Material research for wearables and implantable devices

Soft sensors that can perceive multiaxial forces, such as normal and shear, are of interest for dexterous robotic manipulation and monitoring of human performance. There exists a critical need for wearable sensors that encompass sensing capabilities for external forces with wide-ranging directionality and magnitude. However, planar fabrication techniques that are typically used have significant design constraints that often prohibit the creation of functionally compelling and complex architectures. Moreover, they often require multiple step operations and precise alignment for production, which adversely affect repeatability. Additive manufacturing can address the above challenges by incorporating digital design and automation to broaden the range of intricate structures possible and the reproducibility of samples. Here, we employed an additive manufacturing process based on continuous liquid interface production (CLIP) to create high-resolution (30 μm), three-dimensional, elastomeric polyurethane lattices for use as dielectric layers in capacitive sensors. We showed that the capacitive responses and sensitivities are highly tunable through designs of lattice type, thickness, and material-void volume percentage. These robust devices were able to withstand 850 loading cycles without a deterioration in performance. Microcomputed tomography (microCT) and finite element simulation were employed to elucidate the influence of lattice design on the deformation mechanism and concomitant sensing behavior. The advantage of 3D printing to make volumetric devices that match the natural contours of the human form was exhibited with examples of fully printed representative athletic equipment, including a shoe sole and helmet lining. CLIP enabled simple printing of customizable devices for capacitive sensors responsive to various external loadings.

Arielle Berman is a 5th year doctoral student in mechanical engineering under the guidance of Prof. Zhenan Bao at Stanford University, where she develops skin-inspired, multimodal tactile sensors for human wearable and robotic applications. Arielle’s research is supported by the National Science Foundation Graduate Research Fellowship Program (NSF GRFP), the Stanford Graduate Fellowship (SGF), and the Enhancing Diversity in Graduate Education (EDGE) Doctoral Fellowship. She received her BS with honors from Penn State (2018) and her MS at Georgia Tech (2020), both in mechanical engineering. Outside of research, Arielle is passionate about building community for graduate students, having served as co-President of the Stanford Mechanical Engineering Women’s Group.

The visual appearance of surfaces are influenced by their color and texture. While the creation and tuning of structural colors has been realized with nanostructures, achieving dynamic control over visual texture remains challenging. Inspired by dynamic textural modulation in cephalopod skin, which transforms between a smooth, flat state and topographically complex, three-dimensional (3D) state, we develop polymer films with programmable surface textures. We bring these textures to life through control over local swelling and contraction to spatially encode virtually arbitrary textures that can be hidden and revealed on demand. By simultaneously modulating optical cavity-based color patterns, we further demonstrate independent control of texture and color in a single device, enabling a higher level of dynamic control over visual appearance. This approach may enable new applications ranging from dynamic camouflage and display for soft machines to new types of photonic devices.

Siddharth is a PhD student in Materials Science at Stanford University, where he is a Meta PhD Fellow and Wu Tsai Human Performance Alliance Fellow working with Mark Brongersma and Nicholas Melosh. His research is focused on developing electrically tunable active optical metasurfaces using soft polymers, enabling applications ranging from on-the-fly reconfigurable optical computing devices to wearable photonics. Previously, he received his Bachelor’s degree in Engineering from the University of New South Wales (Sydney, Australia) and spent time in industry designing award-winning consumer products.

Dielectric elastomer actuators (DEAs) enable diverse applications in haptics, soft robotics, and smart optics. However, their use is limited by the lack of stretchable, solid electrodes that are both conductive and easily integrated. In this talk, a transparent and patternable solid electrode based on a conducting polymer composite of PEDOT:PSS and PEG-PPG-PEG diacrylate (P123DA) is presented, offering actuation performance comparable to traditional nonsolid electrodes like carbon grease. By varying the P123DA to PEDOT:PSS ratio, we optimized the electrical and mechanical properties to attain compliant solid electrodes for DEAs. The resulting solid electrode exhibits excellent optical transmittance and enables reliable static and dynamic actuations. This work provides a new materials strategy to advance DEAs toward integration in stretchable and transparent electronic systems.

Eunyoung (Grace) Kim is a Ph.D. candidate in Professor Zhenan Bao group at Stanford University. She received her M.Phil. and B.Eng. in Mechanical Engineering from the Hong Kong University of Science and Technology. During her M.Phil. degree, she worked in Professor Jang-Kyo Kim’s group, focusing on nanocomposites and multifunctional materials for applications including electromagnetic interference shielding, thermal management, and wearable sensors. At Stanford, her current research focuses on developing dielectric elastomer actuators with optimized actuation performance for integration into wearable devices.

Lipid bilayers are essential structures in life sciences and bioelectronics. One of their key features is their ability to compartmentalize electrolyte domains, allowing cells to selectively exchange ions, create concentration gradients, and establish electric potential differences. In the field of organic electronics, supported lipid bilayers (SLBS) on conductive polymer electrodes enable the study of specific ion-channel proteins or the detection of pore-forming toxins. For this purpose, scientists often employ electrochemical impedance spectroscopy (ElS) measurements. However, this technique is very sensitive to membrane leakage and defects, leading to challenges in data reproducibility and quantitative analysis. In successful experiments, the impedance data is numerically fitted with circuit models, and the relative changes in membrane resistance are used as transduction mechanisms.

In this work, we performed extensive finite-element method (FEM) simulations on SLB-coated PEDOT:PSS electrodes to analyze the impact of defects in SLBS on EIS. In particular, we investigate SLB ionic leakage based on the size and spatial distribution of single and multiple pores and compare that to experimental data. Furthermore, we propose and validate circuit analytical models using FEM simulations to shed light on ionic conductive paths and provide understanding of the main contributors to the membrane resistance.

Julian graduated in electrical engineering and received his PhD from the University of Udine (Italy). During his PhD, he worked on electrochemical modeling of performance and noise for electronic biosensors and bioactuators. Then he continued as a postdoctoral scholar in Prof. Palestri’s group, where he focused on modeling and simulations of conjugated polymers for bioelectronic applications. He joined Prof. Salleo’s group in the fall of 2022 where he is contributing to the understanding of the physical operation of organic devices.

Safe and effective neural stimulation requires sufficient charge injection without damaging the electrode and tissue. Soft electronic materials such as the commonly used conducting polymer PEDOT:PSS have been introduced to improve bio-interfaces, however, much remains unknown about its ability to safely inject charge. In this talk, I will discuss the electrical stimulation performance of PEDOT:PSS and more specifically the PEDOT:PSS-electrolyte interface where charge is injected. We studied the influence of electrolyte composition as well as the applied stimulation parameters on reversible and irreversible events. I will discuss important properties of PEDOT:PSS such as the undesired generation of reactive oxygen species, the potential dependent capacitance, and the accessibility of charge that is stored within the conducting polymer network. The results describe the interactions occurring at the electrode-electrolyte interface which is crucial for understanding the stimulation performance of PEDOT:PSS for safe and long-term functional human-machine interfaces.

After his M.S. in Applied Physics, Gerwin joined ASML where he worked on the development of EUV photolithography systems. He switched careers and received his PhD in Microelectronics for his work on organic bioelectronic devices for cancer treatment with electrical stimulation (Écoles des Mines, France). He joined the Salleo group in the fall of 2021 where he explores conducting polymers for bioelectronic applications with a strong focus on microfabrication and material characterization.