3D-Printed Self-Folding Electronics

Driving macro-scale transformations in three-dimensional-printed biopolymers through controlled induction of molecular anisotropy at the nanoscale

Motivated by the need to harness the properties of renewable and biodegradable polymers for the design and manufacturing of multi-scale structures with complex geometries, we have employed our additive manufacturing platform that leverages molecular self-assembly for the production of metre-scale structures characterized by complex geometries and heterogeneous material composition. As a precursor material, we used chitosan, a chemically modified form of chitin, an abundant and sustainable structural polysaccharide. We demonstrate the ability to control concentration-dependent crystallization as well as the induction of the preferred orientation of the polymer chains through the combination of extrusion-based robotic fabrication and directional toolpathing. Anisotropy is demonstrated and assessed through high-resolution micro-X-ray diffraction in conjunction with finite element simulations. Using this approach, we can leverage controlled and user-defined small-scale propagation of residual stresses to induce large-scale folding of the resulting structures.

Bio-Inspired Dynamically Morphing Microelectronics toward High-Density Energy Applications and Intelligent Biomedical Implants

Choreographing the adaptive shapes of patterned surfaces to exhibit designable mechanical interactions with their environment remains an intricate challenge. Here, a novel category of strain-engineered dynamic-shape materials, empowering diverse multi-dimensional shape modulations that are combined to form fine-grained adaptive microarchitectures is introduced. Using micro-origami tessellation technology, heterogeneous materials are provided with strategic creases featuring stimuli-responsive micro-hinges that morph precisely upon chemical and electrical cues. Freestanding multifaceted foldable packages, auxetic mesosurfaces, and morphable cages are three of the forms demonstrated herein of these complex 4-dimensional (4D) metamaterials. These systems are integrated in dual proof-of-concept bioelectronic demonstrations: a soft foldable supercapacitor enhancing its power density (≈108 mW cm-2), and a bio-adaptive device with a dynamic shape that may enable novel smart-implant technologies. This work demonstrates that intelligent material systems are now ready to support ultra-flexible 4D microelectronics, which can impart autonomy to devices culminating in the tangible realization of microelectronic morphogenesis.

Utilizing 4D Printing to Design Smart Gastroretentive, Esophageal, and Intravesical Drug Delivery Systems

The breakthrough of 3D printing in biomedical research has paved the way for the next evolutionary step referred to as four dimensional (4D) printing. This new concept utilizes the time as the fourth dimension in addition to the x, y, and z axes with the idea to change the configuration of a printed construct with time usually in response to an external stimulus. This can be attained through the incorporation of smart materials or through a preset smart design. The 4D printed constructs may be designed to exhibit expandability, flexibility, self-folding, self-repair or deformability. This review focuses on 4D printed devices for gastroretentive, esophageal, and intravesical delivery. The currently unmet needs and challenges for these application sites are tried to be defined and reported on published solution concepts involving 4D printing. In addition, other promising application sites that may similarly benefit from 4D printing approaches such as tracheal and intrauterine drug delivery are proposed.

Raw Materials, Technology, Healthcare Applications, Patent Repository and Clinical Trials on 4D Printing Technology: An Updated Review

After the successful commercial exploitation of 3D printing technology, the advanced version of additive manufacturing, i.e., 4D printing, has been a new buzz in the technology-driven industries since 2013. It is a judicious combination of 3D printing technologies and smart materials (stimuli responsive), where time is the fourth dimension. Materials such as liquid crystal elastomer (LCE), shape memory polymers, alloys and composites exhibiting properties such as self-assembling and self-healing are used in the development/manufacturing of these products, which respond to external stimuli such as solvent, temperature, light, etc. The technologies being used are direct ink writing (DIW), fused filament fabrication (FFF), etc. It offers several advantages over 3D printing and has been exploited in different sectors such as healthcare, textiles, etc. Some remarkable applications of 4D printing technology in healthcare are self-adjusting stents, artificial muscle and drug delivery applications. Potential of applications call for further research into more responsive materials and technologies in this field. The given review is an attempt to collate all the information pertaining to techniques employed, raw materials, applications, clinical trials, recent patents and publications specific to healthcare products. The technology has also been evaluated in terms of regulatory perspectives. The data garnered is expected to make a strong contribution to the field of technology for human welfare and healthcare.

From Traditional to Novel Printed Electrochromic Devices: Material, Structure and Device

Electrochromic materials have been considered as a new way to achieve energy savings in the building sector due to their potential applications in smart windows, cars, aircrafts, etc. However, the high cost of manufacturing ECDs using the conventional manufacturing methods has limited its commercialization. It is the advantages of low cost as well as resource saving, green environment protection, flexibility and large area production that make printing electronic technology fit for manufacturing electrochromic devices. This paper reviews the progress of research on printed electrochromic devices (ECDs), detailing the preparation of ECDs by screen printing, inkjet printing and 3D printing, using the scientific properties of discrete definition printing method. Up to now, screen printing holds the largest share in the electrochromic industry due to its low cost and large ink output nature, which makes it suitable especially for printing on large surfaces. Though inkjet printing has the advantages of high precision and the highest coloration efficiency (CE) can be up to 542 ± 10 cm²C^-1, it has developed smoothly, and has not shown rigid needs. Inkjet printing is suitable for the personalized printing production of high precision and small batch electronic devices. Since 3D printing is a new manufacturing technology in the 21st century, with the characteristics of integrated molding and being highly controllable, which make it suitable for customized printing of complex devices, such as all kinds of sensors, it has gained increasing attention in the past decade. Finally, the possibility of combining screen printing with inkjet printing to produce high performance ECDs is discussed.

Architected Materials for Additive Manufacturing: A Comprehensive Review

One of the main advantages of Additive Manufacturing (AM) is the ability to produce topologically optimized parts with high geometric complexity. In this context, a plethora of architected materials was investigated and utilized in order to optimize the 3D design of existing parts, reducing their mass, topology-controlling their mechanical response, and adding remarkable physical properties, such as high porosity and high surface area to volume ratio. Thus, the current re-view has been focused on providing the definition of architected materials and explaining their main physical properties. Furthermore, an up-to-date classification of cellular materials is presented containing all types of lattice structures. In addition, this research summarized the developed methods that enhance the mechanical performance of architected materials. Then, the effective mechanical behavior of the architected materials was investigated and compared through the existing literature. Moreover, commercial applications and potential uses of the architected materials are presented in various industries, such as the aeronautical, automotive, biomechanical, etc. The objectives of this comprehensive review are to provide a detailed map of the existing architected materials and their mechanical behavior, explore innovative techniques for improving them and highlight the comprehensive advantages of topology optimization in industrial applications utilizing additive manufacturing and novel architected materials.

A new dimension for magnetosensitive e-skins: active matrix integrated micro-origami sensor arrays

Magnetic sensors are widely used in our daily life for assessing the position and orientation of objects. Recently, the magnetic sensing modality has been introduced to electronic skins (e-skins), enabling remote perception of moving objects. However, the integration density of magnetic sensors is limited and the vector properties of the magnetic field cannot be fully explored since the sensors can only perceive field components in one or two dimensions. Here, we report an approach to fabricate high-density integrated active matrix magnetic sensor with three-dimensional (3D) magnetic vector field sensing capability. The 3D magnetic sensor is composed of an array of self-assembled micro-origami cubic architectures with biased anisotropic magnetoresistance (AMR) sensors manufactured in a wafer-scale process. Integrating the 3D magnetic sensors into an e-skin with embedded magnetic hairs enables real-time multidirectional tactile perception. We demonstrate a versatile approach for the fabrication of active matrix integrated 3D sensor arrays using micro-origami and pave the way for new electronic devices relying on the autonomous rearrangement of functional elements in space.

Self-Folding PCB Kirigami: Rapid Prototyping of 3D Electronics via Laser Cutting and Forming

This paper demonstrates laser forming, localized heating with a laser to induce plastic deformation, can self-fold 2D printed circuit boards (PCBs) into 3D structures with electronic function. There are many methods for self-folding but few are compatible with electronic materials. We use a low-cost commercial laser writer to both cut and fold a commercial flexible PCB. Laser settings are tuned to select between cutting and folding with higher power resulting in cutting and lower power resulting in localized heating for folding into 3D shapes. Since the thin copper traces used in commercial PCBs are highly reflective and difficult to directly fold, two approaches are explored for enabling folding: plating with a nickel/gold coating or using a single, high-power laser exposure to oxidize the surface and improve laser absorption. We characterized the physical effect of the exposure on the sample as well as the fold angle as a function of laser passes and demonstrate the ability to lift weights comparable with circuit packages and passive components. This technique can form complex, multifold structures with integrated electronics; as a demonstrator, we fold a commercial board with a common timing circuit. Laser forming to add a third dimension to printed circuit boards is an important technology to enable the rapid prototyping of complex 3D electronics.

Materials for Smart Soft Actuator Systems

In contrast to conventional hard actuators, soft actuators offer many vivid advantages, such as improved flexibility, adaptability, and reconfigurability, which are intrinsic to living systems. These properties make them particularly promising for different applications, including soft electronics, surgery, drug delivery, artificial organs, or prosthesis. The additional degree of freedom for soft actuatoric devices can be provided through the use of intelligent materials, which are able to change their structure, macroscopic properties, and shape under the influence of external signals. The use of such intelligent materials allows a substantial reduction of a device's size, which enables a number of applications that cannot be realized by externally powered systems. This review aims to provide an overview of the properties of intelligent synthetic and living/natural materials used for the fabrication of soft robotic devices. We discuss basic physical/chemical properties of the main kinds of materials (elastomers, gels, shape memory polymers and gels, liquid crystalline elastomers, semicrystalline ferroelectric polymers, gels and hydrogels, other swelling polymers, materials with volume change during melting/crystallization, materials with tunable mechanical properties, and living and naturally derived materials), how they are related to actuation and soft robotic application, and effects of micro/macro structures on shape transformation, fabrication methods, and we highlight selected applications.

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.

In Situ Formation of Ag Nanoparticles for Fiber Strain Sensors: Toward Textile-Based Wearable Applications

Wearable electronic devices have attracted significant attention as important components in several applications. Among various wearable electronic devices, interest in textile electronic devices is increasing because of their high deformability and portability in daily life. To develop textile electronic devices, fiber-based electronic devices should be fundamentally studied. Here, we report a stretchable and sensitive fiber strain sensor fabricated using only harmless materials during an in situ formation process. Despite using a mild and harmless reducing agent instead of typical strong and hazardous reducing agents, the developed fiber strain sensors feature a low initial electrical resistance of 0.9 Ω/cm, a wide strain sensing range (220%), high sensitivity (∼5.8 × 10⁴), negligible hysteresis, and high stability against repeated stretching-releasing deformation (5000 cycles). By applying the fiber sensors to various textiles, we demonstrate that the smart textile system can monitor various gestures in real-time and help users maintain accurate posture during exercise. These results will provide meaningful insights into the development of next-generation wearable applications.

Additive manufacturing landscape and materials perspective in 4D printing

4D printing is inspired by embedded product designs to produce stimuli-responsive consumables fabricated by available commercial 3D printers. Although significant progress on smart material performance has been made and different studies have focused on new strategies and process improvements in typical additive manufacturing. Herein, the proposed review article discusses material arrangements for 4D printing, highlighting the structural evolvement mechanism, the behavior of deformation, and their prospective implementation with respect. Starting from a generalized idea, and fundamental workflow, together with a graphical manifestation of the 4D printing concept, and 4D printing for shape-memory materials (SMMs), self-fitting wearables based on shape memory alloys (SMAs) are reviewed exclusively. Furthermore, the capabilities of single and multiple materials mechanisms for shape-shifting behavior are summarized. Finally, we explored the future application potential under succeeding context: SMA-based knitted garments, transforming food, and relevant sectors wise development and proceedings with the advancement in smart materials. We determined our review by aiming our future directions such as the "dream it and make it feasible" technology.

GRAPHICAL ABSTRACT

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.

Direct 2D-to-3D transformation of pen drawings

Pen drawing is a method that allows simple, inexpensive, and intuitive two-dimensional (2D) fabrication. To integrate such advantages of pen drawing in fabricating 3D objects, we developed a 3D fabrication technology that can directly transform pen-drawn 2D precursors into 3D geometries. 2D-to-3D transformation of pen drawings is facilitated by surface tension-driven capillary peeling and floating of dried ink film when the drawing is dipped into an aqueous monomer solution. Selective control of the floating and anchoring parts of a 2D precursor allowed the 2D drawing to transform into the designed 3D structure. The transformed 3D geometry can then be fixed by structural reinforcement using surface-initiated polymerization. By transforming simple pen-drawn 2D structures into complex 3D structures, our approach enables freestyle rapid prototyping via pen drawing, as well as mass production of 3D objects via roll-to-roll processing.

Remotely Controlled, Reversible, On-Demand Assembly and Reconfiguration of 3D Mesostructures via Liquid Crystal Elastomer Platforms

Three-dimensional (3D) mesostructures are gaining rapidly growing interest due to their potential applications in a broad range of areas. Despite intensive studies, remotely controlled, reversible, on-demand assembly and reconfiguration of 3D mesostructures, which are desired for many applications, including robotics, minimally invasive biomedical devices, and deployable systems, remain a challenge. Here, we introduce a facile strategy to utilize liquid crystal elastomers (LCEs), a soft polymer capable of large, reversible shape changes, as a platform for reversible assembly and programming of 3D mesostructures via compressive buckling of two-dimensional (2D) precursors in a remote and on-demand fashion. The highly stretchable, reversible shape-switching behavior of the LCE substrate, resulting from the soft elasticity of the material and the reversible nematic-isotropic transition of liquid crystal (LC) molecules upon remote thermal stimuli, provides deterministic thermal-mechanical control over the reversible assembly and reconfiguration processes. Demonstrations include experimental results and finite element simulations of 3D mesostructures with diverse geometries and material compositions, showing the versatility and reliability of the approach. Furthermore, a reconfigurable light-emitting system is assembled and morphed between its "on" and "off" status via the LCE platform. These results provide many exciting opportunities for areas from remotely programmable 3D mesostructures to tunable electronic systems.

Highly aligned carbon nanotubes and their sensor applications

Flexible electronics comprising carbon nanotube (CNT) membranes and polymer composites are used in diverse applications, including health monitoring. Devices prepared using such electronics need to exhibit acceptable sensitivity at high strains, with the advantage of negligible hysteresis. Herein, we report a simple, physically robust method to fabricate a highly sensitive and stretchable sensor that enables the detection of pressure, strain, and human activity with facial expressions based on the highly aligned carbon nanotubes embedded in polydimethylsiloxane (PDMS). The aligned CNT network in PDMS modulates the electron conduction path in a unidirectional manner and provides multimodal mechanical sensing ability with a wide sensing range and high sensitivity. The highly aligned CNT sensor demonstrates high-pressure sensitivity (1.29 kPa-1), excellent stability and repeatability (over 10 000 cycles) with negligible hysteresis, and a good strain sensitivity over a wide range (up to 65%) with a good linear response. We confirmed the applicability of the sensor to detect small signals, such as heartbeat and pulse rate, expressions, and voice recognition, and that it could distinguish between various human motions with a very short recovery time of approximately 50 ms.

Programming Shape-Morphing Behavior of Liquid Crystal Elastomers via Parameter-Encoded 4D Printing

Currently, four-dimensional (4D) printing programming methods are mainly structure-based, which usually requires more than one material to endow products with site-specific attributes. Here, we propose a new 4D printing programming approach that enables site-specific shape-morphing behaviors in a single material by regulating the printing parameters. Specifically, a direct ink writing three-dimensional (3D) printer with the ability to change printing parameters (e.g., deposition speed) on the fly is reported. By site-specifically adjusting print speed and print path to control the local nematic arrangements of printed liquid crystal elastomers (LCEs), the shape-morphing behaviors of the LCEs can be successfully programmed. In this way, locally programmed popping-up, self-assembling, and oscillating behaviors can be designed by varying the print speed in specific regions. Snake-like curling is realized by uniformly boosting the print speed in a single line. Furthermore, two theories and an ultrasound image diagnostic apparatus are employed to reveal the mechanism behind this behavior. This work provides a feasible way to realize the gradient transition of material properties through a single material. It broadens the design space and pushes the envelope of 4D printing, which is expected to be helpful in the fabrication of soft robotics and flexible electronics.

Theory and practice of origami in science

No longer just the purview of artists and enthusiasts, origami engineering has emerged as a potentially powerful tool to create three dimensional structures on disparate scales. Whether origami (and the closely related kirigami) engineering can emerge as a useful technology will depend crucially on both fundamental theoretical advances as well as the development of further fabrication tools.

Topology optimization and 3D printing of multimaterial magnetic actuators and displays

Upcoming actuation systems will be required to perform multiple tightly coupled functions analogous to their natural counterparts; e.g., the ability to control displacements and high-resolution appearance simultaneously is necessary for mimicking the camouflage seen in cuttlefish. Creating integrated actuation systems is challenging owing to the combined complexity of generating high-dimensional designs and developing multifunctional materials and their associated fabrication processes. Here, we present a complete toolkit consisting of multiobjective topology optimization (for design synthesis) and multimaterial drop-on-demand three-dimensional printing for fabricating complex actuators (>106 design dimensions). The actuators consist of soft and rigid polymers and a magnetic nanoparticle/polymer composite that responds to a magnetic field. The topology optimizer assigns materials for individual voxels (volume elements) while simultaneously optimizing for physical deflection and high-resolution appearance. Unifying a topology optimization-based design strategy with a multimaterial fabrication process enables the creation of complex actuators and provides a promising route toward automated, goal-driven fabrication.

Responsive, 3D Electronics Enabled by Liquid Crystal Elastomer Substrates

Traditional electronic devices are rigid, planar, and mechanically static. The combination of traditional electronic materials and responsive polymer substrates is of significant interest to provide opportunities to replace conventional electronic devices with stretchable, 3D, and responsive electronics. Liquid crystal elastomers (LCEs) are well suited to function as such dynamic substrates because of their large strain, reversible stimulus response that can be controlled through directed self-assembly of molecular order. Here, we discuss using LCEs as substrates for electronic devices that are flat during processing but then morph into controlled 3D structures. We design and demonstrate processes for a variety of electronic devices on LCEs including deformation-tolerant conducting traces and capacitors and cold temperature-responsive antennas. For example, patterning twisted nematic orientation within the substrate can be used to create helical electronic devices that stretch up to 100% with less than 2% change in resistance or capacitance. Moreover, we discuss self-morphing LCE antennas which can dynamically change the operating frequency from 2.7 GHz (room temperature) to 3.3 GHz (-65 °C). We envision applications for these 3D, responsive devices in wearable or implantable electronics and in cold-chain monitoring radio frequency identification sensors.

Programmable Self-Assembling 3D Architectures Generated by Patterning of Swellable MOF-Based Composite Films

The integration of swellable metal-organic frameworks (MOFs) into polymeric composite films is a straightforward strategy to develop soft materials that undergo reversible shape transformations derived from the intrinsic flexibility of MOF crystals. However, a crucial step toward their practical application relies on the ability to attain specific and programmable actuation, which enables the design of self-shaping objects on demand. Herein, a chemical etching method is demonstrated for the fabrication of patterned composite films showing tunable self-folding response, predictable and reversible 2D-to-3D shape transformations triggered by water adsorption/desorption. These films are fabricated by selective removal of swellable MOF crystals allowing control over their spatial distribution within the polymeric film. Upon exposure to moisture, various programmable 3D architectures, which include a mechanical gripper, a lift, and a unidirectional walking device, are generated. Remarkably, these 2D-to-3D shape transformations can be reversed by light-induced desorption. The reported strategy offers a platform for fabricating flexible MOF-based autonomous soft mechanical devices with functionalities for micromanipulation, automation, and robotics.

Shape-changing polymers for biomedical applications

Smart polymers that are capable of controlled shape transformations under external stimuli have attracted significant attention in the recent years due to the resemblance of this behavior to the biological intelligence observed in nature. In this review, we focus on the recent progress in the field of shape-morphing polymers, highlighting their most promising applications in the biomedical field.

Prospective Design, Rapid Prototyping, and Testing of Smart Dressings, Drug Delivery Patches, and Replacement Body Parts Using Microscopy Aided Design and ManufacturE (MADAME)

Natural materials exhibit smart properties including gradients in biophysical properties that engender higher order functions, as well as stimuli-responsive properties which integrate sensor and/or actuator capacities. Elucidation of mechanisms underpinning such smart material properties (i), and translation of that understanding (ii), represent two of the biggest challenges in emulating natural design paradigms for design and manufacture of disruptive materials, parts, and products. Microscopy Aided Design And ManufacturE (MADAME) stands for a computer-aided additive manufacturing platform that incorporates multidimensional (multi-D) printing and computer-controlled weaving. MADAME enables the creation of composite design motifs emulating e.g., patterns of woven protein fibers as well as gradients in different caliber porosities, mechanical, and molecular properties, found in natural tissues, from the skin on bones (periosteum) to tree bark. Insodoing, MADAME provides a means to manufacture a new genre of smart materials, products and replacement body parts that exhibit advantageous properties both under the influence of as well as harnessing dynamic mechanical loads to activate material properties (mechanoactive properties). This Technical Report introduces the MADAME technology platform and its associated machine-based workflow (pipeline), provides basic technical background of the novel technology and its applications, and discusses advantages and disadvantages of the approach in context of current 3 and 4D printing platforms.

Novel Materials for 3D Printing by Photopolymerization

The field of 3D printing, also known as additive manufacturing (AM), is developing rapidly in both academic and industrial research environments. New materials and printing technologies, which enable rapid and multimaterial printing, have given rise to new applications and utilizations. However, the main bottleneck for achieving many more applications is the lack of materials with new physical properties. Here, some of the recent reports on novel materials in this field, such as ceramics, glass, shape-memory polymers, and electronics, are reviewed. Although new materials have been reported for all three main printing approaches-fused deposition modeling, binder jetting or laser sintering/melting, and photopolymerization-based approaches, apparently, most of the novel physicochemical properties are associated with materials printed by photopolymerization approaches. Furthermore, the high resolution that can be achieved using this type of 3D printing, together with the new properties, has resulted in new implementations such as microfluidic, biomedical devices, and soft robotics. Therefore, the focus here is on photopolymerization-based additive manufacturing including the recent development of new methods, novel monomers, and photoinitiators, which result in previously inaccessible applications such as complex ceramic structures, embedded electronics, and responsive 3D objects.

Functional Polymers and Nanocomposites for 3D Printing of Smart Structures and Devices

Three-dimensional printing (3DP) has attracted a considerable amount of attention during the past years, being globally recognized as one of the most promising and revolutionary manufacturing technologies. Although the field is rapidly evolving with significant technological advancements, materials research remains a spotlight of interest, essential for the future developments of 3DP. Smart polymers and nanocomposites, which respond to external stimuli by changing their properties and structure, represent an important group of materials that hold a great promise for the fabrication of sensors, actuators, robots, electronics, and medical devices. The interest in exploring functional materials and their 3DP is constantly growing in an attempt to meet the ever-increasing manufacturing demand of complex functional platforms in an efficient manner. In this review, we aim to outline the recent advances in the science and engineering of functional polymers and nanocomposites for 3DP technologies. The report covers temperature-responsive polymers, polymers and nanocomposites with electromagnetic, piezoresistive and piezoelectric behaviors, self-healing polymers, light- and pH-responsive materials, and mechanochromic polymers. The main objective is to link the performance and functionalities to the fundamental properties, chemistry, and physics of the materials, and to the process-driven characteristics, in an attempt to provide a multidisciplinary image and a deeper understanding of the topic. The challenges and opportunities for future research are also discussed.

Soft Tendril-Inspired Grippers: Shape Morphing of Programmable Polymer-Paper Bilayer Composites

Nastic movements in plants that occur in response to environmental stimuli have inspired many man-made shape-morphing systems. Tendril is an exemplification serving as a parasitic grasping component for the climbing plants by transforming from a straight shape into a coiled configuration via the asymmetric contraction of internal stratiform plant tissues. Inspired by tendrils, this study using a three-dimensional (3D) printing approach developed a class of soft grippers with preprogrammed deformations being capable of imitating the general motions of plant tendrils, including bending, spiral, and helical distortions for grasping. These grippers initially in flat configurations were tailored from a polymer-paper bilayer composite sheet fabricated via 3D printing a polymer on the paper substrate with different patterns. The rough and porous paper surface provides a printed polymer that is well-adhered to the paper substrate which in turn serves as a passive strain-limiting layer. During printing, the melted polymer filament is stretched, enabling the internal strain to be stored in the printed polymer as memory, and then it can be thermally released, which will be concurrently resisted by the paper layer, resulting in various transformations based on the different printed geometries. These obtained transformations were then used for designing grippers to grasp objects with corresponding motions. Furthermore, a fully equipped robotic tendril with three segments was reproduced, where one segment was used for grasping the object and the other two segments were used for forming a tendril-like twistless spring-like structure. This study further helps in the development of soft robots using active polymer materials for engineered systems.

Catalytically Initiated Gel-in-Gel Printing of Composite Hydrogels

Herein, we describe a method to 3D print robust hydrogels and hydrogel composites via gel-in-gel 3D printing with catalytically activated polymerization to induce cross-linking. A polymerizable shear-thinning hydrogel ink with tetramethylethylenediamine as catalyst was directly extruded into a shear-thinning hydrogel support bath with ammonium persulfate as initiator in a pattern-wise manner. When the two gels came into contact, the free radicals generated by the catalyst initiated the free-radical polymerization of the hydrogel ink. Unlike photocuring, a catalyst-initiated polymerization is suitable for printing hydrogel composites of varying opacity, since it does not depend upon light penetration through the sample. The hydrogel support bath also exhibited a temperature-responsive behavior in which the gel "melted" upon cooling below 16 °C. Therefore, the printed object was easily removed by cooling the gel to a liquid state. Hydrogel composites with graphene oxide and multiwalled carbon nanotubes (MWCNTs) were successfully printed. The printed composites with MWCNTs afforded photothermally active objects, which have utility as stimuli-responsive actuators.