Ultrathin Crystalline Silicon Nano and Micro Membranes with High Areal Density for Low-Cost Flexible Electronics

Integrating Hard Silicon for High-Performance Soft Electronics via Geometry Engineering

Soft electronics, which are designed to function under mechanical deformation (such as bending, stretching, and folding), have become essential in applications like wearable electronics, artificial skin, and brain-machine interfaces. Crystalline silicon is one of the most mature and reliable materials for high-performance electronics; however, its intrinsic brittleness and rigidity pose challenges for integrating it into soft electronics. Recent research has focused on overcoming these limitations by utilizing structural design techniques to impart flexibility and stretchability to Si-based materials, such as transforming them into thin nanomembranes or nanowires. This review summarizes key strategies in geometry engineering for integrating crystalline silicon into soft electronics, from the use of hard silicon islands to creating out-of-plane foldable silicon nanofilms on flexible substrates, and ultimately to shaping silicon nanowires using vapor-liquid-solid or in-plane solid-liquid-solid techniques. We explore the latest developments in Si-based soft electronic devices, with applications in sensors, nanoprobes, robotics, and brain-machine interfaces. Finally, the paper discusses the current challenges in the field and outlines future research directions to enable the widespread adoption of silicon-based flexible electronics.

Flexible silicon for high-performance photovoltaics, photodetectors and bio-interfaced electronics

Silicon (Si) is currently the most mature and reliable semiconductor material in the industry, playing a pivotal role in the development of modern microelectronics, renewable energy, and bio-electronic technologies. In recent years, widespread research attention has been devoted to the development of advanced flexible electronics, photovoltaics, and bio-interfaced sensors/detectors, boosting their emerging applications in distributed energy sources, healthcare, environmental monitoring, and brain-computer interfaces (BCIs). Despite the rigid and brittle nature of Si, a series of new fabrication technologies and integration strategies have been developed to enable a wide range of c-Si-based high-performance flexible photovoltaics and electronics, which were previously only achievable with intrinsically soft organic and polymer semiconductors. More interestingly, programmable geometric engineering of crystalline silicon (c-Si) units and logic circuits has been explored to enable the fabrication of various highly flexible nanoprobes for intracellular sensing and the deployment of soft BCI matrices to record and understand brain neural activities for the development of advanced neuroprosthetics. This review will systematically examine the latest progress in the fabrication of Si-based flexible solar cells, photodetectors, and biological probing interfaces over the past decade, identifying key design principles, mechanisms, and technological milestones achieved through novel geometry, morphology, and composition control. These advancements, when combined, will not only promote the practical applications of sustainable energy and wearable electronics but also spur new breakthroughs in emerging human-machine interfaces (HMIs) and artificial intelligence applications, which hold significant implications for understanding neural activities, implementing more efficient artificial Intelligence (AI) algorithms, and developing new therapies or treatments. Finally, we will summarize and provide an outlook on the current challenges and future opportunities of Si-based electronics, flexible optoelectronics, and bio-sensing.

Ultra-Flexible High-Linearity Silicon Nanomembrane Synaptic Transistor Array

The increasing demand for mobile artificial intelligence applications has elevated edge computing to a prominent research area. Silicon materials, renowned for their excellent electrical properties, are extensively utilized in traditional electronic devices. However, the development of silicon materials for flexible neuromorphic computing devices encounters great challenges. To address these limitations, ultrasoft silicon nanomembranes have emerged as a focal point due to their capability to preserve the superior electrical properties of silicon while providing substantial mechanical flexibility and interfacial tunability. Despite these advantages, difficulties remain in the transfer process of silicon nanomembranes and their integration for flexible synaptic transistors. In this work, an organic-inorganic hybrid polyimide-Al2O3 dielectric layer has been designed for synaptic behavior grown by an atomic layer deposition process, and integrated with a silicon nanomembrane to realize highly flexible synaptic transistors. These transistors demonstrate stable electrical performance even after undergoing 10 000 bending cycles at an extreme curvature radius of 2.2 mm. Furthermore, the silicon nanomembrane transistors effectively emulate synaptic functions, exhibiting exceptional linearity in their long-term characteristics, making them suitable for the application scenarios of detecting subtle signals. When applied to handwritten digit recognition simulations, these synaptic transistors have achieved a high accuracy rate of 93.2%.

A Fully Integrated Conformal Wearable Ultrasound Patch for Continuous Sonodynamic Therapy

Cancer treatment is a continuous process, that the current therapy cannot meet the requirement well, including radiotherapy and chemotherapy. Wearable ultrasound device has the potential for continuous sonodynamic therapy due to its portability. However, the miniaturization of ultrasonic probe, system integration of device, and the strategy of continuous treatment are still urgent issues to be addressed. Herein, a portable wearable antitumor system is introduced, which utilizes a custom-developed multiplexed ultrasonic patch array (CWUS Patch) to accurately focus ultrasound on the lesion site and controllably stimulate sonosensitizer to produce a large amount of toxic reactive oxygen species (ROS). The system enables dynamic control of the ultrasound patches and allows real-time adjustments to optimize their performance in various applications, providing greater flexibility and precision in healthcare technology. Furthermore, the excellent penetration property of ultrasound into tumor tissues that induce synchronous apoptosis of tumor cells from the inside out is verified through a mouse model of breast cancer. This fully integrated conformal wearable ultrasound system provides a promising approach to noninvasively, continuously, and efficiently treat deep tumors.

Closely Packed Stretchable Ultrasound Array Fabricated with Surface Charge Engineering for Contactless Gesture and Materials Detection

Communication with hand gestures plays a significant role in human-computer interaction by providing an intuitive and natural way for humans to communicate with machines. Ultrasound-based devices have shown promising results in contactless hand gesture recognition without requiring physical contact. However, it is challenging to fabricate a densely packed wearable ultrasound array. Here, a stretchable ultrasound array is demonstrated with closely packed transducer elements fabricated using surface charge engineering between pre-charged 1-3 Lead Zirconate Titanate (PZT) composite and thin polyimide film without using a microscope. The array exhibits excellent ultrasound properties with a wide bandwidth (≈57.1%) and high electromechanical coefficient (≈0.75). The ultrasound array can decipher gestures up to 10 cm in distance by using a contactless triboelectric module and identify materials from the time constant of the exponentially decaying impedance based on their triboelectric properties by utilizing the electrostatic induction phase. The newly proposed metric of the areal-time constant is material-specific and decreases monotonically from a highly positive human body (1.13 m2 s) to negatively charged polydimethylsiloxane (PDMS) (0.02 m2 s) in the triboelectric series. The capability of the closely packed ultrasound array to detect material along with hand gesture interpretation provides an additional dimension in the next-generation human-robot interaction.