Crack-inducing Strain Sensor Array using Inkjet-Printed Silver Thin Film for Underplate and Off-centered Force Sensing Applications.

The change in resistance upon bending in metal films as thick as 1 mm used for underpanel force touch applications is limited by the low sensitivity, thus requiring high-performance readout circuitry. In this paper, we report inkjet-printed silver thin films having crack-inducing underlayers, which further increases the sensitivity of their resistance changes under deformation. This allows for detecting weak vertical forces even through the plates (force-receiving layer), such as 0.4 or 1.2 mm thick polyethylene terephthalate or 0.4 mm thick glass. The underplate sensors will detect a force level as low as 10 gf, which corresponds to the amount of force required for fingerprint recognition. Furthermore, such highly sensitive strain sensors can potentially solve the inaccuracy issue of wearable devices, which can occur when misplaced sensors detect relatively weak biosignals, such as heart rate and blood pressure. The sensor detects the accurate pulse patterns of the wrist artery even though it is off-centered from the artery by 6 mm or larger. The crack-based strain sensor and its usage as a hidden underplate force sensing device will create various wearable and user-machine interface applications.

Nonpatterned Soft Piezoresistive Films with Filamentous Conduction Paths for Mimicking Multiple-Resolution Receptors of Human Skin.

Soft pressure sensors play key roles as input devices of electronic skin (E-skin) to imitate real human skin. For efficient data acquisition according to stimulus types such as detailed pressure images or macroscopic strength of stimuli, soft pressure sensors can have variable spatial resolution, just like the uneven spatial distribution of pressure-sensing receptors on the human body. However, previous methods on soft pressure sensors cannot achieve such tunability of spatial resolution because their sensor materials and read-out electrodes need to be elaborately patterned for a specific sensor density. Here, we report a universal soft pressure-sensitive platform based on anisotropically self-assembled ferromagnetic particles embedded in elastomer matrices whose spatial resolution can be facilely tuned. Various spatial densities of pressure-sensing receptors of human body parts can be implemented by simply sandwiching the film between soft electrodes with different pitches. Since the anisotropically aligned nickel particles form independent filamentous conductive paths, the pressure sensors show spatial sensing ability without crosstalk, whose spatial resolution up to 100 dpi can be achieved from a single platform. The sensor array shows a wide dynamic range capable of detecting various pressure levels, such as liquid drops (∼30 Pa) and plantar (∼300 kPa) pressures. Our universal soft pressure-sensing platform would be a key enabling technology for actually imitating the receptor systems of human skin in robot and biomedical applications.

Revisit to three-dimensional percolation theory: Accurate analysis for highly stretchable conductive composite materials.

A percolation theory based on variation of conductive filler fraction has been widely used to explain the behavior of conductive composite materials under both small and large deformation conditions. However, it typically fails in properly analyzing the materials under the large deformation since the assumption may not be valid in such a case. Therefore, we proposed a new three-dimensional percolation theory by considering three key factors: nonlinear elasticity, precisely measured strain-dependent Poisson's ratio, and strain-dependent percolation threshold. Digital image correlation (DIC) method was used to determine actual Poisson's ratios at various strain levels, which were used to accurately estimate variation of conductive filler volume fraction under deformation. We also adopted strain-dependent percolation threshold caused by the filler re-location with deformation. When three key factors were considered, electrical performance change was accurately analyzed for composite materials with both isotropic and anisotropic mechanical properties.

Negatively strain-dependent electrical resistance of magnetically arranged nickel composites: application to highly stretchable electrodes and stretchable lighting devices.

A novel property of the negatively strain-dependent electrical resistance change of nickel conductive composites is presented. The composite shows negatively strain-dependent resistance change when magnetically arranged, while most conductive materials show opposite behavior. This negative dependency is utilized to produce highly stretchable electrodes and to demonstrate a new conceptual resolution-sustainable stretchable lighting/display device.