Flurin Stauffer

Stretchable Silver Nanowire-Elastomer Composite Microelectrodes with Tailored Electrical Properties.

We introduce a photolithography process compatible with soft and rigid substrates, enabling the fabrication of complex 3D interconnected patterns of silver nanowire (AgNW) networks embedded in polydimethylsiloxane (PDMS). Dimensions of the AgNW micropatterns are controlled within the film plane by photolithography, whereas thickness is controlled via a novel and uniform deposition technique using centrifugation. We report the first systematic characterization of the electromechanical properties of such microelectrodes with finest stretchable feature of 15 μm. We observe a geometry-dependent behavior of the gauge factor not only by changing the thickness of the microelectrodes, as it has been commonly reported so far, but also by varying their lateral dimensions. The presented nanocomposites exhibited sheet resistances down to 0.6 Ω/sq, gauge factors ranging from 0.01 to 100, and stretchability above 50% uniaxial strain. This versatile process allows for the production of highly sensitive strain sensors and robust high-density stretchable conductors on a wafer scale with direct implications in mass production of stretchable electronic devices.

Bright Stretchable Alternating Current Electroluminescent Displays Based on High Permittivity Composites

A high permittivity composite is developed to enhance the brightness of stretchable electroluminescent displays. The unique two-step assembly process yields dense layers, in which the voids around the electroluminescent particles are filled with smaller high dielectric particles. A stretchable seven-segment display based on the composite is bright enough to be used under standard indoor lighting conditions.

Exchangeable Colloidal AFM Probes for the Quantification of Irreversible and Long-Term Interactions

An original method is presented to study single-colloid interaction with a substrate in liquid environment. Colloids, either in solution or adsorbed on a surface, are fixed by suction against the aperture of a microchanneled atomic force microscopy cantilever. Their adhesion to the substrate is measured, followed by their release via a short overpressure surge. Such colloid exchange procedure allows for 1), the quick variation of differently functionalized colloids within the same experiment; 2), the investigation of long-term interactions by leaving the colloids on a surface for a defined time before detaching them; and 3), the inspection of irreversible interactions. After validation of the method by reproducing literature results obtained with traditional colloidal atomic force microscopy, the serial use of colloids with different surface functionalization was shown on a micropatterned surface. Finally, concanavalin A-coated colloids were allowed to adsorb on human embryonic kidney cells and then detached one by one. The adhesion between cells and colloids was up to 60 nN, whereas individual cells adhered with 20 nN to the glass substrate. A cellular elastic modulus of 0.8 kPa was determined using the attached colloid as indenter.

Skin Conformal Polymer Electrodes for Clinical ECG and EEG Recordings

Preparation-free and skin compliant biopotential electrodes with high recording quality enable wearables for future healthcare and the Internet of Humans. Here, super-soft and self-adhesive electrodes are presented for use on dry and hairy skin without skin preparation or attachment pressure. The electrodes show a skin-contact impedance of 50 kΩ cm2 at 10 Hz that is comparable to clinical standard gel electrodes and lower than existing dry electrodes. Microstructured electrodes inspired by grasshopper feet adhere repeatedly to the skin with a force of up to 0.1 N cm-2 without further attachment even during strong movement or deformation of the skin. Skin compliance and adhesive properties of the electrodes result in reduction of noise and motion artifacts superior to other dry electrodes reaching the performance of commercial gel electrodes. The signal quality is demonstrated by recording a high-fidelity electrocardiograms of a swimmer in water. Furthermore, an electrode with soft macropillars is used to detect alpha activity in the electroencephalograms from the back of the head through dense hair. Compared to gel electrodes, the soft biopotential electrodes are nearly imperceptible to the wearer and cause no skin irritations even after hours of application. The electrodes presented here could combine unobtrusive and long-term biopotential recordings with clinical-grade signal performance.

Multilayer Patterning of High Resolution Intrinsically Stretchable Electronics

Stretchable electronics can bridge the gap between hard planar electronic circuits and the curved, soft and elastic objects of nature. This has led to applications like conformal displays, electronic skin and soft neuroprosthetics. A remaining challenge, however, is to match the dimensions of the interfaced systems, as all require feature sizes well below 100 μm. Intrinsically stretchable nanocomposites are attractive in this context as the mechanical deformations occur on the nanoscale, although methods for patterning high performance materials have been lacking. Here we address these issues by reporting on a multilayer additive patterning approach for high resolution fabrication of stretchable electronic devices. The method yields highly conductive 30 μm tracks with similar performance to their macroscopic counterparts. Further, we demonstrate a three layer micropatterned stretchable electroluminescent display with pixel sizes down to 70 μm. These presented findings pave the way towards future developments of high definition displays, electronic skins and dense multielectrode arrays.

High‐Density Stretchable Electrode Grids for Chronic Neural Recording

Electrical interfacing with neural tissue is key to advancing diagnosis and therapies for neurological disorders, as well as providing detailed information about neural signals. A challenge for creating long-term stable interfaces between electronics and neural tissue is the huge mechanical mismatch between the systems. So far, materials and fabrication processes have restricted the development of soft electrode grids able to combine high performance, long-term stability, and high electrode density, aspects all essential for neural interfacing. Here, this challenge is addressed by developing a soft, high-density, stretchable electrode grid based on an inert, high-performance composite material comprising gold-coated titanium dioxide nanowires embedded in a silicone matrix. The developed grid can resolve high spatiotemporal neural signals from the surface of the cortex in freely moving rats with stable neural recording quality and preserved electrode signal coherence during 3 months of implantation. Due to its flexible and stretchable nature, it is possible to minimize the size of the craniotomy required for placement, further reducing the level of invasiveness. The material and device technology presented herein have potential for a wide range of emerging biomedical applications.

Easy to Apply Polyoxazoline-Based Coating for Precise and Long-Term Control of Neural Patterns

Arranging cultured cells in patterns via surface modification is a tool used by biologists to answer questions in a specific and controlled manner. In the past decade, bottom-up neuroscience emerged as a new application, which aims to get a better understanding of the brain via reverse engineering and analyzing elementary circuitry in vitro. Building well-defined neural networks is the ultimate goal. Antifouling coatings are often used to control neurite outgrowth. Because erroneous connectivity alters the entire topology and functionality of minicircuits, the requirements are demanding. Current state-of-the-art coating solutions such as widely used poly(l-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) fail to prevent primary neurons from making undesired connections in long-term cultures. In this study, a new copolymer with greatly enhanced antifouling properties is developed, characterized, and evaluated for its reliability, stability, and versatility. To this end, the following components are grafted to a poly(acrylamide) (PAcrAm) backbone: hexaneamine, to support spontaneous electrostatic adsorption in buffered aqueous solutions, and propyldimethylethoxysilane, to increase the durability via covalent bonding to hydroxylated culture surfaces and antifouling polymer poly(2-methyl-2-oxazoline) (PMOXA). In an assay for neural connectivity control, the new copolymers ability to effectively prevent unwanted neurite outgrowth is compared to the gold standard, PLL-g-PEG. Additionally, its versatility is evaluated on polystyrene, glass, and poly(dimethylsiloxane) using primary hippocampal and cortical rat neurons as well as C2C12 myoblasts, and human fibroblasts. PAcrAm-g-(PMOXA, NH2, Si) consistently outperforms PLL-g-PEG with all tested culture surfaces and cell types, and it is the first surface coating which reliably prevents arranged nodes of primary neurons from forming undesired connections over the long term. Whereas the presented work focuses on the proof of concept for the new antifouling coating to successfully and sustainably prevent unwanted connectivity, it is an important milestone for in vitro neuroscience, enabling follow-up studies to engineer neurologically relevant networks. Furthermore, because PAcrAm-g-(PMOXA, NH2, Si) can be quickly applied and used with various surfaces and cell types, it is an attractive extension to the toolbox for in vitro biology and biomedical engineering.

A guide towards long-term functional electrodes interfacing neuronal tissue

Implantable electronics address therapeutical needs of patients with electrical signaling dysfunctions such as heart problems, neurological disorders or hearing impairments. While standard electronics are rigid, planar and made of hard materials, their surrounding biological tissues are soft, wet and constantly in motion. These intrinsic differences in mechanical and chemical properties cause physiological responses that constitute a fundamental challenge to create functional long-term interfaces. Using soft and stretchable materials for electronic implants decreases the mechanical mismatch between implant and biological tissues. As a result, tissue damage during and after implantation is reduced, leading not only to an attenuated foreign body response, but also enabling completely novel applications. However, but for a few exceptions, soft materials are not sufficient to create long-term stable functional implants. In this work, we review recent progress in interfacing both the central (CNS) and peripheral nervous system (PNS) for long-term functional devices. The basics of soft and stretchable devices are introduced by highlighting the importance of minimizing physical as well as mechanical mismatch between tissue and implant in the CNS and emphasizing the relevance of an appropriate surface chemistry for implants in the PNS. Finally, we report on the latest materials and techniques that provide further electronic enhancements while reducing the foreign body reaction. Thus, this review should serve as a guide for creating long-term functional implants to enable future healthcare technologies and as a discussion on current ideas and progress within the field.

Multifunctional 3D electrode platform for real-time in situ monitoring and stimulation of cardiac tissues

Cardiovascular diseases are a major cause of death around the world underlining the importance of efficient treatments including novel medication. In vitro models of cardiac tissues are highly desired for monitoring electrophysiological response to screen pharmaceutical compounds at an early stage. Here, we report a platinum based 3D pillar electrode platform with cell growth guiding channel, which allows integrated, continuous electrical stimulation and recording of the cardiac tissues. This platform was successfully used to culture beating cardiomyocytes over weeks. The Pt-PDMS pillar electrode showed an impedance of 2.5 ± 0.3 kΩ at 10 Hz that is stable for using in cell cultures at 37 °C. Electrically pacing the cells improved maturation of the cardiac tissues. Contraction activities of 3D cardiac tissues were monitored in real-time through the pillar electrodes to evaluate physiological parameters. Adding 100 nM Isoproterenol clearly increased the spontaneous beating rate and decreased the extracellular field potential duration of the microtissues observed with electrical recording through the pillar electrodes. Overall, this platform has a potential to be applied in drug screening for in situ monitoring the biophysical parameters of the heart tissue in real-time.