4D Printing of High-Performance Thermal-Responsive Liquid Metal Elastomers Driven by Embedded Microliquid Chambers

Customizable Single-Layer Programmable Deformation Hydrogel Robots Based on One-Time Fabricating with Near-Infrared-Triggered Responsiveness

Programmable deformation hydrogel robots have garnered significant attention in biomedical fields due to their ability to undergo large-scale reversible deformation. As clinical demand rises, there is a need for hydrogel robots that are easy to process and operate, and can undergo programmable deformation. Here, we propose a method to fabricate single-layer programmable deformation hydrogel robots in one step using a high-precision digital light processing 3D printing system. Two kinds of deformable elements with different structure distribution on the top and bottom sides are produced by using two kinds of focused light with varying intensities. By combining these deformable elements, we create four basic modules with different and fixed deformable shapes. The desired shape deformation in hydrogel robots can be achieved by programming the combination of these four basic modules. The hydrogel robots exhibit reversible repeat deformation under near-infrared light stimulation. We validate our approach by fabricating several scaffolds using combinations of the four basic modules, demonstrating the feasibility of programming deformation and the potential application of these scaffolds in pipeline movement. This research provides the feasibility for the simple programming deformation of hydrogel robots and offers a novel approach for fabricating programmable deformation hydrogel robots in biomedical fields.

A Rolling Light-Driven Pneumatic Soft Actuator Based on Liquid-Gas Phase Change

Light-driven wireless actuators provide obvious advantages for remote control. However, traditional double-layer actuators are restricted to the thin film deformation mode when undertaking complex tasks. Here, an actuator is proposed that employs thermal strain and local photothermal effects induced by low boiling point liquids to generate asymmetry along the fiber axis, thereby causing elastic deformation of the fiber. Under continuous irradiation, the sustained elastic deformation results in dynamic frustration within the fiber, creating torque around its axis. Based on this principle, the fiber actuator fabricated in this study enables rolling translation, while the ring actuator achieves simultaneous rolling and lifting motion for object manipulation. Continuous rolling under light eliminates the need for complex light manipulation. This new movement method offers an insight for various application scenarios.

Soft Bioelectronics for Heart Monitoring

Cardiovascular diseases (CVDs) are a predominant global health concern, accounting for over 17.9 million deaths in 2019, representing approximately 32% of all global fatalities. In North America and Europe, over a million adults undergo cardiac surgeries annually. Despite the benefits, such surgeries pose risks and require precise postsurgery monitoring. However, during the postdischarge period, where monitoring infrastructures are limited, continuous monitoring of vital signals is hindered. In this area, the introduction of implantable electronics is altering medical practices by enabling real-time and out-of-hospital monitoring of physiological signals and biological information postsurgery. The multimodal implantable bioelectronic platforms have the capability of continuous heart sensing and stimulation, in both postsurgery and out-of-hospital settings. Furthermore, with the emergence of machine learning algorithms into healthcare devices, next-generation implantables will benefit artificial intelligence (AI) and connectivity with skin-interfaced electronics to provide more precise and user-specific results. This Review outlines recent advancements in implantable bioelectronics and their utilization in cardiovascular health monitoring, highlighting their transformative deployment in sensing and stimulation to the heart toward reaching truly personalized healthcare platforms compatible with the Sustainable Development Goal 3.4 of the WHO 2030 observatory roadmap. This Review also discusses the challenges and future prospects of these devices.

Recent Advances in 4D Printing of Advanced Materials and Structures for Functional Applications

4D printing has attracted tremendous worldwide attention during the past decade. This technology enables the shape, property, or functionality of printed structures to change with time in response to diverse external stimuli, making the original static structures alive. The revolutionary 4D-printing technology offers remarkable benefits in controlling geometric and functional reconfiguration, thereby showcasing immense potential across diverse fields, including biomedical engineering, electronics, robotics, and photonics. Here, a comprehensive review of the latest achievements in 4D printing using various types of materials and different additive manufacturing techniques is presented. The state-of-the-art strategies implemented in harnessing various 4D-printed structures are highlighted, which involve materials design, stimuli, functionalities, and applications. The machine learning approach explored for 4D printing is also discussed. Finally, the perspectives on the current challenges and future trends toward further development in 4D printing are summarized.

High internal phase emulsions gel ink for direct-ink-writing 3D printing of liquid metal

3D printing of liquid metal remains a big challenge due to its low viscosity and large surface tension. In this study, we use Carbopol hydrogel and liquid gallium-indium alloy to prepare a liquid metal high internal phase emulsion gel ink, which can be used for direct-ink-writing 3D printing. The high volume fraction (up to 82.5%) of the liquid metal dispersed phase gives the ink excellent elastic properties, while the Carbopol hydrogel, as the continuous phase, provides lubrication for the liquid metal droplets, ensuring smooth flow of the ink during shear extrusion. These enable high-resolution and shape-stable 3D printing of three-dimensional structures. Moreover, the liquid metal droplets exhibit an electrocapillary phenomenon in the Carbopol hydrogel, which allows for demulsification by an electric field and enables electrical connectivity between droplets. We have also achieved the printing of ink on flexible, non-planar structures, and demonstrated the potential for alternating printing with various materials.

Miniaturized and untethered McKibben muscles based on photothermal-induced gas-liquid transformation

Pneumatic artificial muscles can move continuously under the power support of air pumps, and their flexibility also provides the possibility for applications in complex environments. However, in order to achieve operation in confined spaces, the miniaturization of artificial muscles becomes crucial. Since external attachment devices greatly hinder the miniaturization and use of artificial muscles, we propose a light-driven approach to get rid of these limitations. In this study, we report a miniaturized fiber-reinforced artificial muscle based on mold editing, capable of bending and axial elongation using gas-liquid conversion in visible light. The minimum volume of the artificial muscle prepared using this method was 15.7 mm3 (d = 2 mm, l = 5 mm), which was smaller than those of other fiber-reinforced pneumatic actuators. This research can promote the development of non-tethered pneumatic actuators for rescue and exploration, and create the possibility of miniaturization of actuators.

Customizable single-layer hydrogel robot with programmable NIR-triggered responsiveness

Hydrogel robots are widely used in biomedical fields due to their excellent biocompatibility and response to external stimuli. However, traditional processing methods cannot rapidly fabricate complex structures, and smart response strategies often rely on double-layer structures fabricated from two materials with significantly different swelling properties. In this study, we present a single-layer hydrogel robot that can be fabricated in one step using a high-precision digital light processing (H-P DLP) 3D printing system. The robot has structural differences and the ability to maintain a repetitive response. Additionally, we fabricated several robot grippers to demonstrate their potential for customization and programming, as well as their potential applications in cargo delivery. Our work provides a new approach to achieve the formation and response of various irregular hydrogels, which is expected to advance the development of biomedical applications.

Four-Dimensional Printing and Shape Memory Materials in Bone Tissue Engineering

The repair of severe bone defects is still a formidable clinical challenge, requiring the implantation of bone grafts or bone substitute materials. The development of three-dimensional (3D) bioprinting has received considerable attention in bone tissue engineering over the past decade. However, 3D printing has a limitation. It only takes into account the original form of the printed scaffold, which is inanimate and static, and is not suitable for dynamic organisms. With the emergence of stimuli-responsive materials, four-dimensional (4D) printing has become the next-generation solution for biological tissue engineering. It combines the concept of time with three-dimensional printing. Over time, 4D-printed scaffolds change their appearance or function in response to environmental stimuli (physical, chemical, and biological). In conclusion, 4D printing is the change of the fourth dimension (time) in 3D printing, which provides unprecedented potential for bone tissue repair. In this review, we will discuss the latest research on shape memory materials and 4D printing in bone tissue repair.

A review on the recent progress, opportunities, and challenges of 4D printing and bioprinting in regenerative medicine

Four-dimensional (4 D) printing is a novel emerging technology, which can be defined as the ability of 3 D printed materials to change their form and functions. The term 'time' is added to 3 D printing as the fourth dimension, in which materials can respond to a stimulus after finishing the manufacturing process. 4 D printing provides more versatility in terms of size, shape, and structure after printing the construct. Complex material programmability, multi-material printing, and precise structure design are the essential requirements of 4 D printing systems. The utilization of stimuli-responsive polymers has increasingly taken the place of cell traction force-dependent methods and manual folding, offering a more advanced technique to affect a construct's adjusted shape transformation. The present review highlights the concept of 4 D printing and the responsive bioinks used in 4 D printing, such as water-responsive, pH-responsive, thermo-responsive, and light-responsive materials used in tissue regeneration. Cell traction force methods are described as well. Finally, this paper aims to introduce the limitations and future trends of 4 D printing in biomedical applications based on selected key references from the last decade.

Direct Ink Writing: A 3D Printing Technology for Diverse Materials

Additive manufacturing (AM) has gained significant attention due to its ability to drive technological development as a sustainable, flexible, and customizable manufacturing scheme. Among the various AM techniques, direct ink writing (DIW) has emerged as the most versatile 3D printing technique for the broadest range of materials. DIW allows printing of practically any material, as long as the precursor ink can be engineered to demonstrate appropriate rheological behavior. This technique acts as a unique pathway to introduce design freedom, multifunctionality, and stability simultaneously into its printed structures. Here, a comprehensive review of DIW of complex 3D structures from various materials, including polymers, ceramics, glass, cement, graphene, metals, and their combinations through multimaterial printing is presented. The review begins with an overview of the fundamentals of ink rheology, followed by an in-depth discussion of the various methods to tailor the ink for DIW of different classes of materials. Then, the diverse applications of DIW ranging from electronics to food to biomedical industries are discussed. Finally, the current challenges and limitations of this technique are highlighted, followed by its prospects as a guideline toward possible futuristic innovations.

High-Performance Liquid Metal/Polyborosiloxane Elastomer toward Thermally Conductive Applications

With the combination of high flexibility and thermal property, thermally conductive elastomers have played an important role in daily life. However, traditional thermally conductive elastomers display limited stretchability and toughness, seriously restricting their further development in practical applications. Herein, a high-performance composite is fabricated by dispersing room-temperature liquid metal microdroplets (LM) into a polyborosiloxane elastomer (PBSE). Due to the unique solid-liquid coupling mechanism, the LM can deform with the PBSE matrix, achieving higher fracture strain (401%) and fracture toughness (2164 J/m²). Meanwhile, the existence of LM microdroplets improves the thermal conductivity of the composite. Interestingly, the LM/PBSE also exhibits remarkable anti-impact, adhesion capacities under complex loading environments. As a novel stretchable elastomer with enhanced mechanical and thermal behavior, the LM/PBSE shows good application prospects in the fields of thermal camouflages, stretchable heat-dissipation matrixes, and multifunctional shells for electronic devices.

On-Demand Programming of Liquid Metal-Composite Microstructures through Direct Ink Write 3D Printing

Soft, elastically deformable composites with liquid metal (LM) droplets can enable new generations of soft electronics, robotics, and reconfigurable structures. However, techniques to control local composite microstructure, which ultimately governs material properties and performance, is lacking. Here a direct ink writing technique is developed to program the LM microstructure (i.e., shape, orientation, and connectivity) on demand throughout elastomer composites. In contrast to inks with rigid particles that have fixed shape and size, it is shown that emulsion inks with LM fillers enable in situ control of microstructure. This enables filaments, films, and 3D structures with unique LM microstructures that are generated on demand and locked in during printing. This includes smooth and discrete transitions from spherical to needle-like droplets, curvilinear microstructures, geometrically complex embedded inclusion patterns, and connected LM networks. The printed materials are soft (modulus < 200 kPa), highly deformable (>600 % strain), and can be made locally insulating or electrically conductive using a single ink by controlling the process conditions. These capabilities are demonstrated by embedding elongated LM droplets in a soft heat sink, which rapidly dissipates heat from high-power LEDs. These programmable microstructures can enable new composite paradigms for emerging technologies that demand mechanical compliance with multifunctional response.

Directly Printed Embedded Metal Mesh for Flexible Transparent Electrode via Liquid Substrate Electric-Field-Driven Jet

Flexible transparent electrodes (FTEs) with embedded metal meshes play an indispensable role in many optoelectronic devices due to their excellent mechanical stability and environmental adaptability. However, low-cost, simple, efficient, and environmental friendly integrated manufacturing of high-performance embedded metal meshes remains a huge challenge. Here, a facile and novel fabrication method is proposed for FTEs with an embedded metal mesh via liquid substrateelectric-field-driven microscale 3D printing process. This direct printing strategy avoids tedious processes and offers low-cost and high-volume production, enabling the fabrication of high-resolution, high-aspect ratio embedded metal meshes without sacrificing transparency. The final manufactured FTEs with 80 mm × 80 mm embedded metal mesh offers excellent optoelectronic performance with a sheet resistance (Rs ) of 6 Ω sq-1 and a transmittance (T) of 85.79%. The embedded metal structure still has excellent mechanical stability and good environmental suitability under different harsh working conditions. The practical feasibility of the FTEs is successfully demonstrated with a thermally driven 4D printing structure and a resistive transparent strain sensor. This method can be used to manufacture large areas with facile, high-efficiency, low-cost, and high-performance FTEs.

Temperature-Stress Bimodal Sensing Conductive Hydrogel-Liquid Metal by Facile Synthesis for Smart Wearable Sensor

Conductive hydrogels have attracted great attention due to their promising applications in wearable sensors. However, developing conductive hydrogels with excellent sensor properties and multiple stimuli responsiveness for smart wearable devices is still a challenge. This paper presents a facile synthetic method of a crosslinked chitosan quaternary ammonium salt and liquid metal (CHACC-LM) composite hydrogel with temperature-stress bimodal sensing for smart wearable sensor. LM as liquid fillers toughen the hydrogel matrix (stress: 1.11 MPa) and enhance the hydrogel extensibility (strain: 233%). The CHACC-LM hydrogel exhibits conductivity , excellent antibacterial properties (> 99%), an electrical self-healing property, and strain sensitivity (GF = 1.6). In addition, the CHACC-LM hydrogel can be used as wearable flexible sensors with the ability of monitoring human activities directly and the distinguished ability of discerning subtle motions (handwriting). It also shows sensitivity in the external environment such as low temperature, thermal response, and water solution. Importantly, the composite hydrogel simultaneous response to different stress and temperature stimuli. Furthermore, the CHACC-LM hydrogel can be used for gesture recognition and to control the manipulator in human-computer interaction. All these properties provide a great scope for researchers to achieve practical advances in smart wearable sensors.

Fractal-Based Stretchable Circuits via Electric-Field-Driven Microscale 3D Printing for Localized Heating of Shape Memory Polymers in 4D Printing

Thermally responsive shape memory polymers (SMPs) used in 4D printing are often reported to be activated by external heat sources or embedded stiff heaters. However, such heating strategies impede the practical application of 4D printing due to the lack of precise control over heating or the limited ability to accommodate the stretching during shape programming. Herein, we propose a novel 4D printing paradigm by fabricating stretchable heating circuits with fractal motifs via electric-field-driven microscale 3D printing of conductive paste for seamless integration into 3D printed structures with SMP components. By regulating the fractal order and printing/processing parameters, the overall electrical resistance and areal coverage of the circuits can be tuned to produce an efficient and uniform heating performance. Compared with serpentine structures, the resistance of fractal-based circuits remains relatively stable under both uniaxial and biaxial stretching. In practice, steady-state and transient heating modes can be respectively used during the shape programming and actuation phases. We demonstrate that this approach is suitable for 4D printed structures with shape programming by either uniaxial or biaxial stretching. Notably, the biaxial stretchability of fractal-based heating circuits enables the shape change between a planar structure and a 3D one with double curvature. The proposed strategy would offer more freedom in designing 4D printed structures and enable the manipulation of the latter in a controlled and selective manner.

3D Printing of Physical Organ Models: Recent Developments and Challenges

Physical organ models are the objects that replicate the patient-specific anatomy and have played important roles in modern medical diagnosis and disease treatment. 3D printing, as a powerful multi-function manufacturing technology, breaks the limitations of traditional methods and provides a great potential for manufacturing organ models. However, the clinical application of organ model is still in small scale, facing the challenges including high cost, poor mimicking performance and insufficient accuracy. In this review, the mainstream 3D printing technologies are introduced, and the existing manufacturing methods are divided into "directly printing" and "indirectly printing", with an emphasis on choosing suitable techniques and materials. This review also summarizes the ideas to address these challenges and focuses on three points: 1) what are the characteristics and requirements of organ models in different application scenarios, 2) how to choose the suitable 3D printing methods and materials according to different application categories, and 3) how to reduce the cost of organ models and make the process simple and convenient. Moreover, the state-of-the-art in organ models are summarized and the contribution of 3D printed organ models to various surgical procedures is highlighted. Finally, current limitations, evaluation criteria and future perspectives for this emerging area are discussed.

Selective Laser Sintering-Based 4D Printing of Magnetism-Responsive Grippers

Components fabricated by four-dimensional (4D) printing hold the potential for applications in soft robotics because of their characteristics of responding to external stimuli. Grippers, being the common structures used in robotics, were fabricated by the selective laser sintering (SLS)-based 4D printing of magnetism-responsive materials and tested for remote-controllable deformation in an external magnetic field. A composite material consisting of magnetic Nd2Fe14B powder and thermoplastic polyurethane powder was selected as the raw material for the SLS; the magnetic particle acquired permanent magnetism by magnetization after the SLS process. Microscopic characterization showed the homogeneous dispersion of magnetic particles inside the polymer matrix. The magnetic induction intensity distribution was systematically investigated by both experiments and numerical simulations. The reliability of the numerical model proposed was justified by the excellent consistency between them. The deformation of the grippers could be regulated by tuning the magnetic particle content and the distance from the external magnet; the deformation mechanism is investigated numerically. The magnetic driving force and the corresponding horizontal displacement are calculated, thus having high accuracy compared with the existing research that obtained the deformation amount by only visual inspection. Mechanical properties of the SLS-fabricated magnetic polymer composite specimens were also studied because of their close relationship with the deformation behaviors. These findings provide guidance for future research on controllable deformation and driving force calculation for 4D printing.

Fused Filament Fabrication 4D Printing of a Highly Extensible, Self-Healing, Shape Memory Elastomer Based on Thermoplastic Polymer Blends

A polymer blend with high extensibility, exhibiting both shape memory and self-healing, was 4D printed using a low-cost fused filament fabrication (FFF, or fused deposition modeling, FDM) 3D printer. The material is composed of two commercially available commodity polymers, polycaprolactone (PCL), a semi-crystalline thermoplastic, and polystyrene-block-poly(ethylene-co-butylene)-block-polystyrene (SEBS), a thermoplastic elastomer. The shape memory and self-healing properties of the blends were studied systematically through thermo-mechanical and morphological characterization, providing insight into the shape memory mechanism useful for tuning the material properties. In 3D-printed articles, the orientation of the semi-crystalline and micro-phase-separated domains leads to improvement of the shape memory property and extensibility of this material compared to compression-molded samples. By controlling the orientation of the printed fibers, we achieved a high strain at break over 1200%, outperforming previously reported flexible 4D-printed materials. The self-healing agent, PCL, enables the material to heal scratches and cracks and adhere two surfaces after annealing at 80 °C for 30 min. The high performance, multi-functionality, and potential scalability make it a promising candidate for a broad spectrum of applications, including flexible electronics, soft actuators, and deployable devices.