Professor Bai Wubin of the University of North Carolina at Chapel Hill, USA team published an article in "Science Advances", reporting a strategy of micro-folding three-dimensional structures, which can form three-dimensional deformable microelectronic systems integrated with various functional materials. can realize the transition state of three-dimensional microelectronic systems through micro-folding strategies, providing an effective design idea for the versatility of composite materials. This deformable microelectronic system can be used in robots including reconfigurable microantennas, sensors for vibration monitoring, and electrocardiogram mapping, demonstrating the broad prospects of this solution in terms of potential functional applications.
DNA and proteins fold in three dimensions (3D) to carry out life-sustaining functions. Folding schemes that mimic such functional materials could unlock huge potential, especially in the fields of robotics, medicine and telecommunications. Traditional 3D micro-nanostructure strategies, including ion beam lithography, layer-by-layer growth, multi-photon lithography, printing-based advanced manufacturing and holographic lithography, can design high-precision 3D structures, but are usually limited to making certain high-performance materials, such as single crystal silicon nanofilms, structurally deformable. The development of deformable 3D structures can enrich the development of existing forms of flexible electronic devices, which remains a research challenge for future device functions and applications. In addition, combining the origami and kirigami design concepts in some existing solutions, there is great promise in forming various deformable structures in 3D through the multi-dimensional freedom of folding.
Figure 1. Folding assembly strategy of single crystal silicon 3D microstructure.
Here, the research team proposes a design strategy that enables origami at the microscale to build deformable 3D microelectronic systems. The system features a wide range of material options, including monocrystalline silicon and metal nanofilms as well as their polymer integration. This solution first integrates the 2D precursor with the folded backbone through a transfer printing process, strategically bending the backbone at different angles, and transforming the 2D precursor into a specially designed 3D mesostructure. This macroscopic folding strategy, precisely changing the structure from angles and directions to integrate the folding trajectories of microscopic precursors into 3D structures, leads to multidimensional control of structure formation and unconventional architectures, such as edge-located single-crystalline silicon inverted pyramids, free-standing microscopic gold cages, and complex 3D forms of various high-performance materials. The fundamental study of strain distribution, structural stability, and folding behavior during microfolding reported here establishes general principles for the design of 3D deformable structures with tunable topology. Furthermore, applications in microantennas for wearable communications, 3D vibration sensors for hand tremor monitoring, and robots for cardiac mapping highlight the practicality and scalability of deformable 3D systems enabled by microfolding. The relevant research results were published in Science Advances under the title "3D morphable systems via deterministic microfolding for vibrational sensing, robotic implants, and reconfigurable telecommunication".
Preparation scheme and design principles of deformable 3D structures
Figure 2. Other 3D deformable structures based on micro-folding assembly.
This micro-folding scheme enables with different feature sizes and geometries in a wide range of materials (Figures 2 and 3). Figure 2 shows a set of 3D deformable mesoscopic structures composed of various functional materials (metallic nanofilms, polymers, and inorganic semiconductor nanofilms) with geometries ranging from simple to complex states. Here, photolithography, etching, transfer printing and laser cutting define the pattern of the Si/PLGA bilayer, and a micro-folding process transforms the 2D precursor into a 3D structure resembling a headband and a butterfly.
Figure 3. 3D deformable mesoscopic structure with deterministic control of geometry via folding alignment.
Among other things, this microfolding scheme enables 3D reconfigurability by switching the folding registration between various folding axes to generate different 3D structures from 2D precursors. The Si/PLGA bilayer with a ribbon-like geometry is folded along the x- and y-axes to form a turtle shell (shape I) and a shield (shape II), respectively. This design strategy also enables the realization of a different set of ribbon-like mesostructures composed of metal films or metal and polymer bilayers.The zigzag copper ribbon can form a single lace with y-axis folds, while the x-axis folds transform to create decorative rings. Even in the case of 2D copper precursors with bilateral symmetry, the resulting 3D structures resemble diamonds or fences, depending on the folding path. Furthermore, folding registration is also suitable for complex deformable 3D structures with mixed strip/circle geometries. The Au/PI bilayer is constructed in a ribbon-like periodic pattern, in which birdcages and spiked rods are folded through the x- and y-axes, respectively. Compared with reconstruction methods of 3D structures activated by smart materials such as shape memory polymers, shape memory alloys, and liquid crystal elastomers, microfolding strategies provide high structural precision, stability, and continuous deformation capabilities by enabling well-controlled folding under various external stimuli (e.g., thermal, chemical, optical, magnetic, electronic, and mechanical strategies).
3D Deformable dipole microantenna
Figure 4. Deformable dipole microantenna controlled by micro-folding.
Antenna miniaturization plays an important role in wireless communication technology. Generally, the origami scheme developed is highly compatible with modern planar device technology and has broad versatility in structural design and modification, providing an effective way for 3D antenna miniaturization. Here, a reconfigurable dipole microantenna was prepared from a double-layer copper nanofilm with a serpentine pattern and a PI substrate via microfolding assembly. The reflection coefficient of the antenna is S11, and the folding angle ranges from 0° to 90°. It can be observed that the operating frequency of the microantenna is 5.20 GHz and the minimum S11 is −38 dB. When it is fully folded (θ = 90°), the center frequency moves only slightly to 5.32 GHz and the minimum S11 is −26 dB, which is in good agreement with the simulation results. The simulated 2D and 3D radiation patterns at 5.20 GHz frequency for all folded states show that the performance of the microantenna during 3D transformation has negligible changes in spatial distribution when the folding angle is from 0° to 90°. The stable performance of the S11 parameters, gain, and radiation pattern of the 3D antenna in various folded configurations demonstrates that microfolding can enable the miniaturization of 3D meandering wire antennas and provide the opportunity to adjust their geometry on demand while maintaining consistent performance, and can be used in biomedical applications ranging from telecommunications to implantable miniaturized devices.
3D vibration sensor for monitoring hand tremor
Figure 5. 3D vibration sensor for monitoring hand tremor.
micro-folding strategy prepares a 3D-structured wearable vibration sensor that can accurately capture tremors in real time to assess the severity of symptoms. A 3D vibration sensor is mounted on the index finger and can detect the presence of tremors as the hand moves in different directions. Due to the piezoresistive effect, the resistance of the Si sensor is linearly proportional to its strain change, that is, the vibration in the x direction plays a dominant role in the resistance change of the SiNM sensor. Both the simulated strain change and the measured resistance change of the SiNM sensor are proportional to the vibration frequency . The 3D vibration sensor was placed on the index finger to verify its ability to capture subtle finger movements. The 3D vibration sensor based on the fully folded structure has higher sensitivity than the vibration sensor based on the partially folded mesostructure, thus highlighting the advantages of micro-folded components. By configuring folding conditions, the adjustability of sensitivity and sensing range can meet the performance requirements (vibration, frequency and amplitude) of various practical applications. In addition, the vibration sensing direction selectivity in 3D vibration sensors can distinguish the vibration directions by integrating multiple 3D vibration sensors.
Flowering robot for cardiac deployment
Figure 6: Robot for cardiac deployment
The robot developed with a micro-folding strategy can be enclosed in a catheter structure and has a minimally invasive method of intrapericardial insertion. The planar form of the epicardial bioelectronic system consists of a pre-cut PI substrate layer in a flower-shaped geometry, four resistive strain sensors composed of Au snake resistors located on the petals, and a parylene encapsulated top layer. Fully bloomed 3D epicardial bioelectronic robots were obtained from 2D integrated electronics via microfolding components. Fully bloomed epicardial bioelectronic robots can be encapsulated into catheters due to their mechanical softness and deformability.The 3D epicardial bioelectronic robot in its closed state can safely enter the chest or reach the heart through a vein. Once it reaches the desired location, the catheter is retracted and a flower-like structure immediately appears, tightly integrating with the target tissue. Demonstrated is a bioelectronic robot with interfaces to geometrically irregular cardiac tissue deployed conformally on the epicardial surface of a living mouse heart. Multiple sensors (labeled C1, C2, C3, and C4) are well arranged on the device's petals and simultaneously distributed in the four chambers of the heart, collecting spatially resolved information, thereby enabling real-time, overall quantification of myocardial contractility, which can assist in the diagnosis and treatment of heart disease. Therefore, the 3D bioelectronic epicardial robot has potential clinical application value in locating dysfunctional tissue through multiple output channels and real-time monitoring of myocardial contractility recovery after cardiac surgery.
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Reference:
1: https://www.science.org/doi/10.1126/sciadv.ade0838
2: https://www.science.org/doi/10.1126/sciadv.abb7417
3: https://www.nature.com/articles/natrevmats201719
Source: Frontiers of Polymer Science
