4D Printing of Hydrogels: A Review

3D printing permits the construction of objects by layer‐by‐layer deposition of material, resulting in precise control of the dimensions and properties of complex printed structures. Although 3D printing fabricates inanimate objects, the emerging technology of 4D printing allows for animated structures that change their shape, function, or properties over time when exposed to specific external stimuli after fabrication. Among the materials used in 4D printing, hydrogels have attracted growing interest due to the availability of various smart hydrogels. The reversible shape‐morphing in 4D printed hydrogel structures is driven by a stress mismatch arising from the different swelling degrees in the parts of the structure upon application of a stimulus. This review provides the state‐of‐the‐art of 4D printing of hydrogels from the materials perspective. First, the main 3D printing technologies employed are briefly depicted, and, for each one, the required physico‐chemical properties of the precursor material. Then, the hydrogels that have been printed are described, including stimuli‐responsive hydrogels, non‐responsive hydrogels that are sensitive to solvent absorption/desorption, and multimaterial structures that are totally hydrogel‐based. Finally, the current and future applications of this technology are presented, and the requisites and avenues of improvement in terms of material properties are discussed.     

Combining 3D Printing with Electrospinning for Rapid Response and Enhanced Designability of Hydrogel Actuators

Porous structures have emerged as a breakthrough of shape‐morphing hydrogels to achieve a rapid response. However, these porous actuators generally suffer from a lack of complexity and diversity in obtained 3D shapes. Herein, a simple yet versatile strategy is developed to generate shape‐morphing hydrogels with both fast deformation and enhanced designability in 3D shapes by combining two promising technologies: electrospinning and 3D printing. Elaborate patterns are printed on mesostructured stimuli‐responsive electrospun membranes, modulating in‐plane and interlayer internal stresses induced by swelling/shrinkage mismatch, and thus guiding morphing behaviors of electrospun membranes to adapt to changes of the environment. With this strategy, a series of fast deformed hydrogel actuators are constructed with various distinctive responsive behaviors, including reversible/irreversible formations of 3D structures, folding of 3D tubes, and formations of 3D structures with multi low‐energy states. It is worth noting that although poly(N‐isopropyl acrylamide) is chosen as the model system in the present research, our strategy is applicable to other stimuli‐responsive hydrogels, which enriches designs of rapid deformed hydrogel actuators.     

Direct 3D Printing of High Strength Biohybrid Gradient Hydrogel Scaffolds for Efficient Repair of Osteochondral Defect

The emerging 3D printing technique allows for tailoring hydrogel‐based soft structure tissue scaffolds for individualized therapy of osteochondral defects. However, the weak mechanical strength and uncontrollable swelling intrinsic to conventional hydrogels restrain their use as bioinks. Here, a high‐strength thermoresponsive supramolecular copolymer hydrogel is synthesized by one‐step copolymerization of dual hydrogen bonding monomers, N‐acryloyl glycinamide, and N‐[tris(hydroxymethyl)methyl] acrylamide. The obtained copolymer hydrogels demonstrate excellent mechanical properties—robust tensile strength (up to 0.41 MPa), large stretchability (up to 860%), and high compressive strength (up to 8.4 MPa). The rapid thermoreversible gel ? sol transition behavior makes this copolymer hydrogel suitable for direct 3D printing. Successful preparation of 3D‐printed biohybrid gradient hydrogel scaffolds is demonstrated with controllable 3D architecture, owing to shear thinning property which allows continuous extrusion through a needle and also immediate gelation of fluid upon deposition on the cooled substrate. Furthermore, this biohybrid gradient hydrogel scaffold printed with transforming growth factor beta 1 and β‐tricalciumphosphate on distinct layers facilitates the attachment, spreading, and chondrogenic and osteogenic differentiation of human bone marrow stem cells (hBMSCs) in vitro. The in vivo experiments reveal that the 3D‐printed biohybrid gradient hydrogel scaffolds significantly accelerate simultaneous regeneration of cartilage and subchondral bone in a rat model.     

3D Printing of Hydrogels for Stretchable Ionotronic Devices

In the booming development of flexible electronics represented by electronic skins, soft robots, and human–machine interfaces, 3D printing of hydrogels, an approach used by the biofabrication community, is drawing attention from researchers working on hydrogel-based stretchable ionotronic devices. Such devices can greatly benefit from the excellent patterning capability of 3D printing in three dimensions, as well as the free design complexity and easy upscale potential. Compared to the advanced stage of 3D bioprinting, 3D printing of hydrogel ionotronic devices is in its infancy due to the difficulty in balancing printability, ionic conductivity, shape fidelity, stretchability, and other functionalities. In this review, a guideline is provided on how to utilize the power of 3D printing in building high-performance hydrogel-based stretchable ionotronic devices mainly from a materials’ point of view, highlighting the systematic approach to balancing the printability, printing quality, and performance of printed devices. Various 3D printing methods for hydrogels are introduced, and then the ink design principles, balancing printing quality, printed functions, such as elastic conductivity, self-healing ability, and device (e.g., flexible sensors, shape-morphing actuators, soft robots, electroluminescent devices, and electrochemical biosensors) performances are discussed. In conclusion, perspectives on the future directions of this exciting field are presented.     

A Self-Thickening and Self-Strengthening Strategy for 3D Printing High-Strength and Antiswelling Supramolecular Polymer Hydrogels as Meniscus Substitutes

3D printing of high-strength and antiswelling hydrogel-based load-bearing soft tissue scaffolds with similar geometric shape to natural tissues remains a great challenge owing to insurmountable trade-off between strength and printability. Herein, capitalizing on the concentration-dependent H-bonding-strengthened mechanism of supramolecular poly(N-acryloyl glycinamide) (PNAGA) hydrogel, a self-thickening and self-strengthening strategy, that is, loading the concentrated NAGA monomer into the thermoreversible low-strength PNAGA hydrogel is proposed to directly 3D printing latently H-bonding-reinforced hydrogels. The low-strength PNAGA serves to thicken the concentrated NAGA monomer, affording an appropriate viscosity for thermal-assisted extrusion 3D printing of soft PNAGA hydrogels bearing NAGA monomer and initiator, which are further polymerized to eventually generate high-strength and antiswelling hydrogels, due to the reconstruction of strong H-bonding interactions from postcompensatory PNAGA. Diverse polymer hydrogels can be printed with self-thickened corresponding monomer inks. Further, the self-thickened high-strength PNAGA hydrogel is printed into a meniscus, which is implanted in rabbit's knee as a substitute with in vivo outcome showing an appealing ability to efficiently alleviate the cartilage surface wear. The self-thickening strategy is applicable to directly printing a variety of polymer-hydrogel-based tissue engineering scaffolds without sacrificing mechanical strength, thus circumventing problems of printing high-strength hydrogels and facilitating their application scope.     

Inherent Photodegradability of Polymethacrylate Hydrogels: Straightforward Access to Biocompatible Soft Microstructures

Hydrogels are important functional materials useful for 3D cell culture, tissue engineering, 3D printing, drug delivery, sensors, or soft robotics. The ability to shape hydrogels into defined 3D structures, patterns, or particles is crucial for biomedical applications. Here, the rapid photodegradability of commonly used polymethacrylate hydrogels is demonstrated without the need to incorporate additional photolabile functionalities. Hydrogel degradation depths are quantified with respect to the irradiation time, light intensity, and chemical composition. It can be shown that these parameters can be utilized to control the photodegradation behavior of polymethacrylate hydrogels. The photodegradation kinetics, the change in mechanical properties of polymethacrylate hydrogels upon UV irradiation, as well as the photodegradation products are investigated. This approach is then exploited for microstructuring and patterning of hydrogels including hydrogel gradients as well as for the formation of hydrogel particles and hydrogel arrays of well‐defined shapes. Cell repellent but biocompatible hydrogel microwells are fabricated using this method and used to form arrays of cell spheroids. As this method is based on readily available and commonly used methacrylates and can be conducted using cheap UV light sources, it has vast potential to be applied by laboratories with various backgrounds and for diverse applications.     

Programmed Deformations of 3D‐Printed Tough Physical Hydrogels with High Response Speed and Large Output Force

Shape‐morphing hydrogels have emerging applications in biomedical devices, soft robotics, and so on. However, successful applications require a combination of excellent mechanical properties and fast responding speed, which are usually a trade‐off in hydrogel‐based devices. Here, a facile approach to fabricate 3D gel constructs by extrusion‐based printing of tough physical hydrogels, which show programmable deformations with high response speed and large output force, is described. Highly viscoelastic poly(acrylic acid‐co‐acrylamide) (P(AAc‐co‐AAm)) and poly(acrylic acid‐co‐N‐isopropyl acrylamide) (P(AAc‐co‐NIPAm)) solutions or their mixtures are printed into 3D constructs by using multiple nozzles, which are then transferred into FeCl₃ solution to gel the structures by forming robust carboxyl–Fe³⁺ coordination complexes. The printed gel fibers containing poly(N‐isopropyl acrylamide) segment exhibit considerable volume contraction in concentrated saline solution, whereas the P(AAc‐co‐AAm) ones do not contract. The mismatch in responsiveness of the gel fibers affords the integrated 3D gel constructs the shape‐morphing ability. Because of the small diameter of gel fibers, the printed gel structures deform and recover with a fast speed. A four‐armed gripper is designed to clamp plastic balls with considerable holding force, as large as 115 times the weight of the gripper. This strategy should be applicable to other tough hydrogels and broaden their applications.     

Engineering Tridimensional Hydrogel Tissue and Organ Phantoms with Tunable Springiness

Biomimicking organ phantoms with vivid biological structures and soft and slippery features are essential for in vitro biomedical applications yet remain hither to unmet challenges in their fabrication such as balancing between spatial structural complexity and matchable mechanical properties. Herein, 3D printable tissue-mimicking elastomeric double network hydrogels with tailorable stiffness are evolved to idiosyncratically match diverse biological soft tissues by regulating the compositions of hydrogel matrix and the density of metal coordination bonds. Relying on digital light processing 3D printing, various mechanically tunable biomimetic volumetric hydrogel organ constructs with structural complexity and fidelity, including kidney, brain, heart, liver, stomach, lung, trachea, intestine, and even the intricate vascularized tissues, are fabricated faultlessly. Proof-of-concept 3D printed hydrogel heart and liver phantoms provide sophisticated internal channels and cavity structures and external realistic anatomical architectures that more closely mimic native organs. For the in vitro application demonstration, a 3D printed hydrogel brain phantom with tortuous cerebral arteries and slippery characters serves as an effective neurosurgical training platform for realistic simulation of endovascular interventions. This platform offers a means to construct mechanically precisely tunable hydrogel-based biomimetic organ phantoms that are expected to be used in surgical training, medical device testing, and organs-on-chips.     

3D Printing of Multifunctional Conductive Polymer Composite Hydrogels

Functional conductive hydrogels are widely used in various application scenarios, such as artificial skin, cell scaffolds, and implantable bioelectronics. However, their novel designs and technological innovations are severely hampered by traditional manufacturing approaches. Direct ink writing (DIW) is considered a viable industrial-production 3D-printing technology for the custom production of hydrogels according to the intended applications. Unfortunately, creating functional conductive hydrogels by DIW has long been plagued by complicated ink formulation and printing processes. In this study, a highly 3D printable poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)-based ink made from fully commercially accessible raw materials is demonstrated. It is shown that complex structures can be directly printed with this ink and then precisely converted into high-performance hydrogels via a post-printing freeze–thawing treatment. The 3D-printed hydrogel exhibits high electrical conductivity of ≈2000 S m⁻¹, outstanding elasticity, high stability and durability in water, electromagnetic interference shielding, and sensing capabilities. Moreover, the hydrogel is biocompatible, showing great potential for implantable and tissue engineering applications. With significant advantages, the fabrication strategy is expected to open up a new route to create multifunctional hydrogels with custom features, and can bring new opportunities to broaden the applications of hydrogel materials.     

3D Electrophoresis‐Assisted Lithography (3DEAL): 3D Molecular Printing to Create Functional Patterns and Anisotropic Hydrogels

The ability to easily generate anisotropic hydrogel environments made from functional molecules with microscale resolution is an exciting possibility for the biomaterials community. This study reports a novel 3D electrophoresis‐assisted lithography (3DEAL) platform that combines elements from proteomics, biotechnology, and microfabrication to print well‐defined 3D molecular patterns within hydrogels. The potential of the 3DEAL platform is assessed by patterning immunoglobulin G, fibronectin, and elastin within nine widely used hydrogels and characterizing pattern depth, resolution, and aspect ratio. Furthermore, the technique's versatility is demonstrated by fabricating complex patterns including parallel and perpendicular columns, curved lines, gradients of molecular composition, and patterns of multiple proteins ranging from tens of micrometers to centimeters in size and depth. The functionality of the printed molecules is assessed by culturing NIH‐3T3 cells on a fibronectin‐patterned polyacrylamide‐collagen hydrogel and selectively supporting cell growth. 3DEAL is a simple, accessible, and versatile hydrogel‐patterning platform based on controlled molecular printing that may enable the development of tunable, chemically anisotropic, and hierarchical 3D environments.     

3D Printing of Resilin in Water by Multiphoton Absorption Polymerization

Resilin is an elastic rubber-like protein found in the cuticles of insects. It incorporates outstanding properties of high resilience and fatigue lifetime, where kinetic energy storage is needed for biological functions such as flight and jumps. Since resilin is rich in tyrosine groups, localized photopolymerization is enabled due to the ability to introduce di-tyrosine bonds by a ruthenium-based photoinitiator. Using Multiphoton Absorption Polymerization 3D printing process, objects containing 100% recombinant resilin protein are printed in water at a submicron length scale. Consequently, protein-based hydrogels with complex structures are printed using space positioning voxel polymerization. The objects are characterized by dynamic mechanical analysis using nanoindentation. Printing parameters such as printing speed and laser power are found to enable tuning the mechanical properties of the printed objects. The printed objects are soft and resilient, similar to native resilin, while presenting the highest resolution of a structure made entirely of a protein and better mechanical properties of common hydrogels and poly(dimethylsiloxane). Moreover, topography and mechanical properties enable cell growth and alignment without cell adhesion primers, thus facilitating biological applications. The fabrication of 3D resilin-based hydrogel will open the way for potential applications based on biomimicking and in creating new functional objects.     

Direct 3D Printed Biomimetic Scaffolds Based on Hydrogel Microparticles for Cell Spheroid Growth

Biocompatible hydrogel inks with shear‐thinning, appropriate yield strength, and fast self‐healing are desired for 3D bioprinting. However, the lack of ideal 3D bioprinting inks with outstanding printability and high structural fidelity, as well as cell‐compatibility, has hindered the progress of extrusion‐based 3D bioprinting for tissue engineering. In this study, novel self‐healable pre‐cross‐linked hydrogel microparticles (pcHμPs) of chitosan methacrylate (CHMA) and polyvinyl alcohol (PVA) hybrid hydrogels are developed and used as bioinks for extrusion‐based 3D printing of scaffolds with high fidelity and biocompatibility. The pcHμPs display excellent shear thinning when injected through a syringe and subsequently self‐heal into gels as shear forces are removed. Numerical simulations indicate that the pcHμPs experience a plug flow in the nozzle with minimal disturbance, which favors a steady and continuous printing. Moreover, the pcHμPs show a self‐supportive yield strength (540 Pa), which is critical for the fidelity of printed constructs. A series of biomimetic constructs with very high aspect ratio and delicate fine structures are directly printed by using the pcHμP ink. The 3D printed scaffolds support the growth of bone‐marrow‐derived mesenchymal stem cells and formation of cell spheroids, which are most important for tissue engineering.     

Surface Patterning of Hydrogels for Programmable and Complex Shape Deformations by Ion Inkjet Printing

Convenient patterning and precisely programmable shape deformations are crucial for the practical applications of shape deformable hydrogels. Here, a facile and versatile computer‐assisted ion inkjet printing technique is described that enables the direct printing of batched, very complicated patterns, especially those with well‐defined, programmable variation in cross‐linking densities, on one or both surfaces of a large‐sized hydrogel sample. A mechanically strong hydrogel containing poly(sodium acrylate) is first prepared, and then digital patterns are printed onto the hydrogel surfaces by using a commercial inkjet printer and an aqueous ferric solution. The complexation between the polyelectrolyte and ferric ions increases the cross‐linking density of the printed regions, and hence the gel sample can undergo shape deformation upon swelling/deswelling. The deformation rates and degrees of the hydrogels can be conveniently adjusted by changing the printing times or the different/gradient grayscale distribution of designed patterns. By printing appropriate patterns on one or both surfaces of the hydrogel sheets, many complex 3D shapes are obtained from shape deformations upon swelling/deswelling, such as cylindrical shell and forsythia flower (patterns on one surface), ding (patterns on both surfaces), blooming flower (different/gradient grayscale distributive patterns on one surface), and non‐Euclidean plates (different/gradient grayscale distributive patterns on both surfaces).     

Fluidic Infiltrative Assembly of 3D Hydrogel with Heterogeneous Composition and Function

3D hydrogels are powerful, multifunctional materials that are poised to become a building block in next-generation systems. Modern schemes to print complex 3D hydrogels are advancing rapidly; however, they possess several limitations including—but not limited to—polymer incompatibility or difficulty in imparting continuous heterogeneity in composition or function. Here, a simple strategy of synthesizing programmable hydrogel systems with tunable form and function in 3D is presented. This approach utilizes commercially available stereolithographic printer/resin to fabricate high-resolution molds followed by the programmed infiltration and gelation of hydrogel prepolymer. This mold is then sacrificed to yield 3D, multifunctional hydrogels exhibiting user-defined heterogeneity. The approach is compatible with numerous in-situ gelling polymers and modifiers ranging from interpenetrating networks of organic or synthetic polymers to functional materials possessing dense concentrations of nanomaterials or fluorescent markers. Accessible and versatile, this approach allows the fabrication of complex, multimaterial constructs with tunable 3D environmental responses inaccessible to well-established hydrogel 3D printing methods.     

Additive Manufacturing of Batteries

Additive manufacturing, i.e., 3D printing, is being increasingly utilized to fabricate a variety of complex‐shaped electronics and energy devices (e.g., batteries, supercapacitors, and solar cells) due to its excellent process flexibility, good geometry controllability, as well as cost and material waste reduction. In this review, the recent advances in 3D printing of emerging batteries are emphasized and discussed. The recent progress in fabricating 3D‐printed batteries through the major 3D‐printing methods, including lithography‐based 3D printing, template‐assisted electrodeposition‐based 3D printing, inkjet printing, direct ink writing, fused deposition modeling, and aerosol jet printing, are first summarized. Then, the significant achievements made in the development and printing of battery electrodes and electrolytes are highlighted. Finally, major challenges are discussed and potential research frontiers in developing 3D‐printed batteries are proposed. It is expected that with the continuous development of printing techniques and materials, 3D‐printed batteries with long‐term durability, favorable safety as well as high energy and power density will eventually be widely used in many fields.     

Photocurable 3D Printing of High Toughness and Self-Healing Hydrogels for Customized Wearable Flexible Sensors

Currently, most customized hydrogels can only be processed via extrusion-based 3D printing techniques, which is limited by printing efficiency and resolution. Here, a simple strategy for the rapid fabrication of customized hydrogels using a photocurable 3D printing technique is presented. This technique has been rarely used because the presence of water increases the molecular distance between the polymer chains and reduces the monomer polymerization rate, resulting in the failure of rapid solid-liquid separation during printing. Although adding cross-linkers to printing inks can effectively accelerate 3D cross-linked network formation, chemical cross-linking may result in reduced toughness and self-healing ability of the hydrogel. Therefore, an interpenetrated-network hydrogel based on non-covalent interactions is designed to form physical cross-links, affording fast solid-liquid separation. Poly(acrylic acid (AA)-N-vinyl-2-pyrrolidone (NVP)) and carboxymethyl cellulose (CMC) are cross-linked via Zn²⁺-ligand coordination and hydrogen bonding; the resulting mixed AA-NVP/CMC solution is used as the printing ink. The printed poly(AA-NVP/CMC) hydrogel exhibited high tensile toughness (3.38 MJ m⁻³) and superior self-healing ability (healed stress: 81%; healed strain: 91%). Some objects like manipulator are successfully customized by photocurable 3D printing using hydrogels with high toughness and complex structures. This high-performance hydrogel has great potential for application in flexible wearable sensors.     

Self-Shaping Soft Electronics Based on Patterned Hydrogel with Stencil-Printed Liquid Metal

Hydrogel-based soft electronics (HSE) is promising as implantable devices due to the similarity of hydrogel substrates to biologic tissues. Most existing HSE devices are based on conducting hydrogels that usually have weak mechanical properties, low conductivity, and poor patternability. Reported here is an HSE with good mechanical performance, high sensitivity, and versatile functions by stencil printing of liquid metal on a tough hydrogel, facilitating integration of multiple sensing units. Self-shaping ability is imparted to the HSE by creating gradient structure in the hydrogel substrate. The resultant HSE actively deforms into 3D configurations with zero or nonzero Gaussian curvature to fix on objects or organs with sophisticated geometries and maintains the sensing functions. The versatilities and potential applications of this HSE are demonstrated by monitoring motions of a rice field eel and beatings of a rabbit heart. Such HSE based on morphing substrate should pave the way for implantable electronics with better fixation and interfacial contact with the organs. The concept of morphing hydrogel devices can be extended to other soft electronics with responsive polymer films or elastomers as the substrates.     

Complex 3D‐Printed Microchannels within Cell‐Degradable Hydrogels

3D‐printing is emerging as a technology to introduce microchannels into hydrogels, for the perfusion of engineered constructs. Although numerous techniques have been developed, new techniques are still needed to obtain the complex geometries of blood vessels and with materials that permit desired cellular responses. Here, a printing process where a shear‐thinning and self‐healing hydrogel “ink” is injected directly into a “support” hydrogel with similar properties is reported. The support hydrogel is further engineered to undergo stabilization through a thiol‐ene reaction, permitting (i) the washing of the ink to produce microchannels and (ii) tunable properties depending on the crosslinker design. When adhesive peptides are included in the support hydrogel, endothelial cells form confluent monolayers within the channels, across a range of printed configurations (e.g., straight, stenosis, spiral). When protease‐degradable crosslinkers are used for the support hydrogel and gradients of angiogenic factors are introduced, endothelial cells sprout into the support hydrogel in the direction of the gradient. This printing approach is used to investigate the influence of channel curvature on angiogenic sprouting and increased sprouting is observed at curved locations. Ultimately, this technique can be used for a range of biomedical applications, from engineering vascularized tissue constructs to modeling in vitro cultures.     

4D Printing of Multi-Stimuli Responsive Protein-Based Hydrogels for Autonomous Shape Transformations

Stimuli responsive hydrogels that can change shape in response to applied external stimuli are appealing for soft robotics, biomedical devices, drug delivery, and actuators. However, existing 3D printed shape morphing materials are non-biodegradable, which limits their use in biomedical applications. Here, 3D printed protein-based hydrogels are developed and applied for programmable structural changes under the action of temperature, pH, or an enzyme. Key to the success of this strategy is the use of methacrylated bovine serum albumin (MA–BSA) as a biodegradable building block to Pickering emulsion gels in the presence of N-isopropylacrylamide or 2-dimethylaminoethyl methacrylate. These shear-thinning gels are ideal for direct ink write (DIW) 3D printing of multi-layered stimuli-responsive hydrogels. While poly(N-isopropylacrylamide) and poly(dimethylaminoethyl methacrylate) introduce temperature and pH-responsive properties into the printed objects, a unique feature of this strategy is an enzyme-triggered shape transformation based on the degradation of the bovine serum albumin network. To highlight this technique, protein-based hydrogels that reversibly change shape based on environmental temperature and pH are fabricated, and irreversibly altered by enzymatic degradation, which demonstrates the complexity that can be introduced into 4D printed systems.     

Botanical‐Inspired 4D Printing of Hydrogel at the Microscale

Botanical systems have evolved the intriguing ability to respond to diverse stimuli due to long‐term survival competition. Mimicking these dynamic behaviors has greatly advanced the developments in wide fields ranging from soft robotics, precision sensors to drug delivery and biomedical devices. However, realization of stimuli‐responsive components at the microscale with high response speed still remains a significant challenge. Herein, the miniature biomimetic 4D printing of pH‐responsive hydrogel is reported in spatiotemporal domain by femtosecond laser direct writing. The dimension of the printed architectures is at the microscale (<10² µm) and the response speed is reduced down to subsecond level (<500 ms). Shape transformation with multiple degrees of freedom is accomplished by taking advantage of pH‐triggered expansion, contraction, and torsion. Biomimetic complex shape‐morphing is enabled by adopting flexible scanning strategies. In addition, application of this 4D‐printed micro‐architecture in selective micro‐object trapping and releasing is demonstrated, showcasing its possibilities in micromanipulation, single‐cell analysis, and drug delivery.