have developed stretchable organic field effect transistors based on low-cost materials and fabrication methods, such as spin-coating and transfer printing. The gate conductor and elastomeric dielectric are deposited by spin-coating, while the semiconductor is deposited using a transfer technique. The transfer technique is strongly affected by the nature and curing conditions of the dielectric. Stretchability is achieved by using intrinsically stretchable components and device architectures that impart stretchability to the non-stretchable components. Previous research has shown that good adhesion to an elastomeric substrate can allow a brittle, plastic material to be stretchable by reducing the extent of stress localization. This principle is used to make the semiconductor and conductors stretchable. One of the key issues is the adhesion between the layers, and measures have been taken to engineer the interfaces to improve the stretchability. The effect of stretching on device parameters such as mobility, drain current, and gate leakage will be described. The materials requirements and materials development efforts will be discussed.

Touch is primarily mapping of the distribution of compressive stress over the area of physical contact between the sensor and the object surfaces. The tactile devices imbedded in human skin can detect stress distribution at a resolution of ~40 microns requiring average contact pressures in 30 to 90 KPa range. To design an artificial tactile sensor on par with human finger both the resolution and sensitivity have to be met with a typical contact area in 1-10 cm2 range. We will describe a self-assembled nanocomposite film composed of nanoparticles and polymer that converts the local stress to electroluminescent light that can be directly focused on a digital camera to obtain a tactile image with 20 micron resolution. The film can be deposited on large area by simple dip coating. Although the film is ~100 nm thin, it behaves like a (nano)sponge with 60% reversible compressibility and modulus below 100 KPa to obtain the desired sensitivity range similar to human skin. We will quantitatively describe the principle of the observed electro-optical properties of the tactile device, and mechanical properties of the thin film including the method to measure stress-strain behavior of these 100 nm thick structures. A model will be discussed to explain the reason for the four orders of magnitude lower modulus of the film compared of its constituents. We will describe the performance of the device to image imbedded palpable structures for potential application such as, breast cancer screening.

Previous work has shown the immense promise of solution-processed semiconducting carbon nanotube network as a competing technology platform alongside organic, metal-oxide semiconductors, and buckled semiconductor nanomembranes for low-cost high-performance flexible electronics. The advantages of superior mobility, room-temperature and scalable processing, and long-term air stability make it possible to proceed from laboratory-scale demonstrations to practical macro-scale electronic systems using carbon nanotubes.
Here in this talk, we report our recent progress on a flexible interactive skin sensor that can simultaneously detect, spatially map, and respond to the pressure stimulus. The interactive skin sensor is realized by seamless integration of our previous work on artificial electronic skin with a flexible full-color active-matrix organic light-emitting diode (AMOLED) display that can display red, green, and blue colors. All the electronics are built upon a 16×16 pixel backplane of solution-processed semiconducting nanotube thin-film transistors that are extremely bendable, robust, and with superior performance and yield. Different pressure profile has been successfully mapped and the applied pressure is directly reflected by the output light intensity of the display. This represents a new class of flexible interactive electronics that may find wide range of applications in control or input devices, robotics, medical applications, and light-weight wearable electronics.

A principle advantage of organic electronic technologies is that devices comprise very thin layer stacks, totaling roughly 600 nm. Typical substrates are, however, much thicker (ge;100 mu;m ) and therefore determine the mechanical properties of the device. In this work we use plastic foil substrates, only 2.2 times thicker than the combined electrodes and light-emitting polymer. In doing so, we demonstrate display-compatible polymer LEDs with unprecedented mechanical properties in opto-electronic devices. They can be operated as free-standing ultrathin films allowing for crumpling during device operation. Furthermore, they may be applied to almost any surface whether rigid or elastomeric, and can withstand the associated mechanical deformation. They are shown to be extremely flexible with radius of curvature under 10 mu;m, and stretch-compatible to 100% tensile strain. Such ultrathin light-emitting foils constitute a step towards integration with malleable materials like textiles and artificial skin.

Electronic device materials and stretchable substrates are mechanically incompatible. Here we propose a generic design for engineered stretchable substrate, which can guarantee nearly no elongation at the device location when overall the substrate is reversibly deformed. The substrate is formed of silicone rubber with embedded, localized, stiff (non deformable) regions. The surface of the engineered substrate is planar and smooth, providing a uniform interface onto which device materials can be processed directly.
The engineered substrate is prepared using the following steps: stiff, SU8 platforms are first lithographically patterned on a silicon wafer then embedded within a matrix of PDMS rubber. After curing of the silicone elastomer, the substrate with embedded SU8 platforms is lifted-off the wafer. The geometry and density of the SU8 platforms are optimized for a given substrate thickness using finite element modeling (FEM) so that the strain in the regions above the SU8 platforms remains below 0.5%, even when macroscopic strain of 20% is applied. Experimental strain maps recorded at the surface of the engineered substrate using digital image correlation (DIC) confirm our model.
To demonstrate our approach, thin, brittle Alumina (Al2O3) disks are evaporated on bare PDMS and on engineered substrate (the alumina is film is patterned immediately above the rigid platforms), and interconnected with stretchable thin gold conductors. Catastrophic cracking occurs at low strain (<1%) in the alumina film on bare PDMS. No cracks are observed across the Al2O3 disks on engineered substrate, even when cyclically stretched to 20% total strain. The stretchable gold interconnects remain electrically stable over 100 cycles, and no mechanical failure occurs.
Engineered elastomeric substrates with in situ strain minimization offer a promising approach for the direct integration of active electronic devices onto stretchable substrates.

Electronic components that are stretchable and soft are being developed for electronic skins that can conform to curvilinear surfaces. We have been studying new methods to control the shape of a micromoldable liquid metal to create conductors that are soft, self-healing, and ultra-stretchable. The metal is a gallium-based metal alloy that is a low-viscosity liquid at room temperature with low toxicity and negligible volatility. Despite the large surface tension of the metal, it can be molded into non-spherical shapes due to the presence of an ultra-thin oxide skin that forms on its surface. The metal can be patterned by injection into microchannels or by direct-write techniques. Because it is a liquid, the metal is extremely soft and flows in response to stress to retain electrical continuity under extreme deformation. By embedding the metal into elastomeric substrates, it is possible to form soft electrodes, stretchable antennas, self-healing vascular networks, reconfigurable interconnects, soft sensors, and ultra-stretchable wires that maintain metallic conductivity up to ~800% strain. The metal may also be integrated with biocompatible hydrogels to create electrodes, memory resistive devices, sensors, and diodes that are composed entirely of soft materials and may interface readily with tissue. This talk will discuss the mechanical and electrical properties of this micromoldable liquid metal within the context of stretchable electronics and electronic skins. We will demonstrate several methods to pattern the metal by taking advantage of the unique properties enabled by the oxide skin. We will also discuss new applications of the metal for stretchable electronics and new methods to control and reconfigure the shape of the metal on demand by taking advantage of stimuli responsive interfacial properties.

Shape-memory polymers (SMPs) are polymeric smart materials able to return from a temporary, deformed state to their permanent, original shape through application of an external stimulus, such as temperature. We explore SMPs as substrates for electronic skin and take advantage of the tailorable range of stimulus temperatures (Tg) at which stiffness (modulus) and geometry (shape) change. Here we describe two methods for fabricating flexible electronics based on the tunable thermomechanical properties of SMPs and processing of thin film conductors thereon. SMP substrates are synthesized and uniaxially pre-strained between 10% and 50%. A 3 nm Ti adhesion layer is then e-beam evaporated onto the stretched substrates before deposition of 30 nm thick gold conductors. Samples are subsequently heated above Tg, which enables recovery of the initial pre-strained shape to compress and wrinkle the gold conductors. Electrical resistivity of the conductors is measured and the samples are characterized through SEM and AFM analysis before and after the transition from the flat to the wrinkled conductor state. Conductors deposited on pre-strained SMP substrates increase in resistivity from approximately 10 to 20 µOmega; cms due to wrinkling of a smooth Au layer and crack formation on the longitudinal direction due to Poisson expansion. SMPs are also utilized as carriers for spun-on and cured poly(dimethylsiloxane) (PDMS) membranes. With conductors deposited on pre-strained PDMS using a SMP carrier, a drop in resistivity from 13 µOmega; cms (prestrained to 50%) to 10 µOmega; cms (recovered) is observed due to the sealing of microcracks in the Au layer.

Amazing progress in flexible and stretchable electronics has been made over the past few years and various applications such as hemispherically curved array of photodetectors, stretchable solar cells, active matrix light emitting diode and dry gel batteries have been demonstrated and those devices can be laminated onto skin and fingers. Previously, reported inorganic stretchable electronic devices used stretchable conductive interconnects or deposited on the pre-stretched elastomeric substrate to overcome stiffness of the devices. In organic electronics, it is also difficult to make stretchable devices because the conjugated polymers in active layer have intrinsically less elasticity than other common polymers such as rubbers. For improving stretchability, we applied simple blending method for fabricating stretchable organic field effect transistor (OTFT) which is blended elastomeric materials such as polydimethylsiloxane (PDMS) with active and dielectric layer materials. However, it was predicted that PDMS disturbs the crystallinity of active layer and charge transport, and the best conditions of blending two components should be found. In this presentation, we demonstrate that OTFT device performances depend on PDMS ratio and many conjugated materials. In addition, we incorporate an additive in active layer for better miscibility of conjugated polymer and PDMS. Detail studies on morphology, crystallinity, distribution of PDMS in mixed composition will be discussed.

Flexible tactile sensors are promising devices for future medical instruments, various electronic components and robot mechatronics. We have developed a sensor with proper properties for flexible piezoresistive strain sensors showing high sensitive and uniform electrical properties, by assembling thin single-crystalline silicon ribbons with polymer substrate.
Silicon ribbons with a thickness of 100 nm and doping concentration of ~1 X 1019 /cm3 with boron, were transferred onto polyimide substrate by using polydimethylsiloxane (PDMS) stamp. We have successfully fabricated a high sensitive and flexible tactile sensor with 8 x 8 array by forming electrodes and encapsulating the device. Finally, we have integrated it with driving circuits, and characterized in terms of gauge factor, output performance and hysteresis.
The device shows stable operation while it is bent with a bending radius of 5.4 mm, gauge factors of 56, uniformity with a standard variation of 0.28, linearity of < 0.1 %, and hysteresis of < 0.2 %. These properties are comparable to those of conventional strain gauge, and the flexibility of device makes it as a good strain sensor for various applications such as electronic skins and smart gloves.

Reversibly stretchable RF antenna has been developed using silver nanowires used for wireless sensing. We have already demonstrated a reversibly stretchable and transparent electrode via spray-deposition of silver nanowires on an elastomeric substrate (PDMS) with dopamine modification [2]. The sprayed stretchable silver nanowire electrode on PDMS showed less than 1 Omega;/square of surface resistivity. The stretchable RF antenna structure was designed as a 1.5GHz microstrip patch antenna using the transmission-line model. The prototype has dimensions of 61mm long, 74mm wide and 1.5mm thick. The microstrip transmission line is 3.63mm thick with an inset of 19.13mm into the antenna to minimize return losses. Both patch antenna and ground plane are composed of silver nanowires via dropcast-deposition on either side of the substrate. The antenna&’s experimental results are compared to the simulated results from the Ansoft HFSS antenna design software. The antenna acts as a strain sensor as it is stretched lengthwise. Transmitted signals differ relative to the magnitude of strain upon the stretchable antenna. Wireless transmitter might be incorporated into the flexible substrate via encapsulation of hybrid integration of rigid active circuitry and flexible printed circuit boards. This reversibly stretchable wireless RF antenna demonstrates various cost-effective commercial applications in RF communications.
Reference
1. S. Cheng, and Z. Wu, “A Microfluidic, Reversibly Stretchable, Large-Area Wireless Strain Sensor” Adv. Func. Mater. 21, pp. 2282-2290 (2011)
2. T. Aktar, and W.S. Kim, “Reversibly Stretchable Transparent Conductive Coatings of Spray-deposited Silver Nanowires” ACS Applied Materials & Interfaces, 4, pp.1855-1859 (2012).

Organic light-emitting devices (OLEDs) have long been perceived as a low-temperature, low-cost technology that can be fabricated through solution-based processes. However, how to achieve all solution-processed OLEDs has been a challenge. Here we report the fabrication of all-solution processed polymer light-emitting electrochemical cells (LECs) comprised of transparent composite electrode as anode and silver paste as cathode. The composite electrode contains a stack of a single-walled carbon nanotube layer and a silver nanowire layer embedded in the surface layer of a flexible polymer substrate. The dense carbon nanotube layer on the outer surface provides the necessary chemical stability and large contact area on the composite electrode surface with the luminescent polymer layer. The underneath AgNW layer yields a high surface conductivity without much sacrifice in optical transmission. All the materials employed in the LECs were deposited from solutions at ambient conditions by spin-coating, rod-coating, and/or blade-coating. The all-solution processed thin film devices based on a red light-emitting polymer, poly[2-methoxy-5-(3,7-dimethyloctyloxy)-p-phenylenevinylene] (OC1C10), exhibited a turn-on voltage of 3.0 V, a maximum current efficiency of 3.02 cd/A at 11.9 V, and a maximum brightness of 2080 cd/m². The device performance was higher than control devices fabricated on indium tin oxide anode coated on glass and using evaporated aluminum as cathode.

Human skin can be modeled as a two-layer composite consisting of a stiff epidermis attached to a soft dermis [1]: the integration of stiff layers on soft and stretchable substrates is of a fundamental ingredient for the fabrication of artificial skin. The mismatch of elastic properties of the two layers with different stiffness and their relative thicknesses give rise to the formation of buckling and wrinkling patterns than can be used for a variety of application ranging from microfabrication to tunable optics [1].
The fabrication of a stiff layer on a stretchable polymer such as PDMS usually is performed by plasma treatment or exposure to ultraviolet/ozone radiation of a stretched polymer film-, alternatively a thin metallic film is deposited on a pre-stretched substrate [2]. After deposition the relief of the soft substrate stress gives rise to the formation mechanical instabilities of the two-layer system resulting in the formation of wavy patterns [2].
Here we demonstrate the possibility to modulate not only the morphology but also the stiffness of the surface of an elastomer by Supersonic Cluster Beam Implantation (SCBI) of neutral metal cluster in a stretchable substrate [3]. Regions of any shape with different stiffness can be printed with high lateral resolution (down to 1 micron) using microstencil masks by exploiting the high collimation typical of supersonic cluster beams. The stiffness of the implanted region depend upon the dose of implanted nanoparticles allowing a fine control of the mechanical properties of the nanocomposite produced by SCBI.
Gold micropatterns obtained using as stencil mask a TEM grid not in contact with the substrate have been produced. The micropatterns do not delaminate under stretching at 35% strain. The fabrication process consists of one step and it does not include any wet chemical treatment, thus avoiding surface contamination.
Using an AFM equipped with a custom micrometer-sized spherical probe, we performed a joint topographical/nano-mechanical characterization of the nanocomposite pattern. From an array of indentation curves (applied force vs elastic deformation) the local surface topography and the map of the effective Young modulus of the nanocomposite material have been obtained showing an increase of the Young modulus of the implanted regions up to 40 times that of the pristine PDMS. By means of large-scale atomistic simulations we calculated the response-to-load curve of cluster-implanted polymer films. Such virtual nano-indentation experiments provide useful insight on the actual microstructure evolution of the implanted film upon loads, as well as on the effective elastic moduli of these systems.
[1] Efimenko K., et al., Nature Mater., 2005, 4, 293-297
[2] Bowden N., et al., Nature, 1998, 393, 146-149
[3] Corbelli G., et al., Adv. Mater. 2011, 23, 4504-4508

Although indium tin oxide (ITO) has been mainly used for transparent electrodes, its brittleness and growing cost of indium limit the use of ITO in large-area applications and flexible electronics. Several conductive materials, especially graphene and metal nanowire (NW) random networks, have been actively studied as promising alternatives to ITO, due to their high transparencies and flexibilities with low sheet resistances. Compared to undoped synthesized graphene, metal NW networks typically have lower sheet resistances, approaching to the value of ITO. However, the NW networks show disadvantages, such as high NW-NW junction resistances to degrade breakdown properties and percolation-dependent conductivities to limit electrode dimensions. This talk presents formation of the graphene-AgNW hybrid structure as stretchable, transparent electrodes. The hybrid pattern showed low sheet resistance (less than 40 Omega;/sq with the transmittance at 94%) and superb mechanical stretchability (maximum strain of 80% with negligible resistance change). Compared to the graphene and AgNW networks, maximum current density and breakdown electric field were increased at least ten times further by forming the hybrid structure, which suggests its potential uses for high-power transparent electronics. Fabrication of flexible, transparent transistors using the hybrid electrodes demonstrates the substantial promise of future electronics.

In flexible electronics and photovoltaic systems, elastomers are often used as a flexible and stretchable substrate, superstrate, or adhesive for device construction and encapsulation. In operation, these elastomeric films are subject to diurnal temperature cycling, mechanical stress, chemical erosion, and UV degradation. Environmental degradation and mechanical loading during operation act in synergy to induce stresses and structural damage in the elastomer layer. In our work, we observe that environmental effects such as temperature can change the adhesion of the elastomer to adjacent layers. More surprisingly, we observe that stress induced in the elastomer layer can have significant effects on adhesion strength of non-adjacent layers in the film structure. Therefore, it is crucial to determine the response of these elastomeric materials to environmental degradation to produce reliable flexible devices. We characterize the synergistic effect of UV, heat and moisture on several organic and inorganic layered structures on elastomeric substrates. The adhesion strength between the elastomer and the underlying layers is determined for samples subject to different duration of UV, heat, and moisture aging. XPS and wafer curvature studies are conducted to characterize the chemical and mechanical change in the elastomeric film. We show that environmental aging and damage in the elastomer layer can develop stresses that not only reduce adhesion strength of the elastomer but also promote delamination within the device layers.

The integration of flexible and transparent characteristics is an important component in the new organic electronic and optoelectronic devices and has been achieved for various applications, including transistors, lithium-ion batteries, supercapacitors, pressure sensors and artificial skins. Indeed, building flexible transparent energy conversion and storage units plays a key role in realizing fully flexible and transparent devices. In 2006, our group demonstrated the first piezoelectric ZnO nanogenerator that successfully converted mechanical energy into electric energy. Since then, various nanogenerators (NGs) based on piezoelectric effect have been demonstrated. As an important part in this field, some studies on fully integrated flexible and transparent NGs have been reported. Almost all of them are based on piezoelectric ZnO nanowires and the entire device requires sophisticated design and a high degree of integration.
In this work, based on the principle of the triboelectric effect and contact electrification, we demonstrate a novel high-output, flexible and transparent nanogenerator by using transparent polymer materials. We have fabricated three types of regular and uniform polymer patterned arrays (line, cube and pyramid) to improve the efficiency of the nanogenerator. The power generation of the pyramid-featured device far surpassed that exhibited by the unstructured films, and gave an output voltage of up to 18 V at a current density of ~0.13 mu;A/cm2. Furthermore, the as-prepared nanogenerator can be applied as a self-powered pressure sensor for sensing a water droplet (8 mg, ~0.1 Pa in contact pressure) and a falling feather (20 mg, ~0.22 Pa in contact pressure) with a low-end detection limit of ~0.13 mPa.

As a critical component of artificial skin and robotics, tactile sensors have been researched intensively in recent years. Traditional piezoresistive and capacitive tactile sensors have limitations in either spatial resolution or force sensitivity, which hinders their applications in the micro and nano scale. To address this issue, we present a single GaN nanowire transverse force sensor using the piezotronic effect. In principle, owing to the piezoelectric property of GaN, a transverse force can induce a piezopotential distribution along the nanowire, where the local piezopotential can modulate the barrier height of the Schottky contact between the GaN nanowire and platinum electrode. As a result, the conductivity of the nanowire is a function of the applied transverse force. A semi-quantitative model was developed to describe the correlation between transverse force and detected current flow. To demonstrate this idea, atomic force microscope was used to precisely position on single nanowire and measure its electric transportation while applying varied forces simultaneously. Our results show that GaN NWs can be used to transduce a shear/transverse force into dramatic current change through the NW due to the piezotronic effect. Because of this transduction mechanism, the transverse force can be correlated to the natural logarithm of the current. Our results indicate that the force sensitivity of the measured sensor is 1.24±0.13 lnA/nN, and a minimum detectable force of 16 nN. Because of their size, GaN nanowires are potential building blocks for future micro/nano tactile sensors.

Vertical silicon nanowire (SiNW) arrays, due to their high carrier mobility, flexibility, chemical and structural stability are promising candidates for advancements in electronic skin devices, ranging from pressure and temperature sensors to energy storage and harvesting devices. However, large scale integration of highly dense, vertical SiNW arrays remains a significant fabrication challenge. Here, we report two simple vertical transfer printing methods (V-TPMs) that enable vertically aligned SiNW arrays to be utilized in electronic skin devices by detaching SiNW arrays from their original silicon substrates and reattaching the detached SiNW arrays onto any receiving device substrates, while preserving the vertically aligned orientation over a large area. The first V-TPM forms a horizontal crack at a controlled position through a SiNW array by inserting a water soaking step between two consecutive Ag-assisted chemical etching (MACE) steps. The crack allows the SiNWs to be easily detached, allowing them to be transferred to foreign substrates without disrupting the SiNWs uniform length or alignment morphology on the receiver substrates. The second V-TPM forms a porous sacrificial handling layer beneath the SiNW arrays prior to being detached from the substrate by electropolishing at the base of the porous layer, which is then subsequently etch away in a weak basic solution. This V-TPM works and preserves the original SiNW quality regardless of the SiNWs fabrication method and dimensions. In addition, our V-TPMs can be utilized to form electronic devices on desired substrates by fabricating metallic contacts on both ends of the SiNW arrays with a polymer filling for mechanical support and electrical insulation in between. Electrical characterization of the SiNW devices exhibited good current-voltage (I-V) characteristics independent of final substrate materials and bending conditions. We believe that our two V-TPMs will greatly enable advancements in electronic skin devices through the integration of highly dense, vertical SiNW arrays, opening up new possibilities for future sensing and energy storage and harvesting devices.

In this work we introduce a novel architecture for the fabrication of tactile sensors that can be operated at voltages as low as 1V. All the described devices have been fabricated on ultrathin, highly flexible, plastic substrates that can be conformably transferred on whatever kind of surface, such as fabrics, or 3D structures and therefore, particularly suitable for the fabrication of electronic skin. The core of the device is a floating gate Organic Field Effect Transistor (OFET) biased through a control capacitor and with a sensing area directly connected to the floating gate. The insulating layer was fabricated using an ultra-thin (average thickness of 25 nm), hybrid organic/inorganic dielectric, composed by a combination of alumina (grown on a pre-deposited aluminum film that acts as the floating gate electrode) and Parylene C. Using this procedure, we have fabricated devices that can be operated at ultra-low operating voltages, in the range of 1 V.
A reference working point can be imposed in the device by applying a certain voltage through the control gate. At the same time, if an additional electrical charge is somehow induced onto the sensing area fabricated on the floating gate, it gives rise to a charge separation in this metal electrode, which, in turns, induces a modulation of the transistor threshold voltage. In order to achieve sensitivity to pressure, a piezoelectric thin film, namely PVDF-TrFE, is deposited by spin coating on the sensing area of the device and properly poled. In this way, when pressure is applied on the PVDF-TrFE, the charges induced in the piezoelectric film, according to the previous described mechanism, lead to a variation of OFET threshold voltage and a current variation can be detected at each pressure event. A detailed electro-mechanical characterization of the system has been performed and the main data regarding the sensitivity and the frequency response of the fabricated devices will be discussed. Moreover, the pyro-electric properties of the PVDF polymer have been also exploited and preliminary results on the fabrication of temperature sensors using the same approach will be presented. This work is particularly interesting as it introduces a novel, low voltage technology for tactile sensing to be employed for a wide range of applications in the biomedical field and it seems to be particularly suitable for the fabrication of artificial electronic skin. Moreover thanks to the properties of the employed polymer, it opens the way for the easy fabrication of multimodal sensing systems on highly flexible and compliant substrates.

The field of organic electronics holds tremendous potential for applications that benefit from the use of organic materials, (e.g. very low cost, flexible and amendable to large-area processing techniques or roll-to-roll printing). Specifically, the design and development of sensors that take advantage of these benefits can lead to manufacturing of cheap electronic units for electronic skin as well as medicinal, food storage, and environmental monitoring applications. The ability to couple the sensory electrical output with on-chip signal processing can overcome the need for bulky, expensive equipment typically required for most optical detection methods. In order to attain commercial viability, chemical sensors based on organic electronics must continue to address the remaining issues in repeatability, reproducibility, stability, and selectivity. In this talk, I will present recent progress in materials and fabrication of chemical, biological and pressure sensors.

Pressure sensitivity and mechanical self-healing are two vital functions of the human skin. Electronic skins are rapidly approaching human skin-like form factors and sensing performance[1-4], yet, the ability to repeatably self-heal has not been demonstrated in electronic skin systems. In order to enable electrical and mechanical self-healing capability, we describe a composite material composed of a supramolecular organic polymer with embedded nickel nano-structured micro-particles[5]. The composite shows mechanical and electrical self-healing properties under ambient conditions. The electrical conductivity of the composite can be tuned by varying the amount of nickel particles and can reach values > 10 S cm-1. The initial conductivity can be repeatably restored with ~90% efficiency after 15 s healing time, whereas the mechanical properties are completely restored after ~10 minutes. In addition, we show that our material is also pressure and flexion sensitive, with resistance varying inversely with applied flexion and tactile forces. These results demonstrate that natural skin's repeatable self-healing capability can be mimicked in conductive and piezo-resistive materials, thereby potentially expanding the scope for applications of current electronic skin systems.
References
1. Mannsfeld, S. C. B., B. C-K. Tee, Z. Bao, et al. Highly sensitive flexible pressure sensors with microstructured rubber dielectric layers. Nature Materials 9, 859-864 (2010).
2. Lipomi, D. J., M*. Vosgueritchian*, B. C-K. Tee*, et al. Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes. Nature Nanotechnology 6, 788-792 (2011). *equal contribution
3. M. Ramuz, B. C.-K. Tee, J. B.-H. Tok & Bao, Z. Transparent, Optical, Pressure-Sensitive Artificial Skin for Large-Area Stretchable Electronics. Advanced Materials 24, 3223-3227 (2012).
4. T. Sekitani, T. Someya, Stretchable, Large area Organic Electronics. Advanced Materials 22, 2228-2246 (2010).
5. B. C-K. Tee*, C. Wang*, R. Allen, Z. Bao, Nature Nanotechnology, accepted.

In this study we present a simple novel approach to prepare ultra-compliant thin metal film conductors. In contrast with previous studies on bulk elastomer substrates, stiff metal thin films (25 nm-thick Au), are here coated on an ultra-soft, flexible polyurethane foam substrate.
The foam substrate is prepared with a manual bar coater to form membranes of about 500µm thickness. Thin gold films are evaporated directly onto the foam. The heterogeneous substrate is an open-celled foam, with void diameters nearly matching the overall membrane thickness, and capped on both sides with a micrometer thick polyurethane skin. This particular “bubble” organization confers highly anisotropic mechanical properties on the surface of the foam. We demonstrate experimentally that upon uni-axial stretching, all the stress is localized in the membrane that rests above the bubbles, while the membrane above the ligaments is almost stress-free. As a result, the metallic film cracks dramatically above the bubbles and a stable percolating path is formed in the gold above the foam ligaments.
This strain localization and symmetry-breaking process results in thin-film metal conductors on foams with enhanced strain to failure and reduced resistance change with strain compared to metal films patterned on homogeneous substrates.

Stretchable electronics proposes circuits and devices that can be repeatedly stretched and return to their original shape just like a rubber band. Even significant mechanical deformations up to 100% uniaxial strain should not affect their electrical performance. These requirements are challenges for most device materials since they fracture at small strains of 1% due to their brittle nature. Some materials such as metals are ductile but they deform plastically and therefore irreversibly. Therefore a reliable way of producing stretchable electrodes that do not alter their performance due to stretching is required.
Here an approach to mechanically highly compliant thin metal electrodes is presented. Metal electrodes are evaporated onto an ultrathin and ultra-flexible polymer substrate. A 1.4 µm thick PET polymer foil is used as the substrate for the metal films and transferred on a pre-stretched elastomer. Upon relaxing, the metal electrodes on the thin PET substrate form wrinkles. We show that such 100nm thick aluminium, copper, gold and silver electrodes can be reversibly stretched uniaxial to 50% for 1000 and even several thousand cycles without failure. Their electrical resistance does not change significantly with cycling, paving ways for making any metal film mechanically stretchable. Further the combination of metal conductors and the ultrathin substrate enables new ways of heat management.

Conducting polymers (CPs) are organic macromolecules with electrical conductivity on the same order as inorganic semiconductors or metals. In addition, CPs exhibit the traditional advantages of polymeric materials: they are light, easy and inexpensive to synthesize, amenable to blending and copolymerization with other materials, and may be doped with a variety of compounds that can be adsorbed and released under desired conditions. As a result, CPs have been investigated for use in implantable biosensors, organic solar cells, water-purification membranes, artificial muscles, advanced textiles, batteries, flexible electronics, and clinical devices for delivering biologically active compounds. However, the brittleness and poor long-term stability of CPs have greatly impeded their widespread application.
To reduce the inherent brittleness of CPs, we have synthesized blends with polyurethane (PU) using emulsion polymerization. The improved mechanical properties of this composite material allow it to be processed into a range of morphologies (skins, films, foams, and fibers). Dynamic light scattering and UV-vis spectroscopy were used to monitor the growth of conductive polypyrrole nanoparticle domains inside the insulating polyurethane matrix, and thereby optimize the synthesis process. The maximum conductivity of the composite material is 0.33 S/cm, the Young&’s modulus is 0.616 MPa, and the elongation at break approaches 200%.

Electronic skins are readily formed by dipcoating objects in dissolved polypyrrole-polyurethane or by spincoating. The material was found to display widely tunable mechanical and electrical properties, depending on the solvent used to dissolve the composite. More polar solvents, such as chloroform and hexafluoroisopropanol, produced ohmic electrochemical behavior, while less polar solvents such as dimethylformamide and tetrahydrofuran yielded capacitive behavior. The observed relationship between conductivity and mechanical strain indicates that the material can be used as a strain sensor. Furthermore, the use of biocompatible polyurethane has allowed us to generate functional biointerfaces using this composite material.

Electroless deposition (ELD) is a chemical process to metalize surfaces of a variety of insulating materials such as glasses, ceramics, and polymers. It is widely used in industrial applications such as microelectronics, printed circuit boards, packaging, corrosion prevention, etc. Especially, this method could have a potential impact for forming interconnects on soft and/or stretchable substrates such as electronic skins and flexible displays. ELD must be preceded by the deposition of a catalytic seed layer for ELD over the substrate. In general, this seed layer deposition is not a simple process and requires the use of complicated, time-consuming, and often environmentally unfriendly procedures. For example, the first developed process consisted of two steps of surface sensitization and activation using stannous chloride, palladium chloride, and hydrochloric acid solution. In order to improve this seeding process, various deposition techniques have been proposed such as sputtering, MOCVD, laser-assisted deposition, atomic layer deposition (ALD), etc. Although these techniques showed improvements and advantages over the traditional processes, they suffer from one or more of those problems: substrate surface treatment, complexity of the tool, coverage or conformability of deposited seed layer, etc. In this presentation, we show that arc plasma deposition (APD) is an effective and simple method to deposit the seed layer for ELD on surfaces of insulating materials. APD is a simple and dry process for direct deposition of metal nanoparticles and eventual forming of metallic thin films with excellent thickness & flatness, and adhesion control. We investigated the APD of Pd on polyimide and silicone rubber surfaces as seed layer for ELD and characterized thickness development, resistivity, adhesion strength, and stretchability of subsequent Cu films grown by ELD on top of the seed layers.

Application of organic electronics in biological applications such as sensing, therapeutics, and prosthetics to expand the human/machine interface has become a widely investigated area of research. As a bio-inspired device, electronic skin (e-skin) interfaces electronics with human touch by sensing pressure, pain, temperature, and chemical and biological signals. Specifically, the organic field-effect transistor (OFET) device can be employed as the basis for e-skin and would greatly benefit from the capacity to stretch, bend, and fold; potentially allowing integration with moving parts, malleability to a variety of complex (i.e. non-planar) surfaces, and the ability to be easily transported. Stretchable and flexible e-skin would enable wearable systems for personal health monitoring, wound-healing, and ameliorate loss of a person&’s sense of touch. While strides have been made toward generating flexible devices, stretchable devices have been more difficult to achieve. Semiconductors are a critical component for organic electronic devices and specifically, function as the active layer in OFETs. For functional biocompatibility (i.e. in e-skin), semiconducting materials that are inherently stretchable or elastic must be generated.
We have modified high performance semiconductors to achieve much higher stretchability while not greatly degrading the mobility. We will report the mechanical and electrical properties in this talk. We will show results of a stretchable transistor.

Flexible, foldable, and stretchable metallic interconnects are inevitable elements to the realization of electronic skins. Current fabrication methods are based on mask lithography and etching technologies that are slow and expensive. This talk will discuss on our recently developed printing method, namely “Matrix Assisted Catalytic Printing” (MACP), for the solution-based fabrication of metallic conductors that are high performance, and compatible with flexible, stretchable, and wearable circuits. In this method, catalytic salts are printed, with the aid of polymer matrixes, by various printing technologies including screen printing, inkjet printing, and scanning probe printing on to compliant substrates including plastics, elastomers, and textiles. Electroless deposition is then carried out to form patterned metal interconnects with sizes ranging from sub-micrometer to many centimeter scales. With proper pattern design, the metal interconnects are highly durable in various bending, twisting, and stretching tests.

Stretchable transparent conductors are an essential component of next generation optoelectronic devices, such as solar cells, displays, and sensors, as they will enable mechanical compliance in these devices and also increase their durability. In this work, we explore the use of films of poly-(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) and single-walled carbon nanotubes (SWNTs) on stretchable substrates for transparent electrodes in next-generation devices. These materials have high intrinsic conductivity, solution processability, flexibility, and the potential for production at low cost. Using PEDOT, we have produced films with a sheet resistance (Rs) of 46 Omega;/sq at 82% transparency (T) that were used as electrodes in the first intrinsically stretchable organic solar cell and as electrodes in transparent, capacitive pressure sensors for mechanically compliant optoelectronic devices. These films were rendered stretchable by depositing them on pre-strained polydimethylsiloxane substrates. Using SWNTs, we have developed transparent, conducting spray-deposited films of the SWNTs that can be rendered stretchable by applying strain along each axis, and then releasing this strain. This process produced films that accommodated strains of up to 150% and are among the most conductive films in the stretched state. We used these films as electrodes in arrays of transparent, stretchable capacitors, which behave as pressure and strain sensors, and can potentially be used as sensors in biofeedback devices and electronics skin.

Realization of the concept of smart skin [1] and interactive textiles fully relies on the development of flexible and stretchable electronics [2]. This vision requires a wide range of electronic components, including interconnects, opto- [3] and magnetoelectronic [4] elements, which are shapeable into nonplanar geometries after fabrication. Ideally, these components should be elastic and withstand many cycles of deformations without degrading in performance. Various functional elements that fulfill these ambitious requirements, e.g. interconnects, transistors, coils, strain sensors [1] have been realized during the last years. However, stretchable magnetic sensorics that can be integrated into smart skins to equip them with magnetic functionalities, currently lack in stretchability (max. 4%) and sensitivity to small magnetic fields [4,5].
In this work we introduce a novel technology platform to realize highly stretchable magnetoelectronics with enhanced sensitivity [6]. In this respect, we successfully fabricated the world&’s first stretchable spin valve sensor, which is an advanced magnetoresistive element superior to classical giant magnetoresistive (GMR) multilayers. Random wrinkling and a periodic fracture mechanism are used to create a highly compliant meander-like microstructure within the rigid magnetic nanomembrane. This approach is unique, as the fracture pattern is pre-determined by an additional wrinkling phenomenon that occurs due to a thermally induced prestrain in the elastomer support. Upon stretching, the GMR ratio is maintained until the electrical contact breaks down. The prepared magnetoresistive elements achieve a high stretchability of up to 29% with a top sensitivity of 0.8% Oeminus;1 in a magnetic field of 12 Oe under ambient conditions. This qualifies these sensors to be appropriate for smart skin applications, where response to small magnetic fields is necessary. In cyclic loading experiments the reliability of the prepared elements is confirmed, as resistance and GMR performance remain constant over more than 500 loading cycles.
In order to overcome lithographic limitations on elastomer surfaces, transferring of structured functional nanomembranes need to be considered [7]. The possibility of a direct transfer of magnetic sensorics from rigid supports to soft membranes will be in the scope of the presentation. This renders magnetoelectronics to be fully integrated into stretchable electronics systems like smart skins and textiles.
The work is supported in part via the German federal ministry of education and research (project Nanett; FKZ: 03IS2011).
References:
[1] D. H. Kim et al., Science 333, 838 (2011).
[2] D. H. Kim et al., Science 320, 507 (2008).
[3] R. H. Kim et al., Nature Mat. 9, 929 (2010).
[4] M. Melzer et al., Nano Lett. 11, 2522 (2011).
[5] M. Melzer et al., RSC Adv. 2, 2284 (2012).
[6] M. Melzer et al., Adv. Mat. DOI: 10.1002/adma.201201898 (2012).
[7] A. Carlson et al., Adv. Mat. 24, 5284 (2012).

There are lots of different approaches to produce topographically patterned surfaces for multiple applications such as for electronic applications, optical purposes or templating. Common procedures are often based on lithographical approaches with several drawbacks like high costs, toxicity and up-scaling. Our lithographic free approach to prepare periodic surface patterns with a defined gradient in wavelength is based on thin film instabilities, respectively wrinkling.
According to the well-known dependency of wrinkle wavelength and amplitude on film thickness and E moduli of the film and substrate, oxidation of a stressed PDMS gradient material leads to a surface with continuously changing topography, when relaxed [1, 2]. Although theoretical studies concerning the elastic buckling of PDMS gradient materials have been carried out PDMS gradient materials with a continuously changing wrinkle wavelength were hitherto unavailable. We report on a gradient surface wrinkled PDMS material based on variation of the substrates E moduli [3]. Moreover, the longitudinal gradient is prepared on a centimeter scale that allows macroscopic surface patterning with sub-micron structures.
Poly(dimethyl siloxane) with a compositional gradient was fabricated via a precision syringe pump setup. Stretching of the substrate and subsequent oxygen plasma oxidation resulted in a continuously changing wrinkle wavelength on the surface upon relaxation. This approach is a powerful tool for designing gradient surfaces with tailored topography. The setup allows the preparation of different PDMS gradient geometries with high reproducibility. This new lithography-free tool could be very interesting for the design of tailored gradient surfaces and their potential application for diffraction gratings, microlenses, microfluidics and cell adhesion studies.
In summary, we developed a facile preparation method for macroscopic surface-wrinkled PDMS gradient materials. The continuously changing E-modulus of the PDMS gradient substrate was transferred into a stepless varying wrinkle wavelength of the SiOx like surface.
1. Schweikart, A. and A. Fery, Microchimica Acta, 2009. 165(3-4): p. 249-263.
2. Schweikart, A., et al., Soft Matter, 2011. 7(9): p. 4093-4100.
3. Claussen, K.U., et al., RSC Advances, 2012.

A visible trend over the past few years involves the application of conducting polymer devices to the interface with biology, with applications both in sensing and in actuation. Examples include biosensors, artificial muscles, and neural interface devices. The latter are of particular interest, as conducting polymers offer several distinct advantages compared to incumbent technologies, including mechanical flexibility, enhanced biocompatibility, better signal-to-noise ratio and capability for drug delivery. As such, they promise to yield new tools for neuroscience and enhance our understanding on how the brain works. After a brief introduction, I will present a few examples of implantable arrays which utilize conducting polymer devices. Their in vivo performance, electrical characteristics and properties such as mechanical flexibility and biocompatibility will be discussed.

A process to fabricate reliable, biocompatible organic thin film transistors on responsive smart polymer substrates by full photolithography was developed. Since the increasing ability to ever more precisely identify and measure neural interactions and other phenomena in the central and peripheral nervous systems is revolutionizing our understanding of the human body and brain. To facilitate further understanding, more sophisticated neural devices, perhaps using microelectronics processing, must be fabricated. Materials often used in these devices, while suitable for these fabrication processes are not optimized for long-term use in the body and are often orders of magnitude stiffer than the tissue with which they interact. Using the smart polymer substrates described in this work, suitability for processing as well as chronic implantation is demonstrated. We explore how to integrate circuitry onto these flexible, physiologically responsive, biocompatible substrates that can withstand the environment of the body while softening to decrease the mechanical mismatch with the body tissue. To increase the capabilities of these devices beyond individual channel sensing and stimulation, active electronics must be included into our systems. In order to add this functionality to these substrates and explore the limits of these devices, this process enables mobilities up to 0.2 cm2/Vs and threshold voltages close to 0V. The transistors were measured on air as fabricated and under mechanical stress conditions, while being bent and after, they can also withstand up to a 100 bending cycles to a radius of curvature of 8.5mm with minimal impact in the electronic properties. These devices enable the development of new applications with more complex circuitry since they are fully fabricated by photolithography process, this showing the compatibility of the substrate smart polymer with other possible applications. Also, to accomplish the long-term use of this technology we use the softening smart polymer as an encapsulation layer, either by itself or combined with other inorganic materials such as Al or Al2O3. This encapsulation is done in order to allow the organic electronics to survive in the body environment. Devices were also measured on air and after exposure to saline at different time intervals. This work will help with the understanding of fundamental problems for biocompatible, long-term electronic devices implants, leading to a new set of tools and devices that will help understand problems in the neuroscience and materials research.

A major challenge in the field of neuroscience is the ability to measure nervous system communications during chronic (>1 year) studies. Recording fidelity can be adversely affected by the mechanical mismatch between the interface and soft nervous system tissue that leads to scar tissue encapsulation of the electrode. Here, the development of chronic, central and peripheral nervous system interfaces based on thiol-ene “click” chemistry will be discussed. The thiol-ene network can be tuned to have tailorable thermomechanical properties ranging from elastomeric to glassy at body temperature. The neural interfaces are fabricated using full photolithography on a thermo- and physiologically responsive polymer substrate with a maximum feature size of 100 mm and a minimum feature size of 5 µm. Changes in surface morphology as a function of processing temperature are studied with optical profilometry. Additionally, these devices are fabricated using a novel flexible electronics processing technique, transfer-by-polymerization, that enables the incorporation of advanced electronic materials such as carbon nanotubes. This material coupling enables devices to be scaled down without sacrificing the stimulating and recording capability at the electrochemical interface as well as enhance the mechanical performance of the electrodes. These smart neural interfaces are stiff enough for insertion (1 GPa), soften in vivo to approach the modulus of cortical tissue (10-100 kPa), and maintain electrical functionality. The change in thermomechanical properties of the polymer substrate and electrochemical properties of the electrode are studied in vitro. Intracortical electrode arrays have recorded driven neural activity for greater than 8 weeks in the auditory cortex of a rat. This smart, softening polymer neural interface shows promise for neuroscientists to study the chronic effects of neural plasticity and nerve regeneration.

This work demonstrates the integration of novel polymer nanorods coated with Titanium Nitride (TiN) to form electrodes with enhanced electrochemical surface area (ESA) into softening neural interfaces. Fabrication of these electrodes is carried out using reactive ion etching surface modification of specially designed polymer surfaces, onto which TiN is subsequently reactively sputtered. Functional neural devices, such as cortical brain probes, are fabricated through standard photolithography techniques adapted for flexible electronics. The resulting neural interfaces offer injection levels an order of magnitude higher than conventional TiN electrodes. Softening devices, in response to physiological environment, have been fabricated in the past. However interface materials typically have not met the electrochemical requirements for chronic applications. High levels of charge transfer between the electrode and tissue, as well as low impedance, are essential to safely stimulate the appropriate cells or nerves. Integration of nanostructured materials into neural interfaces provides a method to increase the electrochemical performance of neural interfaces. The inherent morphology of this type of material can dramatically increase the ratio between ESA and geometric surface area. Electrochemical characterization is performed to characterize injection capacity levels and interface impedance. The surface morphology is investigated with probe and electron microscopy to correlate its morphology with its electrochemical properties. X-ray photoelectron spectroscopy is used to analyze the chemical environment of the electrode surface and its chemical stability. The performance assessment of electrode materials for smart softening substrates indicates its potential for chronic applications. The synergetic integration of new electrode materials on smart softening polymers is crucial for the advancement of neuroscience and the development of neuroprosthetic devices.

Wearable electronics are coming into high demand for next generation electronics. In fact, there are a number of demonstrations of flexible electronics applied to artificial electronic skin (e-skin) and wearable electronics using organic or inorganic materials. We have reported e-skin based on nanowire and nanotube thin film transistor (TFT) active matrix backplanes on flexible and stretchable substrates that can detect tactile pressure distribution like the human hand(1,2). However, for wearable electronics, different types of sensors as well as signal processing circuits are required to be integrated depending on the applications.
To address this, in this presentation, we report macroscale (7×7 cm2) multi-functional e-skin (16×16 arrays), which can detect tactile pressure, temperature, and light intensity on a mechanically flexible active matrix backplane. For the active channel material in our TFTs, we used a carbon nanotube network film, resulting in high mobility TFTs (>30 cm2/Vs), which allow us to operate the device at low voltage. The sensitivities of the tactile and temperature sensors are ~16 %/kPa and 2.6 %/oC, respectively. For the light imaging sensor, due to low drive current, an amplifier on each pixel was integrated to increase the output current up to >100 mu;A at VDS=3V. The device shows high uniformity for each sensor and good mechanical flexibility.
This work presents the largest integration (1024 transistors and 768 sensors) of nanomaterial-based active components for a functional flexible system. The multi-functional artificial skin sensor presented here is one example to prove multi-active component integrations on flexible substrates.
Reference
(1) K. Takei et al. Nature Materials 9, 821-826, 2010.
(2) T. Takahashi et al. Nano Letters 11, 5408-5413, 2011.

Electrophysiological signals control movement of muscles in our body and transfer information from the central to peripheral nervous system or vice versa. The generation of abnormal electrophysiological signals and their distorted influence to surrounding organs, therefore, induce serious health risks, such as arrhythmia and epilepsy. Mapping electrophysiological signals and using them for appropriate therapies are key factors in many clinical procedures and surgeries. Besides electrophysiology, other physiological signals that represent conditions of various body parts are also important and thereby monitoring them, and sometimes applying feedback actuation, is in significant importance. High performance electronics, including sensors and actuators, that can be integrated with soft and curvilinear body parts are one of the most crucial tools in medicine for diagnosis and therapy. High quality single crystal inorganic materials are conventionally used in high speed electronics for the advantage of high intrinsic electron and hole mobilities. However, biological systems are soft and curvilinear, which generates the mechanical mismatch between electronics and target tissues. Materials processing and integration technologies to solve this mechanical mismatch was developed and applied for implantable biomedical devices and skin mounted electronics. The concept of this approach is also demonstrated in in-vivo and in-vitro animal experiments. The resulting bio-integrated electronic technologies contribute for important improvements in diagnostic and therapeutic surgical tools.

Musculoskeletal monitoring is important for primary and secondary prevention of postural related injuries and improving efficiency and accuracy of physical therapy. To-date musculoskeletal assessment is highly dependent on personal skills of specialists and technicians due to the lack of electronic tools that allow real-time and accurate measurements. In line with recent advances in electronic skin and textile technology, this work presents a novel strain-sensing nanocomposite electronic textile embedded in thin stretchable elastomer such as polydimethylsiloxane (PDMS) that can be adhered to human skin for monitoring of planar strains caused by muscle and joint movements. The strain-sensing electronic textile comprises of composite core-shell nanofibers fabricated by scalable electrospining process, demonstrating a piezoresistive longitudinal gauge factor of up to 60 under 2% tensile and compressive strains. The strain response is tested under repeated uniaxial stretching of up to 100% demonstrating the stretchability of the textile. Accurate monitoring of biaxial strain is achieved by patterning the strain-sensing textile and placement of several electrodes for controlling the direction of current flow with respect to the applied strain. We demonstrate that by using an array of strain sensor devices integrated in an electronic textile that follows the anatomical structure of the joint and muscles under study, the textile can provide critical information regarding the range of motion and monitoring of postural condition with accuracy and repeatability. This work highlights the applicability of electronic textile technology as a strong tool for real-time assessment of muscle movements in real-life situations, providing feedback for improved management of postural related musculoskeletal problems and enhancing clinical applicability to physical therapy.

Cortical microelectrodes that can provide an electrical interface with neural tissue are promising for the treatment of patients suffering from neurological deficits. However, the full clinical exploitation of such neural interfaces has been difficult due to limitations of their effective lifetime on account of scar formation and neuronal death around the implanted electrode. One hypothesis is that the mismatch of the mechanical properties of the electrode and the brain tissue is a significant contributor to these events. We recently developed a new approach to chemically-responsive, mechanically adaptive polymer nanocomposites, which are initially highly rigid, but soften considerably upon exposure to physiological conditions and aqueous environments in general. These materials mimic the architecture and function of the outer dermis of sea cucumbers, which can change its stiffness of its skin when needed. The artificial nanocomposites are comprised of a polymer matrix and rigid, high-aspect ratio cellulose nanocrystals. The cellulose nanocrystals form percolating networks within the matrix polymer, wherein stress-transfer is enabled though interactions between the nanocrystals. Upon modest swelling with water, the reinforcing cellulose network is disrupted, presumably due to competitive hydrogen bonding to water. This results in a dramatic modulus reduction. The current understanding of the structure-property relationships in these materials will be discussed. Initial histological evaluations suggest neural prosthetics based on such mechanically adaptive materials can more rapidly stabilize neural cell populations around the implant than rigid reference systems.

With the emergence of distributed and wireless sensor technology readable tags will be able to collect a vast set of data that can be processed to provide new information. Such information could be extremely important for security, health care and location applications. Some examples include mission critical environments such as power plants, airports, military bases and depots, refineries, and access restricted areas to provide the highest quality of security to record trends and provide immediate required actions. In these environments as well as health care, identifying, tracking and monitoring people is vital to interface different services to create a more resilient system.
Current human tagging and monitoring technology, external to the body, typically requires wrist bands or ID batches, electrodes to be mounted on the skin via adhesive tapes, straps, or penetrating needles, often aided by a conductive gel, with terminal connections to separate boxes hosting circuit boards, power supplies and communication components. Solutions which compromise security (bands and badges could be remove and given to another person) and are poorly suited for practical applications outside of research labs or clinical setting. Long and short-term skin mounted electronics, such as UHF RFID tattoo tags, could provide an effective solution to many of the issues described above. However, due to the high relative permittivity of human body tissues, to date there is one example of RFID tag mounted directly on skin.1
To date the electronics industry largely relies on PBC fabrication methods such as photolithography and screen printing. Methods which are time-consuming, expensive and environmentally detrimental. Inkjet printing provides a more versatile, substrate insensitive, eco-friendly, highly scalable technology to prototype and mass produced electronics on plastics, elastomers and paper. The choice of substrate, paper instead of expensive silicon, combined with the inexpensive fabrication method, inkjet printing, will provide a step towards the commercialization of low-cost on-skin electronics.
Here we report the performance of inkjet printed UHF RFID transfer tattoo tags mounted on skin. The importance of antenna electrical properties and image quality will be discussed. The effect of silver nanoparticle ink formulation chosen, printing methodology, substrate treatment and sintering method (thermal, Ar plasma, photonic curing) on the electrical and image quality properties of the RFID antenna will be discussed. Additionally, we will present the on-body performance of various UHF RFID tags designs including read range and mechanical robustness. Finally, properties, performance and price of inkjet printed antennas versus etched and screen printed tags will be presented.
1. M. A. Ziai and J. Batchelor, IEEE Transitions on Antennas and Propag.,2011, 59(10), 3565.

Future generations of robots, electronics, and assistive medical devices will benefit from components that are soft, enabling adaptation in unstructured environments and more natural interaction with biological systems. This will require soft active materials with electrical and mechanical functionalities for actuation, sensing, and control. As the demand for increased elasticity of electrical components heightens, the challenges for functionality revert to basic questions of fabrication, materials, and design for liquid-elastomer composites. This talk will highlight several designs for soft sensory skins (including strain, pressure, curvature and shear sensors) and circuits based on a liquid-embedded-elastomer approach. In addition, we will discuss fundamental interfacial processes between conductive Gallium-Indium alloys and stretchable elastomers that may impact future soft MEMS manufacturing.

Soft robots often rely on the actuation of elastomeric materials typically driven by high voltage. We show, that such large deformations can be achieved by using the large volume change occurring in liquid gaseous phase transition. This transition is simply induced by electrical Joule heating, resulting in a low-voltage driven actuation, compared to dielectric elastomer actuators. By combination of several phase-transition-actuators, linked with flexible connections, soft machines like grabbers or robots are designed. Due to the low-voltage operations, these machines can abstain from external power supplies or other kinds of driving systems. Furthermore the recently shown phase-transition-actuator is combined with elastomeric polymer opal films that change their colour when mechanically stretched, to control and modify the appearance of a surface.

Active robotic or prosthetic hands, e.g. i-limbTM from Touch Bionics or BioTac® from SynTouch LLC, are equipped with tactile sensors mainly located at the fingertips. The latter allow for surface detection and/or texture recognition but do not provide sufficient sensory feedback to enable accurate object&’s manipulation. Here we propose a stretchable artificial skin designed to coat a robotic finger, i.e. two phalanges and two knuckles so that it can accommodate nicely repeated finger movements and monitor contact in multiple locations along the finger.
The sensory skin is a multilayered structure formed of four layers of soft polyurethane PU foam and sub-micron thick metallization. Capacitive sensors are distributed along the finger, i.e. on three sides of the phalanges (two lateral sides and bottom) and above the knuckles. The ground and AC shield planes are patterned on the first and third PU layers, while active sensor electrodes are patterned on the second PU membrane. Lastly, a fourth layer of foam encapsulates the structure.
The whole thickness of the skin ranges from 1.5 to 3 mm. The skin integrates the compliant capacitive sensors with 3.5x1.5 mm2 surface area, and their readout electronics, developed on a flexible substrate (i.e. polyimide). The main steps of the microfabrication process include (1) manufacturing of sub-millimeter thick PU membranes using a bar coater, (2) evaporation and patterning of thin Ti/Au (titanium/gold, 5/25nm thick) films for ground, AC shield and active electrodes, (3) PU foam multilayer assembly using a polyurethane glue, and (4) bonding of the electronic flexible circuit board.
Preliminary characterization of the capacitive sensors shows that they are able to detect contact events. In particular, light contact with a plastic or metallic objet increases the sensor capacitance by 40% with respect to its initial capacitance value, C0 (i.e. no contact event). When the skin is stretched by about 10% strain, C0 increases by about 5% and similar light contact is reliably detected.
Ongoing experiments are focused on extensively characterizing, both statically and dynamically, the skin sensors, when the skin is deployed flat, stretched flat and mounted on a moving robotic finger.

Recently, electronic skin has received a lot of attention for its application in robotics and human prosthetic. Although there are several senses that human skin can detect, touch or pressure sense have been widely implemented in artificial skin, where pressure sensor array has been typically used. Fabrication methods for flexible pressure sensor array are generally based on two types of technology for electronic skin application. Piezoresistive type conductive composite and capacitive-type pressure sensitive flexible sheet are generally used. However, both methods need active-matrix or pixel separation process to reduce cross talk problem and improve sensing resolution. As a result, these methods involve complex and high cost process.
In this study, we have implemented flexible pressure sensor array based-on conductive nickel composite materials by using patterning method based on magnetic field. When conductive materials are embedded in elastomeric medium, its electrical resistance changes with dimensional change with applied pressure. However, when conductive composite material is used in an array form each pixel needs to be separated in order to reduce cross talk. For simple but high-resolution patterning method, we developed magnetically patterning method. Our pressure sensor is composed of poly (dimethylsiloxane) (PDMS) matrix and nickel conductive fillers. Using engineered magnetic field, we can simply pattern conductive filler that is embedded in PDMS. During magnetic field exposure nickel particles show vertical particle alignment and lateral movement (or patterning) at the same time. Magnetically patterned sensor shows better sensing performance than others. Pressure sensitivities are 2mS per kPa. A pressure sensor sheet that has separated 481 pressure sensors in 40 mm2 area without active device was made. It has 1.77 mm tow-point touch threshold, which is comparable to that of human finger tip&’s (typically 2~3 mm). We also used inkjet-printed silver electrodes to provide top and bottom electrodes for the sensing elements in the form of vertical and horizontal cross lines. Pressure sensor pixel is located at each crossing point of the silver electrodes. Our simple patterning and printing process can be easily applied to scaling up of the sensor arrays on flexible and stretchable platform.
This research was supported by the Converging Research Center Program funded by the Ministry of Education, Science and Technology (No. 2012K001368)

Majority of the pressure sensors use either capacitive or resistive touch sensors; however, only a few types of them have been reported on flexible / stretchable substrates.
Besides pressure sensing, it is also desirable to have stretchable touch sensors for better mechanical durability. Stretchable pressure sensors are applicable to curved surfaces, making it useful toward robotics and biomedical applications. One challenge with stretchable electronic pressure sensors is the limited availability of high performance stretchable conductors and semiconductors needed for device fabrication.
Optical pressure sensors are highly responsive and are unaffected by surrounding parameters such as electronic noise, humidity, temperature etc. As the above qualities are attractive for integration toward artificial skin applications, we report herein a new optical pressure sensor amendable for large area fabrication to constitute artificial skin. Our new type of stretchable optical pressure sensor is highly robust and transparent. It consists of waveguides made from polydimethylsiloxane (PDMS), integrated with polymer light-emitting diode light sources and polymer photo-detectors. Nanoscale gratings were embossed for coupling light in and out of the elastic waveguide. The sensor transduces mechanical deformation to a change in the optical intensity at the output of the waveguide, thereby providing a simple and straightforward method to detect changes in mechanical forces. An optimum sensitivity of 0.2 kPa-1 with low hysteresis was observed [1]. Besides its stretchability and high sensitivity, the device presents the advantages to be light-weight, conformable and low-cost.
1. Ramuz, M., et al., Transparent, Optical, Pressure-Sensitive Artificial Skin for Large-Area Stretchable Electronics. Advanced Materials, 2012. 24(24): p. 3223-3227.

Softness is a key aspect in Nature as it enables deformation, which in turn provides vital functions in animals and plants. In natural mechanotransduction, the environment provides mechanical stimuli, the stimuli cause tissues and membranes to deform and deformation gives rise to tactile functions. In addressing artificial skin development soft skin-like tactile sensors are pursued. Such sensing systems must be built with materials that offer intrinsic features like full flexibility, compliance, extensibility and robustness, to let them play the twofold role of fully complying with the environment they come in contact with and, at the same time, of conforming locally to curved surfaces. Since a few years, worldwide research presents superb combinations of materials able to meet the goals mentioned with increasing standards. In parallel to mechanical behaviour, the type of tactile information pursued in biorobotics goes from pressure and strain, to directional force, surface roughness and shape of the object with which the tactile sensing system interacts. Today all these functionalities cannot yet be met with the use of intrinsically smart materials and thus there is an unbroken quest for new smart concepts which make use of soft materials. In our group, we investigated mainly two approaches. One concerns an optical transparent smart electronic skin. The transduction principle is based on the travelling of electromagnetic waves through a waveguide, mediated by mechanical stimuli. The system integrates optical components with a soft and transparent material, like PDMS, to implement a highly flexible and conformable thin sensing skin, that can be built in various 2D shapes. The distribution of contact and pressure exerted from an external object detected, and information about the shape of the contacting object can be resolved by means of ad hoc processing algorithms. The second approach concerns the design of soft and flexible tactile sensors making use of conducting polymers or CNTs as electrodes in strain sensors and in capacitive sensor layouts to retrieve both normal and shear forces which originate at the interface with environment. This poses several requirements to the soft and said conductive materials that need consideration. Concluding, the development of functional artificial skins must take advantage of investigations performed on improving the skin-like characteristics of the building materials and, concurrently, of tactile sensor designs which allow the detection of a number of tactile cues needed in robotics. The creative integration of knowledge between such two main research fields holds the potential for a future leap forward in the development of innovative artificial tactile skins.

In this presentation, we report the development of a new force sensor that has an elastic modulus (2 MPa) comparable to human skin. The force sensor is capable to measure both normal and shear forces, and is therefore well-suited for various biomedical applications, e.g., assistive technologies, robotics, and prosthesis. The sensor consists of two perpendicular microcracked gold conductors on the silicone poly(dimethylsiloxane) (PDMS). The two gold conductors are electrically insulated from each other and encapsulated by PDMS. The contact to the gold conductors is made via metal wires using silver paste. A normal force applied on the sensor causes a compressive stress on the PDMS, thus the gold conductor, in the direction of the force. Because PDMS is incompressible, a tensile stress develops in the perpendicular direction. This tensile stress leads to a tensile strain, which causes the resistance of the conductor to increase. The increase in resistance is proportional to the magnitude of the normal force. In addition to being soft and compliant, the sensor is more sensitive than traditional metal sensors (i.e., has a larger gauge factor) because the increase in resistance is not only caused by changes in the physical dimensions of the conductor, but also by the lengthening of the microcracks. The novel sensor also allows the determination of shear forces. A shear force causes a differential increase in resistance of the two perpendicular conductors because one conductor is sheared along its axis, while the other one will be sheared in the perpendicular direction. The difference in resistance increase between the conductors is a good measure of the shear force. We will present details on the fabrication process as well as normal and shear force measurements, and algorithm development.

In this paper we present a highly flexible and compliant system that can be employed for reproducing the sense of touch. The system is based on organic field effect transistors fabricated on thin plastic substrates by means of inkjet printing. We will demonstrate that morphological deformations induced on the organic semiconductor by an external mechanical stimulus leads to a reversible and reproducible change in the output current of the fabricated devices. Different plastic substrates have been tested such as PET, PEN and Kapton®, with thicknesses as small as 13 mu;m, leading to a very high flexibility and compliance over curvilinear surfaces. All devices have been fabricated using a solution processable organic semiconductor, TIPS-pentacene, deposited by means of inkjet printing and drop casting. All electrodes have been patterned by inkjet printing using a conductive ink based on silver nanoparticles.
In order to fabricate over large areas systems able to reproduce the sense of touch, matrices of inkjet printed OTFTs have been fabricated. In particular we have employed a common source configuration with 8 independent gate electrodes, one per row, and 8 independent drain electrodes, one per column. Therefore, each single element of the matrix can be addressed independently of the others. Before performing the electromechanical characterization, the flexible matrix has been embedded into a thin PDMS film in order to recreate the consistency of the human skin and to protect the sensors from fatal damages. For the electro-mechanical characterization, performed by applying gate voltage pulses in order to minimize bias stress effects, a mechanical indenter with different shapes (i.e. different curvature radii) has been used to apply the deformation.
The results showed a linear and reproducible response for applied forces ranging from 0 to 1N, with a remarkable resolution of 0.05N. Moreover, reliability tests have been also performed in order to evaluate the durability of the fabricated system, revealing a good stability after more than 1000 cycles for 1N of applied force.
This system can be successfully employed as electronic skin for humanoid robots, and because of its flexibility and compliance it is suitable for surfaces with different shape, both flat and curvilinear, and different stiffness, i.e. rigid as well as soft.
In addition we will present results on the realization of ultra-low voltage strain sensors that can be operated at 1V. All devices have been fabricated on a flexible plastic substrate and are based on a proper combination of different, ultra-thin, insulating films (nominal thickness=20nm) employed as the gate dielectric for the final OFET. A full electromechanical characterization of the presented structure will be shown.

In this talk, we will present novel artificial skins that can dynamically change their morphology and roughness under the control of electrical voltage. The skins of many animals use active and dynamic motions and deformations to achieve functions such as perspiration, shedding debris and dirt, and camouflage et al. On the other hand, the surfaces of engineering structures are rarely capable of such activeness and flexibility. Inspired by various active bio-surfaces found in nature and motivated by their unique functions, we develop polymer coatings, or so-call electroactive skins, that can actively and reversibly transform among various patterns such as lines, segments, dots and circles on demand in response to electrical voltages. The actuation mechanism for the electroactive skins is a new type of voltage-induced instability recently discovered in our group. By harnessing the instability and dynamic deformation, these electroactive skins can provide extraordinary functions including antifouling, camouflage and transfer printing. Furthermore, the electroactive skins can be readily made into large areas at relatively low costs.

The integration of diffractive optical elements on stretchable substrates opens the way to the realization of a completely novel class of stretchable photonics systems, characterized by the ability of changing their optical properties upon modification of their shape due to tensile or compressive strain and to be highly conformable to complex surfaces [1]. The interest in such devices is tremendously increasing driven by the need of bio-mimicking adaptive optical devices. Several attempts have been already made to fabricate stretchable diffracting optical elements by depositing reflective metallic films on gratings embossed on PDMS [2]. The bad adhesion of the metallic layer degraded the optical quality of the diffraction grating even upon a very small deformation [2].
Here we present an effective approach to the fabrication of diffractive stretchable optical components, based on Supersonic Cluster Beam Implantation (SCBI) [3]. A stretchable diffraction grating is obtained by replicating the structured surface of a rigid grating on PDMS. The optically active side is then implanted with silver nanoparticles by means of SCBI, in order to create a nanocomposite reflective layer with a thickness of tenths of nanometers. The extremely good resilience upon deformation of the silver-PDMS nanocomposite allows to maintain extremely good optical performance upon substantial deformation of the grating and a large number of deformation cycles.
Optical characterization at a single wavelength shows a good linear proportionality between the stretching of the implanted device (25% stretching) and the decrease of the diffraction angle at the first diffraction order, as expected, due to the increase of the grating pitch. The optical quality of the diffracted beams is good after hundredths of cycles of stretching. As a comparison, a PDMS grating obtained by thermal evaporation of silver atoms shows a poor optical quality prior to stretching because of the observed cracking of the rigid reflecting coating and an increase in intensity of two transversal diffracted spots with a worsening of the diffracted beams quality after few stretching cycles. AFM analysis demonstrates that the implanted grating maintains the diffractive structure of the PDMS replica, while the evaporated one presents many cracks in the reflective layer, mainly in the transversal direction respect to the grooves direction, and a deterioration of the grooves profile.
These results indicate that SCBI is an enabling tool for the fabrication of diffracting optical components on soft, stretchable and highly conformable substrates, preserving their optical properties and qualities after extensive cycles of stretching.
[1] Simonov A.N., et al., Optics Expr. 2007, 15, 9784-9792
[2] Wilbur J.L. et al., Chem. Mater. 1996, 8, 1380-1385.
[3] Corbelli et al., Adv. Mater. 2011, 23, 4504-4508.