Yi Li’s research while affiliated with University of Connecticut and other places

Wearable and implantable bioelectronics that can interface for extended periods with highly mobile organs and tissues across a broad pH range would be useful for various applications in basic biomedical research and clinical medicine. The encapsulation of these systems, however, presents a major challenge, as such devices require superior barrier performance against water and ion penetration in challenging pH environments while also maintaining flexibility and stretchability to match the physical properties of the surrounding tissue. Current encapsulation materials are often limited to near-neutral pH conditions, restricting their application range. In this work, we report a liquid-based encapsulation approach for bioelectronics under extreme pH environments. This approach achieves high optical transparency, stretchability, and mechanical durability. When applied to implantable wireless optoelectronic devices, our encapsulation method demonstrates outstanding water resistance in vitro, ranging from extremely acidic environments (pH = 1.5 and 4.5) to alkaline conditions (pH = 9). We also demonstrate the in vivo biocompatibility of our encapsulation approach and show that encapsulated wireless optoelectronics maintain robust operation throughout 3 months of implantation in freely moving mice. These results indicate that our encapsulation strategy has the potential to protect implantable bioelectronic devices in a wide range of research and clinical applications.

Traditional rigid ocean pressure sensors typically require protection from bulky pressure chambers and complex seals to survive the large hydrostatic pressure and harsh ocean environment. Here, we introduce soft, flexible pressure sensors that can eliminate such a need and measure a wide range of hydrostatic pressures (0.1 MPa to 15 MPa) in environments that mimic the ocean, achieving small size, high flexibility, and potentially low power consumption. The sensors are fabricated from lithographically patterned gold thin films (100 nm thick) encapsulated with a soft Parylene C film and tested in a customized pressure vessel under well-controlled pressure and temperature conditions. Using a rectangular pressure sensor as an example, the resistance of the sensor is found to decrease linearly with the increase of the hydrostatic pressure from 0.1 MPa to 15 MPa. Finite element analysis (FEA) reveals the strain distributions in the pressure sensor under hydrostatic pressures of up to 15 MPa. The effect of geometry on sensor performance is also studied, and radially symmetric pressure sensors (like circular and spike-shaped) are shown to have more uniform strain distributions under large hydrostatic pressures and, therefore, have a potentially enhanced pressure measurement range. Pressure sensors of all geometries show high consistency and negligible hysteresis over 15 cyclic tests. In addition, the sensors exhibit excellent flexibility and operate reliably under a hydrostatic pressure of 10 MPa for up to 70 days. The developed soft pressure sensors are promising for integration with many platforms including animal tags, diver equipment, and soft underwater robotics.

Flexible electrochemical sensors that measure the concentrations of specific analytes (e.g., ions, molecules, and microorganisms) provide valuable information for medical diagnosis, personal health care, and environmental monitoring. However, the conductive electrodes of such sensors need to be exposed to the surrounding environments like chloride-containing aqueous solutions during their operation, where chloride ions (Cl-) can potentially cause corrosion and dissolution of the sensors, negatively impacting their performance and durability. In this work, we develop soft, flexible conductivity sensors made of gold (Au) electrodes and systematically study their electrochemical behaviors in sodium chloride (NaCl) solutions to prevent chloride-induced corrosion and enhance their sensitivity for marine environmental monitoring. The causes of gold chlorination reactions and polarization effects are identified and effectively prevented by analyzing the effects of direct current (DC) and alternating current (AC) voltages, AC frequencies, and exposed sensing areas of the conductivity (salinity) sensors. Accordingly, a performance diagram is constructed to provide guidance for the selection of operation parameters for the salinity sensor. We also convert the varying impedance values of salinity sensors at different salinity levels into output voltage signals using a voltage divider circuit with an AC voltage (0.6 V) source. The results offer an assessment of the accuracy and response time of the salinity sensors, as well as their potential for integration with data transmission components for real-time ocean monitoring. This study has important implications for the development of soft, flexible, Au-based electrochemical sensors that can operate efficiently in diverse biological fluids and marine environments.

The large, reversible shape-changing behaviors of liquid crystal elastomers (LCEs), resulting from liquid crystal-polymer network couplings, are promising for many applications. Despite intensive studies, harnessing molecular-material-structure interactions of LCEs for the design of locally controlled shape-morphing structures remains a challenge. Here, we report a facile and versatile strategy to tailor the stiffness and the morphing behavior of reconfigurable LCE structures via locally controlled mesogen alignment and crosslinking densities. Selective photopolymerization of spatially aligned LCE structures yields well-controlled lightly and highly crosslinked domains of distinct stiffness and selective permanent mesogen programming, which enables various previously inaccessible stiffness-heterogeneous geometries, as demonstrated in diverse morphing LCE structures via integrated experimental and finite element analysis. Furthermore, programming of the non-photopolymerized regions allows for reshaping, as shown in a sequentially shape-morphing LCE rod and “face”. The heterogenous morphing LCE structures have the potential for many applications, including in artificial muscles, soft robotics, and many others.

Functional structures with reversible shape-morphing and color-changing capabilities are promising for applications including soft robotics and biomimetic camouflage devices. Despite extensive studies, there are few reports on achieving both reversible shape-switching and color-changing capabilities within one structure. Here, we report a facile and versatile strategy to realize such capabilities via spatially programmed liquid crystal elastomer (LCE) structures incorporated with thermochromic dyes. By coupling the shape-changing behavior of LCEs resulting from the nematic-to-isotropic transition of liquid crystals with the color-changing thermochromic dyes, 3D thermochromic LCE structures change their shapes and colors simultaneously, which are controlled by the nematic-isotropic transition temperature of LCEs and the critical color-changing temperature of dyes, respectively. Demonstrations, including the simulated blooming process of a resembled flower, the camouflage behavior of a "butterfly"/"chameleon" robot in response to environmental changes, and the underwater camouflage of an "octopus" robot, highlight the reliability of this strategy. Furthermore, integrating micro-ferromagnetic particles into the "octopus" thermochromic LCE robot allows it to respond to thermal-magnetic dual stimuli for "adaptive" motion and diverse biomimetic motion modes, including swimming, rolling, rotating, and crawling, accompanied by color-changing behaviors for camouflage. The reversibly reconfigurable and color-changing thermochromic LCE structures are promising for applications including soft camouflage robots and multifunctional biomimetic devices.

Energy-efficient, miniaturized electronic ocean sensors for monitoring and recording various environmental parameters remain a challenge because conventional ocean sensors require high-pressure chambers and seals to survive the large hydrostatic pressure and harsh ocean environment, which usually entail a high-power supply and large size of the sensor system. Herein, we introduce soft, pressure-tolerant, flexible electronic sensors that can operate under large hydrostatic pressure and salinity environments, thereby eliminating the need for pressure chambers and reducing the power consumption and sensor size. Using resistive temperature and conductivity (salinity) sensors as an example for demonstration, the soft sensors are made of lithographically patterned metal thin films (100 nm) encapsulated with soft oil-infused elastomers and tested in a customized pressure vessel with well-controlled pressure and temperature conditions. The resistance of the temperature and pressure sensors increases linearly with a temperature range of 5-38 °C and salinity levels of 30-40 Practical Salinity Unit (PSU), respectively, relevant for this application. Pressure (up to 15 MPa) has shown a negligible effect on the performance of the temperature and salinity sensors, demonstrating their large pressure-tolerance capability. In addition, both temperature and salinity sensors have exhibited excellent cyclic loading behaviors with negligible hysteresis. Encapsulated with our developed soft oil-infused elastomer (PDMS, poly(dimethylsiloxane)), the sensor has shown excellent performance under a 35 PSU salinity water environment for more than 7 months. The soft, pressure-tolerant and noninvasive electronic sensors reported here are suitable for integration with many platforms including animal tags, profiling floats, diving equipment, and physiological monitoring.

Origami-inspired multistable structures are gaining increasing interest because of their potential applications in fields ranging from deployable structures to reconfigurable microelectronics. The multistability of such structures is critical for their applications but is challenging to manipulate due to the highly nonlinear deformations and complex configurations of the structures. Here, a comprehensive experimental and computational study is reported to tailor the multistable states of origami-inspired, buckled ferromagnetic structures and their reconfiguration paths. Using ribbon structures as an example, a design phase diagram is constructed as a function of the crease number and compressive strain. As the crease number increases from 0 to 7, the number of distinct stable states first increases and then decreases. The multistability is also shown to be actively tuned by varying the strain from 0% to 40%. Furthermore, analyzing energy barriers for reconfiguration among the stable states reveals dynamic changes in reconfiguration paths with increasing strains. Guided by studies above, diverse examples are designed and demonstrated, from programmable structure arrays to a soft robot. These studies lay out the foundation for the rational design of functional, multistable structures.

Three-dimensional (3D) mesostructures that can reversibly change their geometries and thereby their functionalities are promising for a wide range of applications. Despite intensive studies, the lack of fundamental understanding of the highly nonlinear multistable states existing in these structures has significantly hindered the development of reconfigurable systems that can realize rapid, well-controlled shape changes. Herein we exploit systematic energy landscape analysis of deformable 3D mesostructures to tailor their multistable states and least energy reconfiguration paths. We employ a discrete shell model and minimum energy pathway methods to establish design phase diagrams for a controlled number of stable states and their energy-efficient reconfiguration paths by varying essential geometry and material parameters. Concurrently, our experiments show that 3D mesostructures assembled from ferromagnetic composite thin films of diverse geometries can be rapidly reconfigured among their multistable states in a remote, on-demand fashion by using a portable magnet, with the configuration of each stable state well maintained after the removal of the external magnetic field. The number of stable states and reconfigurable paths observed in experiments are in excellent agreement with computational predictions. In addition, we demonstrate a wide breadth of applications including reconfigurable 3D light emitting systems, remotely-controlled release of particles from a multistable structure, and 3D structure arrays that can form desired patterns following the written path of a magnetic “pen”. Our results represent a critical step towards the rational design and development of reconfigurable structures for applications including soft robotics, multifunctional deployable devices, and many others.