Self-folding polymeric containers for encapsulation and delivery of drugs

Roll-to-roll fabrication of three-dimensional self-folding microstructures

Self-folding technology offers a promising alternative to conventional microfabrication techniques. It utilizes controlled and imbalanced stresses to transform specific patterns of flat materials into pre-determined three-dimensional (3D) structures for diverse applications. However, current production methods of self-folding structures are mostly limited to lab-scale production. In this study, a novel roll-to-roll (R2R) production setup is developed to address the limited scalability of self-folding technology. The R2R setup continuously stretches and bonds a pre-cured PDMS (polydimethylsiloxane) film to another PDMS film attached to a stiff PET (polyethylene terephthalate) carrier layer. This creates a bilayer PDMS film with imbalanced stress, causing it to self-fold into pre-determined 3D shapes upon patterning and releasing from the PET carrier layer. The R2R setup achieves a production rate of 96 cm2 min-1, significantly surpassing our previous method based on spin-coating and baking. This demonstrates the potential of R2R technology for industrial-scale production of self-folding microstructures.

Untethered bistable origami crawler for confined applications

Magnetically actuated miniature origami crawlers are capable of robust locomotion in confined environments but are limited to passive functionalities. Here, we propose a bistable origami crawler that can shape-morph to access two separate regimes of folding degrees of freedom that are separated by an energy barrier. Using the modified bistable V-fold origami crease pattern as the fundamental unit of the crawler, we incorporated internal permanent magnets to enable untethered shape-morphing. By modulating the orientation of the external magnetic field, the crawler can reconfigure between an undeployed locomotion state and a deployed load-bearing state. In the undeployed state, the crawler can deform to enable out-of-plane crawling for robust bi-directional locomotion and navigation in confined environments based on friction anisotropy. In the deployed state, the crawler can execute microneedle insertion in confined environments. Through this work, we demonstrated the advantage of incorporating bistability into origami mechanisms to expand their capabilities in space-constraint applications.

Tunable wave coupling in periodically rotated Miura-ori tubes

Origami folding structures are vital in shaping programmable mechanical material properties. Of particular note, tunable dynamical properties of elastic wave propagation in origami structures have been reported. Despite the promising features of origami metamaterials, the influence of the kinematics of tessellated origami structures on elastic wave propagation remain unexplored. This study proposes elastic metamaterials using connected Miura-ori tubes, the kinematics of which are coupled by folding and unfolding motions in a tubular axis; achieved by periodically connecting non-rotated and rotated Miura-ori tubes. The kinematics generate wave modes with localized deformations within the unit cell of the metamaterials, affecting the global elastic deformation of Miura-ori tubes via the coupling of wave modes. Dispersion analysis, using the generalized Bloch wave framework based on bar-and-hinge models, verifies the influence of kinematics in the connected tubes on elastic wave propagation. Furthermore, folding the connected tubes changes the coupling strength of wave modes between the kinematics and global elastic deformation of the tubes by breaking the ideal kinematics. The coupling of wave modescontributes to the formation of the band gaps and their tunability. These findings enable adaptive and in situ tunability of band structures to prohibit elastic waves in the desired frequency ranges.This article is part of the theme issue 'Origami/Kirigami-inspired structures: from fundamentals to applications'.

Illumination-adjustable photoacoustic and harmonic ultrasound for tracking magnetically driven microrobots

The development of microrobots for biomedical applications has enabled tasks such as targeted drug delivery, minimally invasive surgeries, and precise diagnostics. However, effective in vivo navigation and control remain challenging due to their small size and complex body environment. Photoacoustic (PA) and ultrasound (US) imaging techniques, which offer high contrast, high resolution, and deep tissue penetration, are integrated to enhance microrobot visualization and tracking. Traditional imaging systems have a narrow effective illumination area, suffer from severe reflection artifacts, and are affected by strong electromagnetic fields. To address this, we present an illumination-adjustable PA and harmonic US imaging system with a customized pushrod mechanism for real-time focus adjustment. Experiments demonstrate high-resolution imaging and accurate microrobot positioning, showcasing the potential for biomedical applications, especially in minimally invasive procedures.

Engineered Living Structures with Shape-Morphing Capability Enabled by 4D Printing with Functional Bacteria

Engineered living structures with the incorporation of functional bacteria have been explored extensively in recent years and have shown promising potential applications in biosensing, environmental remediation, and biomedicine. However, it is still rare and challenging to achieve multifunctional capabilities such as material production, shape transformation, and sensing in a single-engineered living structure. In this study, we demonstrate bifunctional living structures by synergistically integrating cellulose-generating bacteria with pH-responsive hydrogels, and the entire structures can be precisely fabricated by three-dimensional (3D) printing. Such 3D-printed bifunctional living structures produce cellulose nanofibers in ambient conditions and have reversible and controlled shape-morphing properties (usually referred to as four-dimensional printing). Those functionalities make them biomimetic versions of silkworms in the sense that both can generate nanofibers and have body motion. We systematically investigate the processing-structure-property relationship of the bifunctional living structures. The on-demand separation of 3D cellulose structures from the hydrogel template and the living nature of the bacteria after processing and shape transformation are also demonstrated.

Features of Anisotropic Drug Delivery Systems

Natural materials are anisotropic. Delivery systems occurring in nature, such as viruses, blood cells, pollen, and many others, do have anisotropy, while delivery systems made artificially are mostly isotropic. There is apparent complexity in engineering anisotropic particles or capsules with micron and submicron sizes. Nevertheless, some promising examples of how to fabricate particles with anisotropic shapes or having anisotropic chemical and/or physical properties are developed. Anisotropy of particles, once they face biological systems, influences their behavior. Internalization by the cells, flow in the bloodstream, biodistribution over organs and tissues, directed release, and toxicity of particles regardless of the same chemistry are all reported to be factors of anisotropy of delivery systems. Here, the current methods are reviewed to introduce anisotropy to particles or capsules, including loading with various therapeutic cargo, variable physical properties primarily by anisotropic magnetic properties, controlling directional motion, and making Janus particles. The advantages of combining different anisotropy in one entity for delivery and common problems and limitations for fabrication are under discussion.

4D printing for biomedical applications

While three-dimensional (3D) printing excels at fabricating static constructs, it fails to emulate the dynamic behavior of native tissues or the temporal programmability desired for medical devices. Four-dimensional (4D) printing is an advanced additive manufacturing technology capable of fabricating constructs that can undergo pre-programmed changes in shape, property, or functionality when exposed to specific stimuli. In this Perspective, we summarize the advances in materials chemistry, 3D printing strategies, and post-printing methodologies that collectively facilitate the realization of temporal dynamics within 4D-printed soft materials (hydrogels, shape-memory polymers, liquid crystalline elastomers), ceramics, and metals. We also discuss and present insights about the diverse biomedical applications of 4D printing, including tissue engineering and regenerative medicine, drug delivery, in vitro models, and medical devices. Finally, we discuss the current challenges and emphasize the importance of an application-driven design approach to enable the clinical translation and widespread adoption of 4D printing.

Engineering Kirigami Frameworks Toward Real-World Applications

The surge in advanced manufacturing techniques has led to a paradigm shift in the realm of material design from developing completely new chemistry to tailoring geometry within existing materials. Kirigami, evolved from a traditional cultural and artistic craft of cutting and folding, has emerged as a powerful framework that endows simple 2D sheets with unique mechanical, thermal, optical, and acoustic properties, as well as shape-shifting capabilities. Given its flexibility, versatility, and ease of fabrication, there are significant efforts in developing kirigami algorithms to create various architectured materials for a wide range of applications. This review summarizes the fundamental mechanisms that govern the transformation of kirigami structures and elucidates how these mechanisms contribute to their distinctive properties, including high stretchability and adaptability, tunable surface topography, programmable shape morphing, and characteristics of bistability and multistability. It then highlights several promising applications enabled by the unique kirigami designs and concludes with an outlook on the future challenges and perspectives of kirigami-inspired metamaterials toward real-world applications.

Roll-to-roll, high-resolution 3D printing of shape-specific particles

Particle fabrication has attracted recent attention owing to its diverse applications in bioengineering1,2, drug and vaccine delivery3-5, microfluidics6,7, granular systems8,9, self-assembly5,10,11, microelectronics12,13 and abrasives14. Herein we introduce a scalable, high-resolution, 3D printing technique for the fabrication of shape-specific particles based on roll-to-roll continuous liquid interface production (r2rCLIP). We demonstrate r2rCLIP using single-digit, micron-resolution optics in combination with a continuous roll of film (in lieu of a static platform), enabling the rapidly permutable fabrication and harvesting of shape-specific particles from a variety of materials and with complex geometries, including geometries not possible to achieve with advanced mould-based techniques. We demonstrate r2rCLIP production of mouldable and non-mouldable shapes with voxel sizes as small as 2.0 × 2.0 µm2 in the print plane and 1.1 ± 0.3 µm unsupported thickness, at speeds of up to 1,000,000 particles per day. Such microscopic particles with permutable, intricate designs enable direct integration within biomedical, analytical and advanced materials applications.

Optimal face-to-face coupling for fast self-folding kirigami

Kirigami-inspired designs can enable self-folding three-dimensional materials from flat, two-dimensional sheets. Hierarchical designs of connected levels increase the diversity of possible target structures, yet they can lead to longer folding times in the presence of fluctuations. Here, we study the effect of rotational coupling between levels on the self-folding of two-level kirigami designs driven by thermal noise in a fluid. Naturally present due to hydrodynamic resistance, we find that this coupling parameter can significantly impact a structure's self-folding pathway, thus enabling us to assess the quality of a kirigami design and the possibility for its optimization in terms of its folding rate and yield.

Tailoring of Silicone Urethane Methacrylate Resin for Vat Photopolymerization-Based 3D Printing of Shape Memory Polymers

Polydimethylsiloxane (PDMS) or silicone elastomers have garnered considerable attention in the field of medical device applications due to their superior thermal stability. However, conventional manufacturing techniques for silicone elastomers suffer from drawbacks such as cost, lengthy production time, and inherent difficulties in fabricating complex structures. To address these limitations, photosensitive polydimethylsiloxane urethane methacrylate (PDMSUMA) oligomers were synthesized, and their curing behaviors were specifically investigated for vat photopolymerization 3D printing applications. The study focused on exploring the impact of weight ratios between poly(ethylene glycol) dimethacrylate (PEGDMA) and 2-hydroxyethyl methacrylate (HEMA) in the PDMSUMA resin formulation. The addition of PEGDMA as a reactive diluent was found to enhance the printability of the PDMSUMA resin and decrease its viscosity. Thermal, mechanical, and shape memory properties of the 3D-printed specimens were examined. Our findings demonstrate the potential of PDMSUMA resins for developing customizable shape memory materials with tailored properties.

Microelectronic Morphogenesis: Smart Materials with Electronics Assembling into Artificial Organisms

Microelectronic morphogenesis is the creation and maintenance of complex functional structures by microelectronic information within shape-changing materials. Only recently has in-built information technology begun to be used to reshape materials and their functions in three dimensions to form smart microdevices and microrobots. Electronic information that controls morphology is inheritable like its biological counterpart, genetic information, and is set to open new vistas of technology leading to artificial organisms when coupled with modular design and self-assembly that can make reversible microscopic electrical connections. Three core capabilities of cells in organisms, self-maintenance (homeostatic metabolism utilizing free energy), self-containment (distinguishing self from nonself), and self-reproduction (cell division with inherited properties), once well out of reach for technology, are now within the grasp of information-directed materials. Construction-aware electronics can be used to proof-read and initiate game-changing error correction in microelectronic self-assembly. Furthermore, noncontact communication and electronically supported learning enable one to implement guided self-assembly and enhance functionality. Here, the fundamental breakthroughs that have opened the pathway to this prospective path are reviewed, the extent and way in which the core properties of life can be addressed are analyzed, and the potential and indeed necessity of such technology for sustainable high technology in society is discussed.

Engineering confined fluids to autonomously assemble hierarchical 3D structures

The inherent coupling of chemical and mechanical behavior in fluid-filled microchambers enables the fluid to autonomously perform work, which in turn can direct the self-organization of objects immersed in the solution. Using theory and simulations, we show that the combination of diffusioosmotic and buoyancy mechanisms produce independently controlled, respective fluid flows: one generated by confining surfaces and the other in the bulk of the solution. With both flows present, the fluid can autonomously join 2D, disconnected pieces to a chemically active, "sticky" base and then fold the resulting layer into regular 3D shapes (e.g. pyramids, tetrahedrons, and cubes). Here, the fluid itself performs the work of construction and thus, this process does not require extensive external machinery. If several sticky bases are localized on the bottom surface, the process can be parallelized, with the fluid simultaneously forming multiple structures of the same or different geometries. Hence, this approach can facilitate the relatively low-cost, mass production of 3D micron to millimeter-sized structures. Formed in an aqueous solution, the assembled structures could be compatible with biological environments, and thus, potentially useful in medical and biochemical applications.

Learning to swim efficiently in a nonuniform flow field

Microswimmers can acquire information on the surrounding fluid by sensing mechanical queues. They can then navigate in response to these signals. We analyze this navigation by combining deep reinforcement learning with direct numerical simulations to resolve the hydrodynamics. We study how local and nonlocal information can be used to train a swimmer to achieve particular swimming tasks in a nonuniform flow field, in particular, a zigzag shear flow. The swimming tasks are (1) learning how to swim in the vorticity direction, (2) learning how to swim in the shear-gradient direction, and (3) learning how to swim in the shear-flow direction. We find that access to laboratory frame information on the swimmer's instantaneous orientation is all that is required in order to reach the optimal policy for tasks (1) and (2). However, information on both the translational and rotational velocities seems to be required to accomplish task (3). Inspired by biological microorganisms, we also consider the case where the swimmers sense local information, i.e., surface hydrodynamic forces, together with a signal direction. This might correspond to gravity or, for microorganisms with light sensors, a light source. In this case, we show that the swimmer can reach a comparable level of performance to that of a swimmer with access to laboratory frame variables. We also analyze the role of different swimming modes, i.e., pusher, puller, and neutral.

Nanomaterial-decorated micromotors for enhanced photoacoustic imaging

Micro-and nanorobots have the potential to perform non-invasive drug delivery, sensing, and surgery in living organisms, with the aid of diverse medical imaging techniques. To perform such actions, microrobots require high spatiotemporal resolution tracking with real-time closed-loop feedback. To that end,  photoacoustic imaging has appeared as a promising technique for imaging microrobots in deep tissue with higher molecular specificity and contrast. Here, we present different strategies to track magnetically-driven micromotors with improved contrast and specificity using dedicated contrast agents (Au nanorods and nanostars). Furthermore, we discuss the possibility of improving the light absorption properties of the employed nanomaterials considering possible light scattering and coupling to the underlying metal-oxide layers on the micromotor's surface. For that, 2D COMSOL simulation and experimental results were correlated, confirming that an increased spacing between the Au-nanostructures and the increase of thickness of the underlying oxide layer lead to enhanced light absorption and preservation of the characteristic absorption peak. These characteristics are important when visualizing the micromotors in a complex in vivo environment, to distinguish them from the light absorption properties of the surrounding natural chromophores.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12213-023-00156-7.

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.

Feasibility and potential of laccase-based enzyme in wastewater treatment through sustainable approach: A review

The worldwide increase in metropolitan cities and rise in industrialization have resulted in the assimilation of hazardous pollutants into the ecosystems. Different physical, chemical and biological techniques have been employed to remove these toxins from water bodies. Several bioprocess applications using microbes and their enzymes are utilized to achieve the goal. Biocatalysts, such as laccases, are employed explicitly to deplete a variety of organic pollutants. However, the degradation of contaminants using biocatalysts has many disadvantages concerning the stability and activity of the enzyme. Hence, they are immobilized on different supports to improve the enzyme kinetics and recyclability. Furthermore, standard wastewater treatment methods are not effective in eliminating all the contaminants. As a result, membrane separation technologies have emerged to overcome the limitations of traditional wastewater treatment methods. Moreover, enzymes immobilized onto these membranes have generated new avenues in wastewater purification technology. This review provides the latest information on laccases from diverse sources, their molecular framework and their mode of action. This report also gives information about various immobilization techniques and the application of membrane bioreactors to eliminate and biotransform hazardous contaminants. In a nutshell, laccases appear to be the most promising biocatalysts for green and cost-efficient wastewater treatment technologies.

Microscopic robots with onboard digital control

Autonomous robots-systems where mechanical actuators are guided through a series of states by information processing units to perform a predesigned function-are expected to revolutionize everything from health care to transportation. Microscopic robots are poised for a similar revolution in fields from medicine to environmental remediation. A key hurdle to developing these microscopic robots is the integration of information systems, particularly electronics fabricated at commercial foundries, with microactuators. Here, we develop such an integration process and build microscopic robots controlled by onboard complementary metal oxide semiconductor electronics. The resulting autonomous, untethered robots are 100 to 250 micrometers in size, are powered by light, and walk at speeds greater than 10 micrometers per second. In addition, we demonstrate a microscopic robot that can respond to an optical command. This work paves the way for ubiquitous autonomous microscopic robots that perform complex functions, respond to their environments, and communicate with the outside world.

Transparent and self-healing elastomers for reconfigurable 3D materials

Transparent soft materials have been widely used in applications ranging from packaging to flexible displays, wearable devices, and optical lenses. Nevertheless, soft materials are susceptible to mechanical damages, leading to functional failure and premature disposal. Herein, we introduce a transparent self-healing elastomer that is able to repair the polymer network via exchange reactions of dynamic disulfide bonds. Due to its self-healing ability, the mechanical properties of the elastomer can be recovered, as well as its transparency after multiple cycles of abrasion and healing. The self-healing polymer is fabricated into three-dimensional (3D) structures by folding or modular origami assembly of planar self-healing polymer sheets. The 3D polymer objects are employed as storage containers of solid and liquid substances, reactors for photopolymerization, and cuvettes for optical measurements (exhibiting superior properties to those of commercial cuvettes). These dynamic polymers show outstanding mechanical, optical, and recycling properties that could potentially be further adapted in adaptive smart packaging, reconfigurable materials, optical devices, and recycling of elastomers. This article is protected by copyright. All rights reserved.

Conformational Transition-Driven Self-Folding Hydrogel Based on Silk Fibroin and Gelatin for Tissue Engineering Applications

Self-folding is a rapidly evolving method for converting flat objects into three-dimensional structures. However, because there are few materials with suitable properties, application of self-folding in tissue engineering has been hindered greatly. Herein, a novel self-folding hydrogel using a conformational transition mechanism was developed by employing a photocrosslinkable silk fibroin and gelatin. It was hypothesized that differences in the amount of β-sheet formation between the upper and lower layers would supply additional folding stress and drive the self-folding behaviour of a bilayer patch, which could improve the mechanical properties and long-term stability of the self-folded structure. In this study, the impact of various proportions of β-sheets in composite hydrogels on their swelling, mechanics, and internal microstructures were investigated. Subsequently, the folding process parameters were optimized, and diffusion through the folded tubular structure was studied with a perfusion test. Finally, it was proven that the self-folding hydrogel system is cytocompatible and can be utilized to build a 3D coculture system of "endothelial cells-smooth muscle cells". These findings suggest that the self-folding hydrogel could be a promising candidate for applications in blood vessel tissue engineering and regenerative medicine. This article is protected by copyright. All rights reserved.

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.

Inhibiting Phase Transfer of Protein Nanoparticles by Surface Camouflage-A Versatile and Efficient Protein Encapsulation Strategy

Engineering a system with a high mass fraction of active ingredients, especially water-soluble proteins, is still an ongoing challenge. In this work, we developed a versatile surface camouflage strategy that can engineer systems with an ultrahigh mass fraction of proteins. By formulating protein molecules into nanoparticles, the demand of molecular modification was transformed into a surface camouflage of protein nanoparticles. Thanks to electrostatic attractions and van der Waals interactions, we camouflaged the surface of protein nanoparticles through the adsorption of carrier materials. The adsorption of carrier materials successfully inhibited the phase transfer of insulin, albumin, β-lactoglobulin, and ovalbumin nanoparticles. As a result, the obtained microcomposites featured with a record of protein encapsulation efficiencies near 100% and a record of protein mass fraction of 77%. After the encapsulation in microcomposites, the insulin revealed a hypoglycemic effect for at least 14 d with one single injection, while that of insulin solution was only ∼4 h.

Dual Ultrasound and Photoacoustic Tracking of Magnetically Driven Micromotors: From In Vitro to In Vivo

The fast evolution of medical micro- and nanorobots in the endeavor to perform non-invasive medical operations in living organisms has boosted the use of diverse medical imaging techniques in the last years. Among those techniques, photoacoustic imaging (PAI), considered a functional technique, has shown to be promising for the visualization of micromotors in deep tissue with high spatiotemporal resolution as it possesses the molecular specificity of optical methods and the penetration depth of ultrasound. However, the precise maneuvering and function's control of medical micromotors, in particular in living organisms, require both anatomical and functional imaging feedback. Therefore, herein, the use of high-frequency ultrasound and PAI is reported to obtain anatomical and molecular information, respectively, of magnetically-driven micromotors in vitro and under ex vivo tissues. Furthermore, the steerability of the micromotors is demonstrated by the action of an external magnetic field into the uterus and bladder of living mice in real-time, being able to discriminate the micromotors' signal from one of the endogenous chromophores by multispectral analysis. Finally, the successful loading and release of a model cargo by the micromotors toward non-invasive in vivo medical interventions is demonstrated.

Adhesion directed capillary origami

Capillary origami takes advantage of the surface forces of a liquid drop to assemble thin film structures. After a structure is assembled, the drop then evaporates away. The transient nature of the liquid drop means that the creation of dry and stable structures is impossible. Work presented in this paper shows that adhesion is, in fact, a key tool that enables the creation of stable, complex, capillary assembled origami structures, rather than a problem to be avoided. Here, polydimethylsiloxane thin films were used in several simple experiments designed to identify the balance between substrate-film adhesion and film-film adhesion in the context of capillary assembly. We then demonstrate how directional adhesion can be used to direct film peeling in order to create non-trivial patterned folds after a fluid drop is deposited. A minimal complex structure, a "double-fold" was created to demonstrate how adhesion uniquely facilitates multiple-step capillary assembly. Finally, a familiar "origami airplane" was created with these methods, demonstrating that adhesion aided capillary origami can be used to assemble complex, functional structures.

Crystal capillary origami capsule with self-assembled nanostructures

The self-assembling mechanism of elasto-capillaries opens new applications in micro and nanotechnology by providing 3D assembly structures with 2D planar unit cells, so-called capillary origami. To date, the final structure has been designed based on the predetermined shape and size of the unit cell. Here, we show that plate-like salt crystallites grow and cover the emulsion interface, which is driven by Laplace pressure. Eventually, it creates a spherical capsule with self-assembled nanostructures. The capsule and the crystallite are investigated by scanning electron microscopy and X-ray diffraction analysis. To explain the mechanism, we develop a theoretical model to estimate the capsule size, the shell thickness, and the conditions necessary to form the shell based on a thin-walled pressure vessel. The proposed crystal capillary origami can fabricate a three-dimensional self-assembled salt capsule without any complicated procedures. We believe that it can offer a new physicochemical avenue for the spontaneous and facile fabrication of water-soluble carrier particles.

Compressive Buckling Fabrication of 3D Cell-Laden Microstructures

Tissue architecture is a prerequisite for its biological functions. Recapitulating the three-dimensional (3D) tissue structure represents one of the biggest challenges in tissue engineering. Two-dimensional (2D) tissue fabrication methods are currently in the main stage for tissue engineering and disease modeling. However, due to their planar nature, the created models only represent very limited out-of-plane tissue structure. Here compressive buckling principle is harnessed to create 3D biomimetic cell-laden microstructures from microfabricated planar patterns. This method allows out-of-plane delivery of cells and extracellular matrix patterns with high spatial precision. As a proof of principle, a variety of polymeric 3D miniature structures including a box, an octopus, a pyramid, and continuous waves are fabricated. A mineralized bone tissue model with spatially distributed cell-laden lacunae structures is fabricated to demonstrate the fabrication power of the method. It is expected that this novel approach will help to significantly expand the utility of the established 2D fabrication techniques for 3D tissue fabrication. Given the widespread of 2D fabrication methods in biomedical research and the high demand for biomimetic 3D structures, this method is expected to bridge the gap between 2D and 3D tissue fabrication and open up new possibilities in tissue engineering and regenerative medicine.

Multiresponsive Cellulose Nanocrystal Cross-Linked Copolymer Hydrogels for the Controlled Release of Dyes and Drugs

Multiresponsive hydrogels have attracted tremendous interest due to their promising applications in tissue engineering, wearable devices, and flexible electronics. In this work, we report a multiresponsive upper critical solution temperature (UCST) composite hydrogel based on poly (acrylic acid-co-acrylamide), PAAc-co-PAAm, sequentially cross-linked by acid-hydrolysis cellulose nanocrystals (CNCs). Scanning electron microscopy (SEM) observations demonstrated that the hydrogels are formed by densely cross-linked porous structures. The PAAc/PAAm/CNC hybrid hydrogels exhibit swelling and shrinking properties that can be induced by multiple stimuli, including temperature, pH, and salt concentration. The driving force of the volume transition is the formation and dissociation of hydrogen bonds in the hydrogels. A certain content of CNCs can greatly enhance the shrinkage capability and mechanical strength of the hybrid hydrogels, but an excess addition may impair the contractility of the hydrogel. Furthermore, the hydrogels can be used as a matrix to adsorb dyes, such as methylene blue (MB), for water purification. MB may be partly discharged from hydrogels by saline solutions, especially by those with high ionic strength. Notably, through temperature-controlled hydrogel swelling and shrinking, doxorubicin hydrochloride (DOX-HCl) can be controllably adsorbed and released from the prepared hydrogels.

Lattice model for self-folding at the microscale

Three-dimensional shell-like structures can be obtained spontaneously at the microscale from the self-folding of 2D templates of rigid panels. At least for simple structures, the motion of each panel is consistent with a Brownian process and folding occurs through a sequence of binding events, where pairs of panels meet at a specific closing angle. Here, we propose a lattice model to describe the dynamics of self-folding. As an example, we study the folding of a pyramid of N lateral faces. We combine analytical and numerical Monte Carlo simulations to find how the folding time depends on the number of faces, closing angle, and initial configuration. Implications for the study of more complex structures are discussed.

Electroactive material-based biosensors for detection and drug delivery

Electroactive materials are employed at the interface of biology and electronics due to their advantageous intrinsic properties as soft organic electronics. We examine the most recent literature of electroactive material-based biosensors and their emerging role as theranostic devices for the delivery of therapeutic agents. We consider electroactive materials through the lens of smart drug delivery systems as materials that enable the release of therapeutic cargo in response to specific physiological and external stimuli and discuss the way these mechanisms are integrated into medical devices with examples of the latest advances. Studies that harness features unique to conductive polymers are emphasized; lastly, we highlight new perspectives and future research direction for this emerging technology and the challenges that remain to overcome.

Biodegradable Defined Shaped Printed Polymer Microcapsules for Drug Delivery

This work describes the preparation and characterization of printed biodegradable polymer (polylactic acid) capsules made in two different shapes: pyramid and rectangular capsules about 1 and 11 μm in size. Obtained core-shell capsules are described in terms of their morphology, loading efficiency, cargo release profile, cell cytotoxicity, and cell uptake. Both types of capsules showed monodisperse size and shape distribution and were found to provide sufficient stability to encapsulate small water-soluble molecules and to retain them for several days and ability for intracellular delivery. Capsules of 1 μm size can be internalized by HeLa cells without causing any toxicity effect. Printed capsules show unique characteristics compared with other drug delivery systems such as a wide range of possible cargoes, triggered release mechanism, and highly controllable shape and size.

Biomedical nanoparticle design: What we can learn from viruses

Viruses are nanomaterials with a number of properties that surpass those of many synthetic nanoparticles (NPs) for biomedical applications. They possess a rigorously ordered structure, come in a variety of shapes, and present unique surface elements, such as spikes. These attributes facilitate propitious biodistribution, the crossing of complex biological barriers and a minutely coordinated interaction with cells. Due to the orchestrated sequence of interactions of their stringently arranged particle corona with cellular surface receptors they effectively identify and infect their host cells with utmost specificity, while evading the immune system at the same time. Furthermore, their efficacy is enhanced by their response to stimuli and the ability to spread from cell to cell. Over the years, great efforts have been made to mimic distinct viral traits to improve biomedical nanomaterial performance. However, a closer look at the literature reveals that no comprehensive evaluation of the benefit of virus-mimetic material design on the targeting efficiency of nanomaterials exists. In this review we, therefore, elucidate the impact that viral properties had on fundamental advances in outfitting nanomaterials with the ability to interact specifically with their target cells. We give a comprehensive overview of the diverse design strategies and identify critical steps on the way to reducing them to practice. More so, we discuss the advantages and future perspectives of a virus-mimetic nanomaterial design and try to elucidate if viral mimicry holds the key for better NP targeting.

Shape memory effect and rapid reversible actuation of nanocomposite hydrogels with electrochemically controlled local metal ion coordination and crosslinking

Rapid and reversible actuation and shape memory effects are critical for biomimetic soft actuators based on polymer hydrogels. However, most conventional hydrogel actuators show very slow actuation or deformation rates in water. It remains a challenge to realize rapid actuations, particularly for hydrogels to actuate in air. Here, a novel strategy to create diverse hydrogel devices with shape memory effects and rapid reversible actuations even in air was demonstrated. This strategy relies on a precise definition of local crosslinking by using multivalent metal ion coordination. This is demonstrated by infiltrating Fe3+ ions into stretchable nanocomposite polyacrylamide hydrogels with the amide groups converted into primary amine groups for multivalent coordination and crosslinking. The Fe3+ coordination with amine groups enhanced the crosslink density and modulus, leading to deswelling. By using an iron rod electrode, the Fe3+ coordination and crosslinking were precisely controlled to generate hydrogels with heterogeneous local crosslinking, including Janus hydrogels, S-shaped hydrogels, and cross-shaped hydrogel grippers. These soft devices were reversibly actuated in tens of seconds when cyclically dehydrated in ethanol and rehydrated in water. Most interestingly, very rapid reversible actuations of a hydrogel device in air were demonstrated by using electro-redox reaction of Fe3+ and Fe2+ in the hydrogel, where the reversible local coordination crosslinking and decomposition served as a hinge to actuate the hydrogel. This strategy based on reversible local coordination and crosslinking may open an avenue for rapid fabrication of hydrogel devices with well-defined structures and actuation properties.

Selective Release of Doxorubicin from Cucurbit[8]uril Stabilized Gold Supra-Pyramid Host at pH of Small Intestine

Gold supra-pyramid structures were obtained by the addition of acidic solution of cucurbit[8]uril (CB[8]) to an aqueous solution of citrate stabilized gold nanoparticles (AuNP). The reaction resulted in the precipitation of supra-pyramid from the solution after just 1 min of shaking. Microscopic images confirmed formation of the supra-pyramid. The stepwise structural transformation towards the supra-pyramid was examined with variable concentrations of CB[8] to AuNP solution. Anionic counter parts of these acids (Br^- , NO₃ ^- , SO₄ ^2- and Cl^- ) controlled the size of the synthesized supra-pyramids. These supra-pyramid hosts showed uptake of three anticancer drugs: oral drugs etoposide, prednisolone and intravenous drug doxorubicin. Releases of drugs from these hosts were emulated at acidic stomach pH, basic small intestinal pH and in the presence of human serum albumin (HSA). The specific release of doxorubicin was confirmed at small intestinal pH 7.4. Poor release of drugs in presence of CB[8] specific guest 1-adamantanamine confirmed the role of the supra-pyramid as the exclusive host. The release of doxorubicin from the supra-pyramid at pH 7.4 was confirmed by fluorescence microscopic imaging with prostate cancer DU-145 cell line.

Multi-responsive hydrogel structures from patterned droplet networks

Responsive hydrogels that undergo controlled shape changes in response to a range of stimuli are of interest for microscale soft robotic and biomedical devices. However, these applications require fabrication methods capable of preparing complex, heterogeneous materials. Here we report a new approach for making patterned, multi-material and multi-responsive hydrogels, on a micrometre to millimetre scale. Nanolitre aqueous pre-gel droplets were connected through lipid bilayers in predetermined architectures and photopolymerized to yield continuous hydrogel structures. By using this droplet network technology to pattern domains containing temperature-responsive or non-responsive hydrogels, structures that undergo reversible curling were produced. Through patterning of gold nanoparticle-containing domains into the hydrogels, light-activated shape change was achieved, while domains bearing magnetic particles allowed movement of the structures in a magnetic field. To highlight our technique, we generated a multi-responsive hydrogel that, at one temperature, could be moved through a constriction under a magnetic field and, at a second temperature, could grip and transport a cargo.

Active matter therapeutics

Nanotherapies based on micelles, liposomes, polymersomes, nanocapsules, magnetic nanoparticles, and noble metal nanoparticles have been at the forefront of drug delivery in the past few decades. Some of these nanopharmaceuticals have been commercially applied to treat a wide range of diseases, from dry eye syndrome to cancer. However, the majority involve particles that are passive, meaning that they do not change shape, and they lack motility; the static features can limit their therapeutic efficacy. In this review, we take a critical look at an emerging field that seeks to utilize active matter for therapeutics. In this context, active matter can be broadly referred to as micro or nanosized constructs that energetically react with their environment or external fields and translate, rotate, vibrate or change shape. Essentially, the recent literature suggests that such particles could significantly augment present-day drug delivery, by enhancing transport and increasing permeability across anatomical barriers by transporting drugs within solid tumor microenvironments or disrupting cardiovascular plaque. We discuss examples of such particles and link the transport and permeability properties of active matter to potential therapeutic applications in the context of two major diseases, namely cancer and heart disease. We also discuss potential challenges, opportunities, and translational hurdles.

Soft-Matter Nanotubes: A Platform for Diverse Functions and Applications

Self-assembled organic nanotubes made of single or multiple molecular components can be classified into soft-matter nanotubes (SMNTs) by contrast with hard-matter nanotubes, such as carbon and other inorganic nanotubes. To date, diverse self-assembly processes and elaborate template procedures using rationally designed organic molecules have produced suitable tubular architectures with definite dimensions, structural complexity, and hierarchy for expected functions and applications. Herein, we comprehensively discuss every functions and possible applications of a wide range of SMNTs as bulk materials or single components. This Review highlights valuable contributions mainly in the past decade. Fifteen different families of SMNTs are discussed from the viewpoints of chemical, physical, biological, and medical applications, as well as action fields (e.g., interior, wall, exterior, whole structure, and ensemble of nanotubes). Chemical applications of the SMNTs are associated with encapsulating materials and sensors. SMNTs also behave, while sometimes undergoing morphological transformation, as a catalyst, template, liquid crystal, hydro-/organogel, superhydrophobic surface, and micron size engine. Physical functions pertain to ferro-/piezoelectricity and energy migration/storage, leading to the applications to electrodes or supercapacitors, and mechanical reinforcement. Biological functions involve artificial chaperone, transmembrane transport, nanochannels, and channel reactors. Finally, medical functions range over drug delivery, nonviral gene transfer vector, and virus trap.

Encoding kirigami bi-materials to morph on target in response to temperature

Shape morphing in response to an environmental stimulus, such as temperature, light, and chemical cues, is currently pursued in synthetic analogs for manifold applications in engineering, architecture, and beyond. Existing strategies mostly resort to active, namely smart or field responsive, materials, which undergo a change of their physical properties when subjected to an external stimulus. Their ability for shape morphing is intrinsic to the atomic/molecular structure as well as the mechanochemical interactions of their constituents. Programming shape changes with active materials require manipulation of their composition through chemical synthesis. Here, we demonstrate that a pair of off-the-shelf passive solids, such as wood and silicone rubber, can be topologically arranged in a kirigami bi-material to shape-morph on target in response to a temperature stimulus. A coherent framework is introduced to enable the optimal orchestration of bi-material units that can engage temperature to collectively deploy into a geometrically rich set of periodic and aperiodic shapes that can shape-match a predefined target. The results highlight reversible morphing by mechanics and geometry, thus contributing to relax the dependence of current strategies on material chemistry and fabrication.

4D biofabrication of skeletal muscle microtissues

Skeletal muscle is one of the most abundant tissues in the body. Although it has a relatively good regeneration capacity, it cannot heal in the case of disease or severe damage. Many current tissue engineering strategies fall short due to the complex structure of skeletal muscle. Biofabrication techniques have emerged as a popular set of methods for increasing the complexity of tissue-like constructs. In this paper, 4D biofabrication technique is introduced for fabrication of the skeletal muscle microtissues. To this end, a bilayer scaffold consisting of a layer of anisotropic methacrylated alginate fibers (AA-MA) and aligned polycaprolactone (PCL) fibers were fabricated using electrospinning and later induced to self-fold to encapsulate myoblasts. Bilayer mats undergo shape-transformation in an aqueous buffer, a process that depends on their overall thickness, the thickness of each layer and the geometry of the mat. Proper selection of these parameters allowed fabrication of scroll-like tubes encapsulating myoblasts. The myoblasts were shown to align along the axis of the anisotropic PCL fibers and further differentiated into aligned myotubes that contracted under electrical stimulation. Overall the significance of this approach is in the fabrication of hollow tubular constructs that can be further developed for the formation of a vascularized and functional muscle.

Fabrication of polymer and carbon polyhedra through controlled cross-linking and capillary deformations

Fabrication of polymer polyhedral structures is achieved by first producing origami sheets with dissimilar stiffness levels at their folds and faces via multi-step photolithography. Subsequent capillary folding of the sheets towards permanently folded target shapes is realized by thermally controlling, simultaneously, the compliance of the sheets and the volume of the deposited droplets. This fabrication method allows us to create millimeter and sub-millimeter polyhedral structures with arbitrary levels of folding, to manufacture permanently folded polymer polyhedra using single-material monolayer sheets, and to produce carbon shapes from these carbon-rich polymer polyhedra through pyrolysis.

Capillary Origami with Atomically Thin Membranes

Small-scale optical and mechanical components and machines require control over three-dimensional structure at the microscale. Inspired by the analogy between paper and two-dimensional materials, origami-style folding of atomically thin materials offers a promising approach for making microscale structures from the thinnest possible sheets. In this Letter, we show that a monolayer of molybdenum disulfide (MoS₂) can be folded into three-dimensional shapes by a technique called capillary origami, in which the surface tension of a droplet drives the folding of a thin sheet. We define shape nets by patterning rigid metal panels connected by MoS₂ hinges, allowing us to fold micron-scale polyhedrons. Finally, we demonstrate that these shapes can be folded in parallel without the use of micropipettes or microfluidics by means of a microemulsion of droplets that dissolves into the bulk solution to drive folding. These results demonstrate controllable folding of the thinnest possible materials using capillary origami and indicate a route forward for design and parallel fabrication of more complex three-dimensional micron-scale structures and machines.

Geometric and Kinematic Analyses and Novel Characteristics of Origami-Inspired Structures

In recent years, origami structures have been gradually applied in aerospace, flexible electronics, biomedicine, robotics, and other fields. Origami can be folded from two-dimensional configurations into certain three-dimensional structures without cutting and stretching. This study first introduces basic concepts and applications of origami, and outlines the common crease patterns, whereas the design of crease patterns is focused. Through kinematic analysis and verification on origami structures, origami can be adapted for practical engineering. The novel characteristics of origami structures promote the development of self-folding robots, biomedical devices, and energy absorption members. We briefly describe the development of origami kinematics and the applications of origami characteristics in various fields. Finally, based on the current research progress of crease pattern design, kinematic analysis, and origami characteristics, research directions of origami-inspired structures are discussed.

Enabling Robust Self-Folding Origami by Pre-Biasing Vertex Buckling Direction

Self-folding is a powerful approach to fabricate materials with complex 3D forms and advanced properties using planar patterning steps, but suffers from intrinsic limitations in robustness due to the highly bifurcated nature of configuration space around the flat state. Here, a simple mechanism is introduced to achieve robust self-folding of microscale origami by separating actuation into two discrete steps using different thermally responsive hydrogels. First, the vertices are pre-biased to move in the desired direction from the flat state by selectively swelling one of the two hydrogels at high temperature. Subsequently, the creases are folded toward their target angles by activating swelling of the second hydrogel upon cooling to room temperature. Since each vertex can be individually programmed to move upward or downward, it is possible to robustly select the desired branch even in multi-vertex structures with reasonably high complexity. This strategy provides key new principles for designing shaping-morphing materials that avoid undesired distractor states, expanding their potential applications in areas such as soft robotics, sensors, mechanical metamaterials, and deployable devices.

Recent Strategies in Extrusion-Based Three-Dimensional Cell Printing toward Organ Biofabrication

Reconstructing human organs is one of the ultimate goals of the medical industry. Organ printing utilizing three-dimensional cell printing technology to fabricate artificial living organ equivalents has shed light on the advancement of this field into a new era. Among three currently applied techniques (inkjet, laser-assisted, and extrusion-based), extrusion-based cell printing (ECP) has evoked the majority of interest due to its low cost, wide range of applicable materials, and ease of spatial and depositional controllability. Major challenges in organ reconstruction include difficulties in precisely fabricating complex structural features, creating perfusable and functional vasculatures, and mimicking biophysical and biochemical characteristics in the printed constructs. In this review, we describe the merits and limitations of ECP for organ fabrication and discuss its recent advances aimed at overcoming these challenges. In addition, we delineate the expected future techniques for printing live tissue or organ substitutes.

A finite deformation theory of desolvation and swelling in partially photo-cross-linked polymer networks for 3D/4D printing applications

Photopolymerization is a process strongly dependent on the light field in the resin. This typically results in a non-uniformly crosslinked network where some parts of the network are fully cross-linked while other parts are partially crosslinked. The partially crosslinked part could exhibit a high volume expansion upon swelling and a high volume shrinkage upon desolvation. Through control over the light field in the photopolymer resin, this feature has been used to create solvent responsive shape changing structures as well as 3D/4D printed smart devices, showing promising application potential. In this paper, we develop a finite deformation theory to consider the nonuniform crosslink density of the network and the interaction between different species inside the network. The mechanical properties of the network are correlated with the reaction process and the existence of residual uncrosslinked monomers is included in the partially crosslinked network. The efficiency of the theory is proved by the finite element simulations of two special applications of the partially crosslinked network.

Twistable Origami and Kirigami: from Structure-Guided Smartness to Mechanical Energy Storage

For achieving active shape transformable materials and structures, smart materials with shape memory effects along with deliberate structure design are generally used as the critical parameters in realizing structure transformation. Beyond such conventional approaches, here a novel structure-guided multimaterial three-dimensional (3D) printing strategy based on twistable origami structures is demonstrated to realize dynamic smart shape transformation. By thermally or photothermally triggering the prestored energy in the twisted structures, the 3D-printed integrated origami structures based on Miura and square-twist origami structures coupled with modifying by kirigami approaches are enabled to present a variable multistep transformable feature as well as a manipulatable stimulus-response behavior. Such shape transformation configuration allows the integrated origami and kirigami structures for constructing smart structures in delivering dynamic multifunction. More importantly, the shape transformation mechanism also suggests a unique capability in mechanical energy storage and release, promising a novel prototype of mechanical actuators. Implication of the results offers a great platform to construct smart and active structures using structure-guided strategies.