Chi Pong Tsui

This study evaluates the rapid tooling feasibility and structural significance of utilizing digital light processing (DLP)-printed alumina as a near-net-shape ceramic mold-insert preform route for replication of curved polymer optics. While conventional production tooling for precision optics demands immediate optical-grade tolerances, the fundamental mechanisms governing polymer replication close to additively manufactured ceramic interfaces remain insufficiently understood. To isolate these multi-factor processing signatures, alumina specimens incorporating concave and convex parabolic surfaces were synthesized via lithography-based ceramic manufacturing. Our design acts as a geometric control lens, ensuring that thermal shrinkage trends, slicing kinematics, and interfacial replication behaviors are clearly exposed and quantified under uniform boundary conditions. Following debinding and sintering, exploratory hot-pressing cycles were executed to evaluate gross profile transfer and surface inheritance on poly(methyl methacrylate) (PMMA) replicas. Quantitative laser scanning confocal microscopy confirmed successful gross curvature generation and revealed geometry-dependent post-sintering shrinkage trends. The convex inserts exhibited an average peak-to-valley (PV) error of 123.48 ± 3.30 µm and an RMS error of 29.76 ± 1.23 µm, whereas the concave alumina inserts showed an average PV error of 137.98 ± 5.80 µm and an RMS error of 34.68 ± 1.20 µm. The PMMA replicas also showed substantial form deviation, with an average PV error of 163.72 ± 15.64 µm and RMS error of 27.37 ± 2.03 µm. Our work presents a route for producing near-net-shape ceramic mold-insert preforms that transforms complex processing variations into a predictable, mathematically addressable roadmap. A geometry-specific CAD pre-compensation can then be performed while the remaining precision gap can be selectively closed via targeted post-polishing depending on the desired optical application tier.

Photoisomerization-based liquid crystal elastomer (LCE) actuators offer precise spatiotemporal control over mechanical deformation, yet their reliance on ultraviolet (UV) irradiation limits biomedical applicability due to phototoxicity and poor tissue penetration. Herein, we report an 808 nm near-infrared (NIR) light-driven photoisomerization actuator based on azobenzene-cross-linked LCEs (Azo-LCEs) integrated with NaYF₄:Yb/Tm@NaYF₄:Yb/Nd@NaYF₄ core-shell-shell upconversion nanoparticles (CSS-UCNPs). The Nd³⁺-sensitized CSS-UCNPs convert 808 nm NIR light into UV/blue upconversion emissions (345-476 nm), driving trans-to-cis isomerization of azobenzene units and inducing macroscopic bending of the LCE films. To evaluate the actuation performance, the UCNPs/Azo-LCE films were tested under continuous-wave 808 nm irradiation for 20 s at power densities of 4-24 W cm^-2, reaching a maximum bending angle of 42.8 ± 2.6° at 24 W cm^-2 and exhibiting stable cyclic actuation over 50 cycles at 16 W cm^-2. For biologically relevant operation, thermal assessment at lower irradiation intensities revealed only a limited temperature rise in the culture medium under 4-8 W cm^-2 irradiation (ΔT = 2.29-4.36 °C), with no obvious cumulative heating during cyclic operation (20 s on/20 s off, 50 cycles). In addition, microgroove-patterned UCNPs/Azo-LCE substrates supported the adhesion and spreading of rat cardiomyoblast cells (H9c2) and guided groove-width-dependent uniaxial alignment. This work establishes a strategy for 808 nm-excited photoisomerization-driven LCE actuation and highlights its potential as an NIR-addressable soft actuator platform for future studies of dynamic cell-guidance, while operating with a modest thermal burden under the tested conditions.

Additive manufacturing (AM), with its high flexibility, cost-effectiveness, and customization, significantly accelerates the advancement of nanogenerators, contributing to sustainable energy solutions and the Internet of Things. In this review, an in-depth analysis of AM for piezoelectric and triboelectric nanogenerators is presented from the perspectives of fundamental mechanisms, recent advancements, and future prospects. It highlights AM-enabled advantages of versatility across materials, structural topology optimization, microstructure design, and integrated printing, which enhance critical performance indicators of nanogenerators, such as surface charge density and piezoelectric constant, thereby improving device performance compared to conventional fabrication. Common AM techniques for nanogenerators, including fused deposition modeling, direct ink writing, stereolithography, and digital light processing, are systematically examined in terms of their working principles, improved metrics (output voltage/current, power density), theoretical explanation, and application scopes. Hierarchical relationships connecting AM technologies with performance optimization and applications of nanogenerators are elucidated, providing a solid foundation for advancements in energy harvesting, self-powered sensors, wearable devices, and human–machine interaction. Furthermore, the challenges related to fabrication quality, cross-scale manufacturing, processing efficiency, and industrial deployment are critically discussed. Finally, the future prospects of AM for nanogenerators are explored, aiming to foster continuous progress and innovation in this field. Graphical Abstract

The growing field of 4D printing has spurred extensive exploration into applications of stimulus-responsive materials, such as hydrogels for micro-actuators. However, the hydrogel-based micro-actuators fabricated by one-step, single-material printing are typically bilayer, and their actuation capabilities are limited. This study proposes a novel gradient printing strategy via two-photon polymerization (2PP) based 4D printing to enhance the actuation performance of stimulus-responsive hydrogel micro-actuators. The feasibility of this approach was demonstrated by investigating the shrinkage rates and elastic moduli of the poly( N -isopropylacrylamide) (PNIPAm) hydrogel micro-cuboids printed at different laser doses using the confocal laser scanning microscope and atomic force microscopy based nano-indentation respectively. The 2PP-based gradient printing was used to fabricate bilayer and trilayer PNIPAm hydrogel micro-actuators, with the laser dose programmed to modulate the crosslinking degree of each layer. These micro-actuators were actuated by near-infrared (NIR) light in the gold nanorods (AuNRs) solutions. The effects of the NIR light powers, micro-actuator sizes, and layer thicknesses on the actuation behaviors were systematically investigated. Compared with 12-µm-thickness bilayer micro-actuation, the introduction of the transitional layer into the gradient trilayer one significantly enhanced the actuation amplitude and speed (the bending angle and curvature increased by about 150 and 70%, respectively, and the cycle time of actuation and recovery shortened by 35%). These advancements have significant implications for printing microscale gradient materials and enhancing their applications.

4D printing, the fabrication of dynamic 3D objects, has emerged as a frontier in additive manufacturing, benefiting from rapid advancements in 3D printing technologies and the development of new stimuli-responsive materials. Among the diverse materials explored for 4D printing, the hydrogel, renowned for its exceptional flexibility, biocompatibility, and tunable mechanical properties, is a class of soft materials well-suited for 4D printing. In addition to selecting and developing appropriate stimuli-responsive materials, it is important to devise suitable printing strategies to enable the fabrication of hydrogel-based structures that can perform complex shape-changing under external stimuli in various applications, such as soft robotics and biomedical areas. In view of this, various printing strategies, including structural design, printing scheme, and stimuli control are systematically summarized. This review aims to provide an up-to-date evolution of 4D-printed hydrogels and insights into the utilization of these printing strategies and printing techniques, such as direct ink writing, stereolithography, and two-photon polymerization, in the 4D printing of hydrogel structures for specific functions and applications.

The impact of 5G communication is expected to be widespread and transformative. It promises to provide faster mobile broadband speeds, lower latency, improved network reliability and capacity, and more efficient use of wireless technologies. The Schottky diode, a BN/GaN layered composite contacting bulk aluminum, is theoretically plausible to harvest wireless energy above X-band. According to our first principle calculation, the insertion of GaN layers dramatically influences the optical properties of the layered composite. The relative dielectric constant of BN/GaN layered composite as a function of layer-to-layer separation is investigated where the optimized dielectric constant is ~2.5. To push the dielectric constant approaching ~1 for high-speed telecommunication, we upgrade our BN-based Schottky diode via nanostructuring, and we find that the relative dielectric constant of BN monolayer (semiconductor side) can be minimized to ~1.5 only if it is deposited on an aluminum monolayer (metal side). It is rare to find a semiconductor with a dielectric constant close to 1, and our findings may push the cut-off frequency of the Al/BN-based rectenna to the high-band 5G network.

Light-responsive liquid crystal elastomers (LCEs) are one kind of attractive and fascinating materials in developing untethered and dexterous bioinspired robotic systems. Unlike widely explored azobenzene-containing LCE actuators utilizing photoisomerization, near-infrared (NIR) light-actuators based on photothermal effects from inorganic nanoparticles offer many benefits in terms of the response time, magnitude of actuation, and manipulation flexibility. However, unwanted aggregation of these nanoparticles in the LCE network tends to bring about uneven local heating, which is undesirable for repeated controllable shape morphing and motions. In this study, we synthesized a novel organic photothermal dopant, conjugated polymers (CPs), via palladium-catalyzed Stille cross-coupling polymerization by using donor monomer diketopyrrolopyrrole (DPP58) and acceptor monomer (TBZ12), and introduced them in the LCE for forming soft CPs/LCE actuators. These CPs with planar molecular structures exhibited good dispersion in the LCE mixture. A low content (0.3 wt%) of the conjugated polymers in the LCE matrix could result in an ultrafast increase in local temperature under 808 nm NIR irradiation. Using a surface-patterned glass substrate, a single film with splay alignments was allowed to be fabricated via UV photopolymerization. The 60 μm-thick CPs/LCE film actuators exhibited a superior photothermal effect, repeatable shape change, and ultrafast response time with actuation within 1 s upon 808 nm laser irradiation. Tunable shape deformation, including bending, “Swiss” rolling and helical twisting, was realized by choosing a cutting strategy at angles oblique to the alignment direction and unsymmetric NIR light illumination. The film could bend up to 180° within 3 s and recover its initial shape within 4 s. Photomotility of the millimeter-scale CPs/LCE actuator was also demonstrated through successive left-hand helical twisting at an average motion speed of 0.47 mm s⁻¹. The development of soft, NIR light-driven CP-doped LCE actuators would offer a promising path toward micromachines and untethered soft robotics.

Micro/nano-fabrication technology via two-photon polymerization (TPP) nanolithography is a powerful and useful manufacturing tool that is capable of generating two dimensional (2D) to three dimensional (3D) arbitrary micro/nano-structures of various materials with a high spatial resolution. This technology has received tremendous interest in cell and tissue engineering and medical microdevices because of its remarkable fabrication capability for sophisticated structures from macro- to nano-scale, which are difficult to be achieved by traditional methods with limited microarchitecture controllability. To fabricate precisely designed 3D micro/nano-structures for biomedical applications via TPP nanolithography, the use of photoinitiators (PIs) and photoresists needs to be considered comprehensively and systematically. In this review, widely used commercially available PIs are first discussed, followed by elucidating synthesis strategies of water-soluble initiators for biomedical applications. In addition to the conventional photoresists, the distinctive properties of customized stimulus-responsive photoresists are discussed. Finally, current limitations and challenges in the material and fabrication aspects and an outlook for future prospects of TPP for biomedical applications based on different biocompatible photosensitive composites are discussed comprehensively. In all, this review provides a basic understanding of TPP technology and important roles of PIs and photoresists for fabricating high-precision stimulus-responsive micro/nano-structures for a wide range of biomedical applications.

Two-photon polymerization direct laser writing (TPP DLW) is an emerging technology for producing advanced functional devices with complex three-dimensional (3D) micro-structures. Tremendous efforts have been devoted to developing two-photon polymerizable photo-sensitive nanocomposites with tailored properties. Light-induced reconfigurable smart materials such as liquid crystalline elastomers (LCEs) are promising materials. However, due to the difficulties in designing two-photon polymerizable liquid crystal monomer (LCM) nanocomposite photoresists, it is challenging to fabricate true 3D LCE micro-structures. In this paper, we report the preparation of photo-sensitive LCE nanocomposites containing photothermal nanomaterials, including multiwalled carbon nanotubes, graphene oxide and gold nanorods (AuNRs), for TPP DLW. The printability of the LCE nanocomposites is assessed by the fidelity of the micro-structures under different laser writing conditions. DLW of GO/LCM photoresist has shown a vigorous bubble formation. This may be due to the excessive heat generation upon rapid energy absorption of 780 nm laser energy. Compared to pure LCM photoresists, AuNR/LCM photoresists have a lower laser intensity threshold and higher critical laser scanning speed, due to the high absorption of AuNRs at 780 nm, which enhanced the photo-sensitivity of the photoresist. Therefore, a shorter printing time can be achieved for the AuNR/LCM photoresist.