Controlled heterogeneous stem cell differentiation on a shape memory hydrogel surface

NIR-Mediated Deformation from a CNT-Based Bilayer Hydrogel

Shape-shifting polymers are widely used in various fields such as intelligent switches, soft robots and sensors, which require both multiple stimulus-response functions and qualified mechanical strength. In this study, a novel near-infrared-light (NIR)-responsible shape-shifting hydrogel system was designed and fabricated through embedding vinylsilane-modified carbon nanotubes (CNTs) into particle double-network (P-DN) hydrogels by micellar copolymerisation. The dispersed brittle Poly(sodium 2-acrylamido-2-methylpropane-1-sulfonate) (PNaAMPS) network of the microgels can serve as sacrificial bonds to toughen the hydrogels, and the CNTs endow it with NIR photothermal conversion ability. The results show that the CNTs embedded in the P-DN hydrogels present excellent mechanical strength, i.e., a fracture strength of 312 kPa and a fracture strain of 357%. Moreover, an asymmetric bilayer hydrogel, where the active layer contains CNTs, can achieve 0°-110° bending deformation within 10 min under NIR irradiation and can realise complex deformation movement. This study provides a theoretical and experimental basis for the design and manufacture of photoresponsive soft actuators.

Preshaped 4D Photocurable Ultratough Organogel Microcoils for Personalized Endovascular Embolization

Endovascular embolization using microcoils can be an effective technique to treat artery aneurysms. However, microcoils with fixed designs are difficult to adapt to all aneurysm types. In this paper, a photocurable ultratough shape memory organogel with a curing time of only 2 s and megapascal-level mechanical properties is proposed. Then, it is used to manufacture the personalized 4D microcoil with a wire diameter of only 0.3 mm. The improved mechanical modulus (511.63 MPa) can reduce the possibility of microcoils' fracture during embolization. Besides, the fast body-temperature-triggering shape memory ability makes the 4D microcoil applicable in vivo. These 4D microcoils are finally delivered into the rabbit, and successfully blocked the blood flow inside different aneurysms, with neoendothelial cells and collagen fibers growing on the microcoil surface snugly, indicating full aneurysm recovery. This 4D organogel microcoil can potentially be used in personalized clinical translation on human beings.

Microchip encapsulation and microRNA-7 overexpression of trabecular meshwork mesenchymal stem/stromal cells improve motor function after spinal cord injury

Manipulation of stem cells and microencapsulation through microfluidic chips has shown more promising results in treating complex conditions, such as spinal cord injury (SCI), than traditional treatments. This study aimed to investigate the potency of neural differentiation and its therapeutic role in SCI animal model of trabecular meshwork mesenchymal stem/stromal cells (TMMSCs) via miR-7 overexpression and microchip-encapsulated. TMMSCs are transduced with miR-7 via a lentiviral vector (TMMSCs-miR-7[+]) and encapsulated in alginate-reduced graphene oxide (alginate-rGO) hydrogel via a microfluidic chip. Neuronal differentiation of transduced cells in hydrogel (3D) and tissue cultures plate (2D) was assessed by expressing specific mRNAs and proteins. Further evaluation is being carried out through 3D and 2D TMMSCs-miR-7(+ and -) transplantation into the rat contusion SCI model. TMMSCs-miR-7(+) encapsulated in the microfluidic chip (miR-7-3D) increased nestin, β-tubulin III, and MAP-2 expression compared with 2D culture. Moreover, miR-7-3D could improve locomotor behavior in contusion SCI rats, decrease cavity size, and increase myelination. Our results revealed that miR-7 and alginate-rGO hydrogel were involved in the neuronal differentiation of TMMSCs in a time-dependent manner. In addition, the microfluidic-encapsulated miR-7 overexpression TMMSCs represented a better survival and integration of the transplanted cells and the repair of SCI. Collectively, the combination of miR-7 overexpression and encapsulation of TMMSCs in hydrogels may represent a promising new treatment for SCI.

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.

Biomechanical Interfaces of Corticotomies on Periodontal Tissue Remodeling during Orthodontic Tooth Movement

Corticotomy is an effective approach in accelerating orthodontic tooth movement (OTM) in clinical treatment. Corticotomy causes regional acceleratory phenomenon (RAP) in the alveolar bone of surgical sites. However, the molecular mechanism of RAP after corticotomy remains unclear. Herein, we established a mouse model to study the biomechanical interfaces of corticotomy-assisted OTM and to investigate the histological responses and underlying cellular mechanism. A total of 144 male C57BL/6 mice were randomly assigned into four groups: corticotomy alone (Corti), sham operation (Sham), corticotomy with tooth movement (Corti + TM), and sham operation with tooth movement (Sham + TM). Nickel–titanium orthodontic springs were applied to trigger tooth movement. Mice were sacrificed on Post-Surgery Day (PSD) 3, 7, 14, 21, and 28 for radiographic, histological, immunohistochemical, and molecular biological analyses. The results reveal that corticotomy significantly promoted alveolar bone turnover and periodontal tissue remodeling. During orthodontic tooth movement, corticotomy significantly promoted osteogenic and proliferative activity, accelerated tooth movement, and eliminated root resorption by upregulating Wnt signal pathway.

Ultrafast and Programmable Shape Memory Hydrogel of Gelatin Soaked in Tannic Acid Solution

Shape memory hydrogels have been paid plenty of attention as a kind of intelligent soft material. However, complicated preparation and slow and uncontrollable shape change have hindered their applications in smart actuators. In this work, a temperature-responsive strong hydrogel was prepared by a facial soaking method without any chemical reactions, i.e., soaking gelatin hydrogel in aqueous tannic acid solution. The hydrogel was constructed by hydrogen bonding between gelatin and tannic acid beside the triple helix of gelatin chains without any chemical cross-linkers. The hydrogel showed ultrafast shape memory and body-temperature response. The hydrogel can be fixed in temporary shape in only 1 s at 25 °C and recover to the original shape in also 1 s at 37 °C, superior to the reported shape memory hydrogels. Furthermore, the hydrogel shape change can be programmed by fixing the temperature, and the designed shape is achieved stepwise by adjusting the recovery temperature. In addition, the hydrogel is stable in water without further swelling. These excellent features will initiate new prosperity of the shape memory hydrogel in biomedical technology, underwater actuators, and soft robots.

Photoresponsive Shape Memory Hydrogels for Complex Deformation and Solvent-Driven Actuation

A new design for photoresponsive shape memory hydrogels and their possible applications are demonstrated in the present study. We show that the photodissociable Fe³⁺-carboxylate coordination can be utilized as a molecular switch to realize photocontrol of shape memory on both macroscopic and microscopic scales and enable a number of functions. Indeed, Fe³⁺-carboxylate coordination can fix a large tensile strain (up to 680%) of the sodium alginate/polyacrylamide hydrogel through cross-linking of sodium alginate chains, and subsequent UV irradiation allows strain energy release in spatially selected regions through reduction of Fe³⁺ to Fe²⁺. By manipulating light irradiation, complex 3D structures are obtained from 2D hydrogel sheets, and they exhibit complex solvent-driven actuation behaviors due to a light-changeable modulus and cross-linking density in the hydrogel. Based on the same approach, micropatterns can be inscribed on the hydrogel surface using mask-assisted irradiation, and they exhibit chain orientation-mediated anisotropic topography change upon solvent exchange. Moreover, light-controlled strain energy release also enables changing hydrogel surface wettability by solvent replacement. The demonstrated mechanism for photoresponsive hydrogels is highly efficient and applicable to many systems, which offers new perspectives in developing hydrogels with multiple photoresponsive functions.

4D Biofabrication: Materials, Methods, and Applications

The mission of regenerative medicine is the development of methods to regrow, repair, or replace damaged or diseased cells, organs, or tissues. 3D bioprinting techniques are one of the most promising approaches for engineering the design of artificial tissues. Current 3D bioprinting technologies possess, however, several intrinsic limitations. 4D biofabrication, a recently developed technology with the embedded ability of shape transformation upon response to intrinsic and/or external stimuli, may solve challenges of 3D bioprinting as well as more accurately mimic the dynamics of the native tissues. This article covers recent advances in 4D biofabrication. It gives a detailed picture of used materials and technologies, provides critical comparisons of methods, discusses possibilities and limitations of different 4D biofabrication technologies, and gives examples of applications.

Advances in Regenerative Medicine and Tissue Engineering: Innovation and Transformation of Medicine

Humans and animals lose tissues and organs due to congenital defects, trauma, and diseases. The human body has a low regenerative potential as opposed to the urodele amphibians commonly referred to as salamanders. Globally, millions of people would benefit immensely if tissues and organs can be replaced on demand. Traditionally, transplantation of intact tissues and organs has been the bedrock to replace damaged and diseased parts of the body. The sole reliance on transplantation has created a waiting list of people requiring donated tissues and organs, and generally, supply cannot meet the demand. The total cost to society in terms of caring for patients with failing organs and debilitating diseases is enormous. Scientists and clinicians, motivated by the need to develop safe and reliable sources of tissues and organs, have been improving therapies and technologies that can regenerate tissues and in some cases create new tissues altogether. Tissue engineering and/or regenerative medicine are fields of life science employing both engineering and biological principles to create new tissues and organs and to promote the regeneration of damaged or diseased tissues and organs. Major advances and innovations are being made in the fields of tissue engineering and regenerative medicine and have a huge impact on three-dimensional bioprinting (3D bioprinting) of tissues and organs. 3D bioprinting holds great promise for artificial tissue and organ bioprinting, thereby revolutionizing the field of regenerative medicine. This review discusses how recent advances in the field of regenerative medicine and tissue engineering can improve 3D bioprinting and vice versa. Several challenges must be overcome in the application of 3D bioprinting before this disruptive technology is widely used to create organotypic constructs for regenerative medicine.

Hydrogel scaffolds for differentiation of adipose-derived stem cells

Natural extracellular matrices (ECMs) have been widely used as a support for the adhesion, migration, differentiation, and proliferation of adipose-derived stem cells (ADSCs). However, poor mechanical behavior and unpredictable biodegradation properties of natural ECMs considerably limit their potential for bioapplications and raise the need for different, synthetic scaffolds. Hydrogels are regarded as the most promising alternative materials as a consequence of their excellent swelling properties and their resemblance to soft tissues. A variety of strategies have been applied to create synthetic biomimetic hydrogels, and their biophysical and biochemical properties have been modulated to be suitable for cell differentiation. In this review, we first give an overview of common methods for hydrogel preparation with a focus on those strategies that provide potential advantages for ADSC encapsulation, before summarizing the physical properties of hydrogel scaffolds that can act as biological cues. Finally, the challenges in the preparation and application of hydrogels with ADSCs are explored and the perspectives are proposed for the next generation of scaffolds.

Nanoinitiator for enzymatic anaerobic polymerization and graft enhancement of gelatin-PAAM hydrogel

As an emerging method for mildly molding polymer hydrogel bioscaffolds, the enzymatically polymerized system is mainly based on the screening of various oxidoreductases to produce radicals, but the design of multifunctional nanoinitiators to facilitate hydrogel performance remains challenging. Here, we utilize N-hydroxyimide-modified silica nanoparticles as nanoinitiators to simultaneously trigger glucose oxidase anaerobic polymerization and nanoparticle-grafting enhancement of the gelatin-polyacrylamide (PAAM) hydrogel. The enzyme-nanoinitiator system produced nitrogen radicals, which were further converted into carbon radicals via GOx-catalyzed glucose reduction, as confirmed by electron paramagnetic resonance (EPR) analysis. Our stretchable hydrogel has a 12-fold increased fracture energy relative to traditional hydrogel due to grafting enhancement by the nanoinitiator. The temperature-dependent physical crosslinking of gelatin endowed our hydrogel facile printing ability. Cytotoxicity assay and 3D cell culture demonstrated the low toxicity of our hydrogel. As the first example of the use of nanoinitiators for enzymatic polymerization, this work provides a biocompatible platform to prepare or print hydrogel bioscaffolds with the required mechanical strength.

Heterogeneous and self-organizing mineralization of bone matrix promoted by hydroxyapatite nanoparticles

The mineralization process is crucial to the load-bearing characteristics of the bone extracellular matrix. In this work, we have studied the spatiotemporal dynamics of mineral deposition by human bone marrow mesenchymal stem cells differentiating toward osteoblasts promoted by the presence of exogenous hydroxyapatite nanoparticles. At the molecular level, the added nanoparticles positively modulated the expression of bone-specific markers and enhanced calcified matrix deposition during osteogenic differentiation. The nucleation, growth and spatial arrangement of newly deposited hydroxyapatite nanocrystals have been evaluated using scanning micro X-ray diffraction and scanning micro X-ray fluorescence. As leading results, we have found the emergence of a complex scenario where the spatial organization and temporal evolution of the process exhibit heterogeneous and self-organizing dynamics. At the same time the possibility of controlling the differentiation kinetics, through the addition of synthetic nanoparticles, paves the way to empower the generation of more structured bone scaffolds in tissue engineering and to design new drugs in regenerative medicine.

Biodegradable Shape Memory Polymers in Medicine

Shape memory materials have emerged as an important class of materials in medicine due to their ability to change shape in response to a specific stimulus, enabling the simplification of medical procedures, use of minimally invasive techniques, and access to new treatment modalities. Shape memory polymers, in particular, are well suited for such applications given their excellent shape memory performance, tunable materials properties, minimal toxicity, and potential for biodegradation and resorption. This review provides an overview of biodegradable shape memory polymers that have been used in medical applications. The majority of biodegradable shape memory polymers are based on thermally responsive polyesters or polymers that contain hydrolyzable ester linkages. These materials have been targeted for use in applications pertaining to embolization, drug delivery, stents, tissue engineering, and wound closure. The development of biodegradable shape memory polymers with unique properties or responsiveness to novel stimuli has the potential to facilitate the optimization and development of new medical applications.

Shape Memory Polymers Based on Supramolecular Interactions

Shape memory polymers (SMPs), with the capability to change from one or more temporary shapes to predetermined shapes in response to an external stimulus, have attracted much interest from both academia and industries. When introducing supramolecular interactions that have been featured as dynamic and reversible into the design of novel SMPs, intriguing and unique functionalities have been engendered and thereby broaden the potential applications of the SMPs to new territories. In this review, we summarize recent progress made in SMPs based on supramolecular interactions, provide insight into the material design and shape memory mechanism, elucidate and evaluate their properties and performance, and point out opportunities and applications of SMPs.

Shape-Memory Hydrogels: Evolution of Structural Principles To Enable Shape Switching of Hydrophilic Polymer Networks

The ability of hydrophilic chain segments in polymer networks to strongly interact with water allows the volumetric expansion of the material and formation of a hydrogel. When polymer chain segments undergo reversible hydration depending on environmental conditions, smart hydrogels can be realized, which are able to shrink/swell and thus alter their volume on demand. In contrast, implementing the capacity of hydrogels to switch their shape rather than volume demands more sophisticated chemical approaches and structural concepts. In this Account, the principles of hydrogel network design, incorporation of molecular switches, and hydrogel microstructures are summarized that enable a spatially directed actuation of hydrogels by a shape-memory effect (SME) without major volume alteration. The SME involves an elastic deformation (programming) of samples, which are temporarily fixed by reversible covalent or physical cross-links resulting in a temporary shape. The material can reverse to the original shape when these molecular switches are affected by application of a suitable stimulus. Hydrophobic shape-memory polymers (SMPs), which are established with complex functions including multiple or reversible shape-switching, may provide inspiration for the molecular architecture of shape-memory hydrogels (SMHs), but cannot be identically copied in the world of hydrophilic soft materials. For instance, fixation of the temporary shape requires cross-links to be formed also in an aqueous environment, which may not be realized, for example, by crystalline domains from the hydrophilic main chains as these may dissolve in presence of water. Accordingly, dual-shape hydrogels have evolved, where, for example, hydrophobic crystallizable side chains have been linked into hydrophilic polymer networks to act as temperature-sensitive temporary cross-links. By incorporating a second type of such side chains, triple-shape hydrogels can be realized. Considering the typically given light permeability of hydrogels and the fully hydrated state with easy permeation by small molecules, other types of stimuli like light, pH, or ions can be employed that may not be easily used in hydrophobic SMPs. In some cases, those molecular switches can respond to more than one stimulus, thus increasing the number of opportunities to induce actuation of these synthetic hydrogels. Beyond this, biopolymer-based hydrogels can be equipped with a shape switching function when facilitating, for example, triple helix formation in proteins or ionic interactions in polysaccharides. Eventually, microstructured SMHs such as hybrid or porous structures can combine the shape-switching function with an improved performance by helping to overcome frequent shortcomings of hydrogels such as low mechanical strength or volume change upon temporary cross-link cleavage. Specifically, shape switching without major volume alteration is possible in porous SMHs by decoupling small volume changes of pore walls on the microscale and the macroscopic sample size. Furthermore, oligomeric rather than short aliphatic side chains as molecular switches allow stabilization of the sample volumes. Based on those structural principles and switching functionalities, SMHs have already entered into applications as soft actuators and are considered, for example, for cell manipulation in biomedicine. In the context of those applications, switching kinetics, switching forces, and reversibility of switching are aspects to be further explored.

Shaped stimuli-responsive hydrogel particles: syntheses, properties and biological responses

This review summarizes a pool of current experimental approaches and discusses perspectives in the development of the synergistic combination of shape and stimuli-response in particulate hydrogels.

NIR-Triggered Rapid Shape Memory PAM-GO-Gelatin Hydrogels with High Mechanical Strength

Shape memory hydrogels containing over 76 wt % of water were synthesized in a one-pot method, and interpenetrating double network was formed by physically cross-linked gelatin network and chemically cross-linked polyacrylamide (PAM) network with graphene oxide (GO). The temporary shape was quickly fixed by cooling in ice water for 30 s after deformation at 80 °C for 10 s. Shape recovery started in 10 s under near-infrared (NIR) irradiation and almost completed within 60 s depending on the curling angle. A small amount of GO in the hydrogels (≤1.5 mg/mL) played a key role in NIR energy absorption and transformation into thermal energy. The hydrogel without GO showed no response to the NIR irradiation and cannot recover to its permanent shape by NIR irradiation. Temperature sweep was conducted in the cycle of 20 °C → 80 °C → 20 °C, and the structure change in the hydrogels with temperature was investigated according to the storage modulus G' and tangent of the loss angle tan δ as a function of the hydrogel composition. The shape-memory capability was confirmed as the contribution from the triple-helix cross-linking network of gelatin. High mechanical toughness (strength > 400 kPa and broken strain > 500%) was achieved by the double-network with the sacrificial gelatin network and GO bridging to dissipate deformation energy. The optimized composition of the hydrogel was found to be a key point to realize stable temporary shape and rapid recovery to the permanent shape controlled by NIR irradiation with reasonable strength. The facile preparation and noncontact gentle stimulus of the present hydrogel hold great potential to be used in soft actuator materials.

Recent Advances in Shape Memory Soft Materials for Biomedical Applications

Shape memory polymers (SMPs) are smart and adaptive materials able to recover their shape through an external stimulus. This functionality, combined with the good biocompatibility of polymers, has garnered much interest for biomedical applications. In this review, we discuss the design considerations critical to the successful integration of SMPs for use in vivo. We also highlight recent work on three classes of SMPs: shape memory polymers and blends, shape memory polymer composites, and shape memory hydrogels. These developments open the possibility of incorporating SMPs into device design, which can lead to vast technological improvements in the biomedical field.

Tough and biocompatible hydrogels based on in situ interpenetrating networks of dithiol-connected graphene oxide and poly(vinyl alcohol)

An interpenetrating network (IPN) strategy has been widely facilitated to construct strong and tough hydrogels, but most of the efforts have been focused on organic/organic networks. Herein, aqueous dispersible 2,2'-(ethylenedioxy)-diethanethiol (EDDET) cross-linked graphene oxide (E-cGO) skeleton was in situ incorporated into a PVA matrix, resulting in novel inorganic/organic IPN hydrogels with super mechanical and chondrocyte cell-adhesion properties. The unique interpenetrating structure and hydrogen bonding were demonstrated to play critical roles in enhancing the compressive property of the IPN hydrogels, in comparison to the GO and thermally reduced graphene oxide (T-rGO) filled hydrogels. It is critical that the E-cGO/PVA hydrogels have been demonstrated as being biocompatible, which make the E-cGO/PVA hydrogels promising candidate biomaterials for load-bearing biotissue substitution.

Hydrogen bonded and ionically crosslinked high strength hydrogels exhibiting Ca2+-triggered shape memory properties and volume shrinkage for cell detachment

Diaminotriazine hydrogen bonding reinforced and Ca²⁺-crosslinked high strength shape memory hydrogels are fabricated. Ca^2+_induced dramatic volume shrinkage is utilized to trigger unharmful cell detachment.

Non-covalent interaction cooperatively induced stretchy, tough and stimuli-responsive polyurethane–urea supramolecular (PUUS) hydrogels

Hydrogen-bonding and hydrophobic effect cooperatively induced stretchy, tough and stimuli-responsive polyurethane–urea supramolecular hydrogels were easily prepared and studied in detail.