Reactive electrospinning of degradable poly(oligoethylene glycol methacrylate)-based nanofibrous hydrogel networks

Quantification and visual inspection of adipic dihydrazide in textile using a xanthylium-based ratiometric fluorescent probe

Residual adipic dihydrazide (ADH) in textiles may react with formaldehyde to form hydrazone, which is a source of the formaldehyde pollutant indoors. In this paper, a xanthylium-based ratiometric fluorescent probe SH-Py was developed for ADH detection. The emission of SH-Py at 680 nm decreased, whereas that at 463 nm increased upon the addition of ADH in pure DMSO. The LOD was as low as 24.5 nM, and the possible sensing mechanism is presented. Good recoveries were observed in the standard recovery experiments of SH-Py for ADH detection in textiles. Moreover, the visualization of ADH in textiles was successfully achieved by spraying the probe solution and then inspecting it under a UV lamp. Additionally, SH-Py has the potential to quantify other aliphatic hydrazides in various samples.

Electrospinning Nanofibers as Stretchable Sensors for Wearable Devices

Wearable devices attract great attention in intelligent medicine, electronic skin, artificial intelligence robots, and so on. However, boundedness of traditional sensors based on rigid materials unconstrained self-multilayer structure assembly and dense substrate in stretchability and permeability limits their applications. The network structure of the elastomeric nanofibers gives them excellent air permeability and stretchability. By introducing metal nanofillers, intrinsic conductive polymers, carbon materials, and other methods to construct conductive paths, stretchable conductors can be effectively prepared by elastomeric nanofibers, showing great potential in the field of flexible sensors. This perspective briefly introduces the representative preparations of conductive thermoplastic polyurethane, nylon, and hydrogel nanofibers by electrospinning and the application of integrated electronic devices in biological signal detection. The main challenge is to unify the stretchability and conductivity of the fiber structure.

Overview of Tissue Engineering and Drug Delivery Applications of Reactive Electrospinning and Crosslinking Techniques of Polymeric Nanofibers with Highlights on Their Biocompatibility Testing and Regulatory Aspects

Traditional electrospinning is a promising technique for fabricating nanofibers for tissue engineering and drug delivery applications. The method is highly efficient in producing nanofibers with morphology and porosity similar to the extracellular matrix. Nonetheless, and in many instances, the process has faced several limitations, including weak mechanical strength, large diameter distributions, and scaling-up difficulties of its fabricated electrospun nanofibers. The constraints of the polymer solution's intrinsic properties are primarily responsible for these limitations. Reactive electrospinning constitutes a novel and modified electrospinning techniques developed to overcome those challenges and improve the properties of the fabricated fibers intended for various biomedical applications. This review mainly addresses reactive electrospinning techniques, a relatively new approach for making in situ or post-crosslinked nanofibers. It provides an overview of and discusses the recent literature about chemical and photoreactive electrospinning, their various techniques, their biomedical applications, and FDA regulatory aspects related to their approval and marketing. Another aspect highlighted in this review is the use of crosslinking and reactive electrospinning techniques to enhance the fabricated nanofibers' physicochemical and mechanical properties and make them more biocompatible and tailored for advanced intelligent drug delivery and tissue engineering applications.

Reactive Cell Electrospinning of Anisotropically Aligned and Bilayer Hydrogel Nanofiber Networks

Structured hydrogels that incorporate aligned nanofibrous morphologies have been demonstrated to better replicate the structures of native extracellular matrices and thus their function in guiding cell responses. However, current techniques for nanofiber fabrication are limited in their ability to create hydrogel scaffolds with tunable directional alignments and cell types/densities, as required to reproduce more complex native tissue structures. Herein, we leverage a reactive cell electrospinning technique based on the dynamic covalent cross-linking of poly(ethylene glycol methacrylate (POEGMA) precursor polymers to fabricate aligned hydrogel nanofibers that can be directly loaded with cells during the electrospinning process. The scaffolds were found to support high C2C12 myoblast viabilities greater than 85% over 14 days, with changes in the electrospinning collector allowing for the single-step fabrication of nonaligned, aligned, or cross-aligned nanofibrous networks. Cell aspect ratios on aligned scaffolds were found on average to be 27% higher compared to those on nonaligned scaffolds; furthermore, cell-loaded bilayer scaffolds with perpendicular fiber alignments showed evidence of enabling localized directional cell responses to individual layer fiber directions while avoiding delamination between the layers. This fabrication approach thus offers potential for better mimicking the structure and thus function of aligned and multilayered tissues (e.g., smooth muscle, neural, or tendon tissues).

Multicellular Layered Nanofibrous Poly(Oligo Ethylene Glycol Methacrylate) (POEGMA)-Based Hydrogel Scaffolds via Reactive Cell Electrospinning

While hydrogels are demonstrated to be effective scaffolds for soft tissue engineering, existing fabrication techniques pose limitations in terms of being able to reproduce both the micro/nanofibrous structures of native extracellular matrix as well as the spatial arrangement of different cell types inherent of more complex tissues. Herein, a reactive cell electrospinning strategy is described using hydrazide and aldehyde-functionalized poly(oligoethylene glycol methacrylate) precursor polymers that can create nanofibrous hydrogel scaffolds with controllable local cell gradients using a sequential all-aqueous process that does not require additives or external energy. Cells can be encapsulated directly during the fabrication process in different layers within the scaffold, enabling localized segregation of different cell types within the structures without compromising their capacity to proliferate (≈4-fold increase in cell density over a 14 day incubation period). This sequential reactive electrospinning approach thus offers promise to generate coculture fibrous hydrogel networks in which both the nanoscale architecture and the cell distribution can be controlled, as it is essential to recreate more complex types of tissues.

Hydrogels for Tissue Engineering: Addressing Key Design Needs Toward Clinical Translation

Collapse

Osteon-mimetic 3D nanofibrous scaffold enhances stem cell proliferation and osteogenic differentiation for bone regeneration

The scaffold microstructure is important for bone tissue engineering. Failure to synergistically imitate the hierarchical microstructure of the components of bone, such as an osteon with concentric multilayers assembled by nanofibers, hinders the performance for guiding bone regeneration. Here, a 2D bilayer nanofibrous membrane (BLM) containing poly(lactide-co-glycolide) (PLGA)/polycaprolactone (PCL) composite membranes in similar compositions (PCL15 and PCL20), but possessing different degrees of shrinkage, was fabricated via sequential electrospinning. Upon incubation in phosphate buffered saline (PBS) (37 °C), the 2D BLM spontaneously deformed into a 3D shape induced by PCL crystallization within the PLGA matrix, and the PCL15 and PCL20 layer formed a concave and convex surface, respectively. The 3D structure contained curved multilayers with an average diameter of 776 ± 169 μm, and on the concave and convex surface the nanofiber diameters were 792 ± 225 and 881 ± 259 nm, respectively. The initial 2D structure facilitated the even distribution of seeded cells. Adipose-derived stem cells from rats (rADSCs) proliferated faster on a concave surface than on a convex surface. For the 3D BLM, the osteogenic differentiation of rADSCs was significantly higher than that on 2D surfaces, even without osteogenic supplements, which resulted from the stretched cell morphology on the curved sublayer leading to increased expression of lamin-A. After being implanted into cranial defects in Sprague Dawley (SD) rats, 3D BLM significantly accelerated bone formation. In summary, 3D BLM with an osteon-like structure provides a potential strategy to repair bone defects.

Recent advances of 3D hydrogel culture systems for mesenchymal stem cell-based therapy and cell behavior regulation

Mesenchymal stem cells (MSCs) have been increasingly recognized as resources for disease treatments and regenerative medicine. Meanwhile, the unique chemical and physical properties of hydrogels provide innate advantages to achieve...

A Review of Sustained Drug Release Studies from Nanofiber Hydrogels

Polymer nanofibers have exceptionally high surface area. This is advantageous compared to bulk polymeric structures, as nanofibrils increase the area over which materials can be transported into and out of a system, via diffusion and active transport. On the other hand, since hydrogels possess a degree of flexibility very similar to natural tissue, due to their significant water content, hydrogels made from natural or biodegradable macromolecular systems can even be injectable into the human body. Due to unique interactions with water, hydrogel transport properties can be easily modified and tailored. As a result, combining nanofibers with hydrogels would truly advance biomedical applications of hydrogels, particularly in the area of sustained drug delivery. In fact, certain nanofiber networks can be transformed into hydrogels directly without the need for a hydrogel enclosure. This review discusses recent advances in the fabrication and application of biomedical nanofiber hydrogels with a strong emphasis on drug release. Most of the drug release studies and recent advances have so far focused on self-gelling nanofiber systems made from peptides or other natural proteins loaded with cancer drugs. Secondly, polysaccharide nanofiber hydrogels are being investigated, and thirdly, electrospun biodegradable polymer networks embedded in polysaccharide-based hydrogels are becoming increasingly popular. This review shows that a major outcome from these works is that nanofiber hydrogels can maintain drug release rates exceeding a few days, even extending into months, which is an extremely difficult task to achieve without the nanofiber texture. This review also demonstrates that some publications still lack careful rheological studies on nanofiber hydrogels; however, rheological properties of hydrogels can influence cell function, mechano-transduction, and cellular interactions such as growth, migration, adhesion, proliferation, differentiation, and morphology. Nanofiber hydrogel rheology becomes even more critical for 3D or 4D printable systems that should maintain sustained drug delivery rates.

Fast Thermoresponsive Poly(oligoethylene glycol methacrylate) (POEGMA)-Based Nanostructured Hydrogels for Reversible Tuning of Cell Interactions

Reactive electrospinning is demonstrated as a viable method to create fast-responsive and degradable macroporous thermoresponsive hydrogels based on poly(oligoethylene glycol methacrylate) (POEGMA). Hydrazide- and aldehyde-functionalized POEGMA precursor polymers were coelectrospun to create hydrazone cross-linked nanostructured hydrogels in a single processing step that avoids the need for porogens, phase separation-driving additives, or scaffold postprocessing. The resulting nanostructured hydrogels can respond reversibly and repeatedly to changes in external temperature within seconds, in contrast to the minutes-to-hours response time observed with bulk hydrogels. Furthermore, nearly quantitative cell delamination can be achieved within 2 min of incubation at 4 °C, resulting in the recovery of as many or more (as well as more proliferatively active) cells from the substrate relative to the conventional trypsinization protocol. The combined macroporosity, nanoscale feature size, and interfacial switching potential of these nanostructured hydrogels thus offer promise for manipulating cell-hydrogel interactions as well as other applications in which rapid responses to external stimuli are desirable.

The effects of surface topography modification on hydrogel properties

Hydrogel has been an attractive biomaterial for tissue engineering, drug delivery, wound healing, and contact lens materials, due to its outstanding properties, including high water content, transparency, biocompatibility, tissue mechanical matching, and low toxicity. As hydrogel commonly possesses high surface hydrophilicity, chemical modifications have been applied to achieve the optimal surface properties to improve the performance of hydrogels for specific applications. Ideally, the effects of surface modifications would be stable, and the modification would not affect the inherent hydrogel properties. In recent years, a new type of surface modification has been discovered to be able to alter hydrogel properties by physically patterning the hydrogel surfaces with topographies. Such physical patterning methods can also affect hydrogel surface chemical properties, such as protein adsorption, microbial adhesion, and cell response. This review will first summarize the works on developing hydrogel surface patterning methods. The influence of surface topography on interfacial energy and the subsequent effects on protein adsorption, microbial, and cell interactions with patterned hydrogel, with specific examples in biomedical applications, will be discussed. Finally, current problems and future challenges on topographical modification of hydrogels will also be discussed.

Tuning the load of gold and magnetic nanoparticles in nanogels through their design for enhanced dual magneto-photo-thermia

We describe a novel synthesis allowing one to enhance the load of magnetic nanoparticles and gold nanorods in nanogels. Two different structures, simple cores and core-shell, were synthesized and their heating properties upon alternating magnetic field or laser exposure are compared. Remarkably, the core-shell structure showed a greater heating capacity in the two modalities.

Multi-scale structuring of cell-instructive cellulose nanocrystal composite hydrogel sheets via sequential electrospinning and thermal wrinkling

Structured hydrogel sheets offer the potential to mimic the mechanics and morphology of native cell environments in vitro; however, controlling the morphology of such sheets across multiple length scales to give cells consistent multi-dimensional cues remains challenging. Here, we demonstrate a simple two-step process based on sequential electrospinning and thermal wrinkling to create nanocomposite poly(oligoethylene glycol methacrylate)/cellulose nanocrystal hydrogel sheets with a highly tunable multi-scale wrinkled (micro) and fibrous (nano) morphology. By varying the time of electrospinning, rotation speed of the collector, and geometry of the thermal wrinkling process, the hydrogel nanofiber density, fiber alignment, and wrinkle geometry (biaxial or uniaxial) can be independently controlled. Adhered C2C12 mouse myoblast muscle cells display a random orientation on biaxially wrinkled sheets but an extended morphology (directed preferentially along the wrinkles) on uniaxially wrinkled sheets. While the nanofiber orientation had a smaller effect on cell alignment, parallel nanofibers promoted improved cell alignment along the wrinkle direction while perpendicular nanofibers disrupted alignment. The highly tunable structures demonstrated are some of the most complex morphologies engineered into hydrogels to-date without requiring intensive micro/nanofabrication approaches and offer the potential to precisely regulate cell-substrate interactions in a "2.5D" environment (i.e. a surface with both micro- and nano-structured topographies) for in vitro cell screening or in vivo tissue regeneration. STATEMENT OF SIGNIFICANCE: While structured hydrogels can mimic the morphology of natural tissues, controlling this morphology over multiple length scales remains challenging. Furthermore, the incorporation of secondary morphologies within individual hydrogels via simple manufacturing techniques would represent a significant advancement in the field of structured biomaterials and an opportunity to study complex cell-biomaterial interactions. Herein, we leverage a two-step process based on electrospinning and thermal wrinkling to prepare structured hydrogels with microscale wrinkles and nanoscale fibers. Fiber orientation/density and wrinkle geometry can be independently controlled during the electrospinning and thermal wrinkling processes respectively, demonstrating the flexibility of this technique for creating well-defined multiscale hydrogel structures. Finally, we show that while wrinkle geometry is the major determinant of cell alignment, nanofiber orientation also plays a role in this process.

Electrospun hydrogels for dynamic culture systems: advantages, progress, and opportunities

The extracellular matrix (ECM) is a water-swollen, tissue-specific material environment in which biophysiochemical signals are organized and influence cell behaviors. Electrospun nanofibrous substrates have been pursued as platforms for tissue engineering and cell studies that recapitulate features of the native ECM, in particular its fibrous nature. In recent years, progress in the design of electrospun hydrogel systems has demonstrated that molecular design also enables unique studies of cellular behaviors. In comparison to the use of hydrophobic polymeric materials, electrospinning hydrophilic materials that crosslink to form hydrogels offer the potential to achieve the water-swollen, nanofibrous characteristics of endogenous ECM. Although electrospun hydrogels require an additional crosslinking step to stabilize the fibers (allowing fibers to swell with water instead of dissolving) in comparison to their hydrophobic counterparts, researchers have made significant advances in leveraging hydrogel chemistries to incorporate biochemical and dynamic functionalities within the fibers. Consequently, dynamic biophysical and biochemical properties can be engineered into hydrophilic nanofibers that would be difficult to engineer in hydrophobic systems without strategic and sometimes intensive post-processing techniques. This Review describes common methodologies to control biophysical and biochemical properties of both electrospun hydrophobic and hydrogel nanofibers, with an emphasis on highlighting recent progress using hydrogel nanofibers with engineered dynamic complexities to develop culture systems for the study of biological function, dysfunction, development, and regeneration.

3D Electrospun Nanofiber-Based Scaffolds: From Preparations and Properties to Tissue Regeneration Applications

Electrospun nanofibers have been frequently used for tissue engineering due to their morphological similarities with the extracellular matrix (ECM) and tunable chemical and physical properties for regulating cell behaviors and functions. However, most of the existing electrospun nanofibers have a closely packed two-dimensional (2D) membrane with the intrinsic shortcomings of limited cellular infiltration, restricted nutrition diffusion, and unsatisfied thickness. Three-dimensional (3D) electrospun nanofiber-based scaffolds can provide stem cells with 3D microenvironments and biomimetic fibrous structures. Thus, they have been demonstrated to be good candidates for in vivo repair of different tissues. This review summarizes the recent developments in 3D electrospun nanofiber-based scaffolds (ENF-S) for tissue engineering. Three types of 3D ENF-S fabricated using different approaches classified into electrospun nanofiber 3D scaffolds, electrospun nanofiber/hydrogel composite 3D scaffolds, and electrospun nanofiber/porous matrix composite 3D scaffolds are discussed. New functions for these 3D ENF-S and properties, such as facilitated cell infiltration, 3D fibrous architecture, enhanced mechanical properties, and tunable degradability, meeting the requirements of tissue engineering scaffolds were discovered. The applications of 3D ENF-S in cartilage, bone, tendon, ligament, skeletal muscle, nerve, and cardiac tissue regeneration are then presented with a discussion of current challenges and future directions. Finally, we give summaries and future perspectives of 3D ENF-S in tissue engineering and clinical transformation.

Enzyme-Crosslinked Electrospun Fibrous Gelatin Hydrogel for Potential Soft Tissue Engineering

Soft tissue engineering has been seeking ways to mimic the natural extracellular microenvironment that allows cells to migrate and proliferate to regenerate new tissue. Therefore, the reconstruction of soft tissue requires a scaffold possessing the extracellular matrix (ECM)-mimicking fibrous structure and elastic property, which affect the cell functions and tissue regeneration. Herein, an effective method for fabricating nanofibrous hydrogel for soft tissue engineering is demonstrated using gelatin-hydroxyphenylpropionic acid (Gel-HPA) by electrospinning and enzymatic crosslinking. Gel-HPA fibrous hydrogel was prepared by crosslinking the electrospun fibers in ethanol-water solution with an optimized concentration of horseradish peroxidase (HRP) and H2O2. The prepared fibrous hydrogel held the soft and elastic mechanical property of hydrogels and the three-dimensional (3D) fibrous structure of electrospun fibers. It was proven that the hydrogel scaffolds were biocompatible, improving the cellular adhesion, spreading, and proliferation. Moreover, the fibrous hydrogel showed rapid biodegradability and promoted angiogenesis in vivo. Overall, this study represents a novel biomimetic approach to generate Gel-HPA fibrous hydrogel scaffolds which have excellent potential in soft tissue regeneration applications.

Electrospinning of a Near Gel Resin To Produce Cross-Linked Fibrous Matrices

Electrospun fibers and matrices have been researched for their utility in various fields; however, because of poor mechanical strength and loss of structural integrity, their commercial viability is limited. A near gel resin (nGR) of polystyrene (PS) was used in the present approach to fabricate cross-linked fibrous matrices of better mechanical strength and oil adsorption while retaining the structural integrity. Electrospinnability of nGR was assessed in bulk (i.e., in styrene monomer) and solution (i.e., in dimethyl formamide) forms with variations in formulation and electrospinning conditions. Ultimately, a uniform cross-linked fibrous matrix of PS was prepared using an oil-in-water emulsion, where the oil phase composed of a monomer (styrene), an initiator (benzoyl peroxide), and a cross-linker (divinylbenzene) was dispersed in a continuous phase of aqueous poly(vinyl alcohol) (PVA). The monomer conversion in the oil phase was carried out below the gel point, and the nGR of PS formed in dispersed droplets was electrospun to fabricate uniform fibrous matrices with the help of a template polymer, that is, PVA. The effect of various material and process parameters on the gelation behavior, electrospinnability, and fiber uniformity was studied and optimized to produce uniform core-sheath fibrous matrices of cross-linked PS. Postelectrospinning heat treatment of matrices was carried out to achieve complete monomer conversion and cross-linking. Fiber formation behavior of the emulsion was assessed using ionic and nonionic surfactants. The cross-link density of the matrices was optimized to achieve the desired structural morphology and dimensional stability. The process of fabrication of emulsion electrospun cross-linked fibers can be further extended to a variety of other monomers in order to enhance the suitability of fibrous matrices for many applications.

A Self-Healing Nanofiber-Based Self-Responsive Time-Temperature Indicator for Securing a Cold-Supply Chain

Perishable foods at undesired temperatures can generate foodborne illnesses that present significant societal costs. To certify refrigeration succession in a food-supply chain, a flexible, easy-to-interpret, damage-tolerant, and sensitive time-temperature indicator (TTI) that uses a self-healing nanofiber mat is devised. This mat is opaque when refrigerated due to nanofiber-induced light scattering, but becomes irreversibly transparent at room temperature through self-healing-induced interfibrillar fusion leading to the appearance of a warning sign. The mat monitors both freezer (-20 °C) and chiller (2 °C) successions and its timer is tunable over the 0.5-22.5 h range through control of the polymer composition and film thickness. The thin mat itself serves as both a temperature sensor and display; it does not require modularization, accurately measures localized or gradient heat, and functions even after crushing, cutting, and when weight-loaded in a manner that existing TTIs cannot. It also contains no drainable chemicals and is attachable to various shapes because it operates through an intrinsic physical response.

Reduction of thrombotic and inflammatory complications of polystyrene-block-polyisoprene-block-polystyrene (SIS) with one-step electrospinning

Polystyrene-block-polyisoprene-block-polystyrene (SIS) has been used as biomaterials due to its soft and stable properties under physiological conditions. However, the thrombotic and inflammatory complications caused by SIS restrain its application as blood-contacting implant. To overcome this problem, the hydrophilic core-shell structured SIS-based microfiber with antioxidant encapsulation is fabricated with one-step reactive electrospinning. We demonstrate that the phase separation of SIS and acylated Pluronic F127 (F127-DA) components and crosslinking during electrospinning renders the microfiber blood compatible and stable under physiological condition; the encapsulation of 2-O-d-glucopyranosyl-l-ascorbic acid (AA-2G) in microfiber and subsequent release of AA-2G detoxifies the excess reactive oxygen species (ROS). The microfibers are nontoxic to cells and promote the fast growth and proliferation of human umbilical vein endothelial cells (HUVECs) in the presence of ROS; the thrombotic and inflammatory complications are effectively reduced with implant evaluation in vivo. Therefore, our work paves a new way to improve the biocompatibility of SIS, making it a promising candidate for blood contact materials.

2.5D Hierarchical Structuring of Nanocomposite Hydrogel Films Containing Cellulose Nanocrystals

Although two-dimensional hydrogel thin films have been applied across many biomedical applications, creating higher dimensionality structured hydrogel interfaces would enable potentially improved and more biomimetic hydrogel performance in biosensing, bioseparations, tissue engineering, drug delivery, and wound healing applications. Herein, we present a new and simple approach to control the structure of hydrogel thin films in 2.5D. Hybrid suspensions containing cellulose nanocrystals (CNCs) and aldehyde- or hydrazide-functionalized poly(oligoethylene glycol methacrylate) (POEGMA) were spin-coated onto prestressed polystyrene substrates to form cross-linked hydrogel thin films. The films were then structured via thermal shrinking, with control over the direction of shrinking leading to the formation of biaxial, uniaxial, or hierarchical wrinkles. Notably, POEGMA-only hydrogel thin films (without CNCs) did not form uniform wrinkles due to partial dewetting from the substrate during shrinking. Topographical feature sizes of CNC-POEGMA films could be tuned across 2 orders of magnitude (from ∼300 nm to 20 μm) by varying the POEGMA concentration, the length of poly(ethylene glycol) side chains in the polymer, and/or the overall film thickness. Furthermore, by employing adhesive masks during the spin-coating process, structured films with gradient wrinkle sizes can be fabricated. This precise control over both wrinkle size and wrinkle topography adds a level of functionality that to date has been lacking in conventional hydrogel networks.

Electrospun superhydrophilic membranes for effective removal of Pb(ii) from water

Nanofibrous membranes have a high specific surface area and large porosity, which are beneficial for being used as adsorbents to remove heavy metal ions from water. In this work, electrospun nanofibers were wrapped with a hydrogel layer with a tunable thickness, which endowed the membrane with excellent superhydrophilic performance. Because of good water-retention properties and abundant functional groups originating from the hydrogel layer, as a static adsorbent, the maximum adsorption capacity of Pb(ii) was up to 146.21 mg g^-1 according to the Langmuir model. Meanwhile, the electrospun membrane also possessed water permeability as a flow-through membrane for dynamic adsorption, which was obviously different from traditional hydrogel adsorbents. As a result, the rejection ratio of Pb(ii) can remain over 55% after running for 72 h under high pH conditions and at low initial ion concentrations. Apart from these, cycle operations confirmed the regeneration of the membrane, and competitive adsorption experiments illustrated the selective removal of Pb(ii) in a mixed ion solution.

Fabricating Degradable Thermoresponsive Hydrogels on Multiple Length Scales via Reactive Extrusion, Microfluidics, Self-assembly, and Electrospinning

While various smart materials have been explored for a variety of biomedical applications (e.g., drug delivery, tissue engineering, bioimaging, etc.), their ultimate clinical use has been hampered by the lack of biologically-relevant degradation observed for most smart materials. This is particularly true for temperature-responsive hydrogels, which are almost uniformly based on polymers that are functionally non-degradable (e.g., poly(N-isopropylacrylamide) (PNIPAM) or poly(oligoethylene glycol methacrylate) (POEGMA)). As such, to effectively translate the potential of thermoresponsive hydrogels to the challenges of remote-controlled or metabolism-regulated drug delivery, cell scaffolds with tunable cell-material interactions, theranostic materials with the potential for both imaging and drug delivery, and other such applications, a method is required to render the hydrogels (if not fully degradable) at least capable of renal clearance following the required lifetime of the material. To that end, this protocol describes the preparation of hydrolytically-degradable hydrazone-crosslinked hydrogels on multiple length scales based on the reaction between hydrazide and aldehyde-functionalized PNIPAM or POEGMA oligomers with molecular weights below the renal filtration limit. Specifically, methods to fabricate degradable thermoresponsive bulk hydrogels (using a double barrel syringe technique), hydrogel particles (on both the microscale through the use of a microfluidics platform facilitating simultaneous mixing and emulsification of the precursor polymers and the nanoscale through the use of a thermally-driven self-assembly and cross-linking method), and hydrogel nanofibers (using a reactive electrospinning strategy) are described. In each case, hydrogels with temperature-responsive properties similar to those achieved via conventional free radical cross-linking processes can be achieved, but the hydrazone cross-linked network can be degraded over time to re-form the oligomeric precursor polymers and enable clearance. As such, we anticipate these methods (which may be generically applied to any synthetic water-soluble polymer, not just smart materials) will enable easier translation of synthetic smart materials to clinical applications.

Structured Macroporous Hydrogels: Progress, Challenges, and Opportunities

Structured macroporous hydrogels that have controllable porosities on both the nanoscale and the microscale offer both the swelling and interfacial properties of bulk hydrogels as well as the transport properties of "hard" macroporous materials. While a variety of techniques such as solvent casting, freeze drying, gas foaming, and phase separation have been developed to fabricate structured macroporous hydrogels, the typically weak mechanics and isotropic pore structures achieved as well as the required use of solvent/additives in the preparation process all limit the potential applications of these materials, particularly in biomedical contexts. This review highlights recent developments in the field of structured macroporous hydrogels aiming to increase network strength, create anisotropy and directionality within the networks, and utilize solvent-free or additive-free fabrication methods. Such functional materials are well suited for not only biomedical applications like tissue engineering and drug delivery but also selective filtration, environmental sorption, and the physical templating of secondary networks.

Bioinspired Antioxidant Defense System Constructed by Antioxidants-Eluting Electrospun F127-Based Fibers

Cells were continuously exposed to oxidative damage by overproduction of reactive oxygen species (ROS) when they contacted implanted biomaterials. The strategy to prevent cells from oxidative injures remains a challenge. Inspired by the antioxidant defense system of cells, we constructed a biocompatible and ROS-responsive architecture on the substrate of styrene-b-(ethylene-co-butylene)-b-styrene elastomer (SEBS). The strategy was based on fabrication of architectures through reactive electrospinning of mixture including SEBS, acylated Pluronic F127, copolymer of poly(ethylene glycol) diacrylate and 1,2-ethanedithiol (PEGDA-EDT), and antioxidants (AA-2G) and ROS-triggered release of AA-2G from microfibers to detoxify the excess ROS. We demonstrated that the stable and hydrophilic architecture was constructed by phase separation of SEBS/F127 components and cross-linking between polymer chains during electrospinning; the ROS-responsive fibers controlled the release of AA-2G and the interaction of AA-2G with ROS reduced the oxidative damage to cells. The bioinspired architecture not only reduced mechanical and oxidative damage to cells but also maintained normal ROS level for physiological hemostasis. This work provides basic principles to design and develop antioxidative biomaterials for implantation in vivo.

Manufacturing of hydrogel biomaterials with controlled mechanical properties for tissue engineering applications

Hydrogels have been recognized as crucial biomaterials in the field of tissue engineering, regenerative medicine, and drug delivery applications due to their specific characteristics. These biomaterials benefit from retaining a large amount of water, effective mass transfer, similarity to natural tissues and the ability to form different shapes. However, having relatively poor mechanical properties is a limiting factor associated with hydrogel biomaterials. Controlling the biomechanical properties of hydrogels is of paramount importance. In this work, firstly, mechanical characteristics of hydrogels and methods employed for characterizing these properties are explored. Subsequently, the most common approaches used for tuning mechanical properties of hydrogels including but are not limited to, interpenetrating polymer networks, nanocomposites, self-assembly techniques, and co-polymerization are discussed. The performance of different techniques used for tuning biomechanical properties of hydrogels is further compared. Such techniques involve lithography techniques for replication of tissues with complex mechanical profiles; microfluidic techniques applicable for generating gradients of mechanical properties in hydrogel biomaterials for engineering complex human tissues like intervertebral discs, osteochondral tissues, blood vessels and skin layers; and electrospinning techniques for synthesis of hybrid hydrogels and highly ordered fibers with tunable mechanical and biological properties. We finally discuss future perspectives and challenges for controlling biomimetic hydrogel materials possessing proper biomechanical properties.

STATEMENT OF SIGNIFICANCE

Hydrogels biomaterials are essential constituting components of engineered tissues with the applications in regenerative medicine and drug delivery. The mechanical properties of hydrogels play crucial roles in regulating the interactions between cells and extracellular matrix and directing the cells phenotype and genotype. Despite significant advances in developing methods and techniques with the ability of tuning the biomechanical properties of hydrogels, there are still challenges regarding the synthesis of hydrogels with complex mechanical profiles as well as limitations in vascularization and patterning of complex structures of natural tissues which barricade the production of sophisticated organs. Therefore, in addition to a review on advanced methods and techniques for measuring a variety of different biomechanical characteristics of hydrogels, the new techniques for enhancing the biomechanics of hydrogels are presented. It is expected that this review will profit future works for regulating the biomechanical properties of hydrogel biomaterials to satisfy the demands of a variety of different human tissues.

Attachment, Proliferation, and Morphological Properties of Human Dermal Fibroblasts on Ovine Tendon Collagen Scaffolds: A Comparative Study

INTRODUCTION

Collagen type I is widely used as a biomaterial for tissue-engineered substitutes. This study aimed to fabricate different three-dimensional (3D) scaffolds using ovine tendon collagen type I (OTC-I), and compare the attachment, proliferation and morphological features of human dermal fibroblasts (HDF) on the scaffolds.

METHODS

This study was conducted between the years 2014 to 2016 at the Tissue Engineering Centre, UKM Medical Centre. OTC-I was extracted from ovine tendon, and fabricated into 3D scaffolds in the form of sponge, hydrogel and film. A polystyrene surface coated with OTC-I was used as the 2D culture condition. Genipin was used to crosslink the OTC-I. A non-coated polystyrene surface was used as a control. The mechanical strength of OTC-I scaffolds was evaluated. Attachment, proliferation and morphological features of HDF were assessed and compared between conditions.

RESULTS

The mechanical strength of OTC-I sponge was significantly higher than that of the other scaffolds. OTC-I scaffolds and the coated surface significantly enhanced HDF attachment and proliferation compared to the control, but no differences were observed between the scaffolds and coated surface. In contrast, the morphological features of HDF including spreading, filopodia, lamellipodia and actin cytoskeletal formation differed between conditions.

CONCLUSION

OTC-I can be moulded into various scaffolds that are biocompatible and thus could be suitable as scaffolds for developing tissue substitutes for clinical applications and in vitro tissue models. However, further study is required to determine the effect of morphological properties on the functional and molecular properties of HDF.

Electrospun Poly(acrylic acid)/Silica Hydrogel Nanofibers Scaffold for Highly Efficient Adsorption of Lanthanide Ions and Its Photoluminescence Performance

Combined with the features of electrospun nanofibers and the nature of hydrogel, a novel choreographed poly(acrylic acid)-silica hydrogel nanofibers (PAA-S HNFs) scaffold with excellent rare earth elements (REEs) recovery performance was fabricated by a facile route consisting of colloid-electrospinning of PAA/SiO2 precursor solution, moderate thermal cross-linking of PAA-S nanofiber matrix, and full swelling in water. The resultant PAA-S HNFs with a loose and spongy porous network structure exhibited a remarkable adsorption capacity of lanthanide ions (Ln(3+)) triggered by the penetration of Ln(3+) from the nanofiber surface to interior through the abundant water channels, which took full advantage of the internal adsorption sites of nanofibers. The effects of initial solution pH, concentration, and contact time on adsorption of Ln(3+) have been investigated comprehensively. The maximum equilibrium adsorption capacities for La(3+), Eu(3+), and Tb(3+) were 232.6, 268.8, and 250.0 mg/g, respectively, at pH 6, and the adsorption data were well-fitted to the Langmuir isotherm and pseudo-second-order models. The resultant PAA-S HNFs scaffolds could be regenerated successfully. Furthermore, the proposed adsorption mechanism of Ln(3+) on PAA-S HNFs scaffolds was the formation of bidentate carboxylates between carboxyl groups and Ln(3+) confirmed by FT-IR and XPS analysis. The well-designed PAA-S HNFs scaffold can be used as a promising alternative for effective REEs recovery. Moreover, benefiting from the unique features of Ln(3+), the Ln-PAA-S HNFs simultaneously exhibited versatile advantages including good photoluminescent performance, tunable emission color, and excellent flexibility and processability, which also hold great potential for applications in luminescent patterning, underwater fluorescent devices, sensors, and biomaterials, among others.

"Click" Chemistry-Tethered Hyaluronic Acid-Based Contact Lens Coatings Improve Lens Wettability and Lower Protein Adsorption

Improving the wettability of and reducing the protein adsorption to contact lenses may be beneficial for improving wearer comfort. Herein, we describe a simple "click" chemistry approach to surface functionalize poly(2-hydroxyethyl methacrylate) (pHEMA)-based contact lenses with hyaluronic acid (HA), a carbohydrate naturally contributing to the wettability of the native tear film. A two-step preparation technique consisting of laccase/TEMPO-mediated oxidation followed by covalent grafting of hydrazide-functionalized HA via simple immersion resulted in a model lens surface that is significantly more wettable, more water retentive, and less protein binding than unmodified pHEMA while maintaining the favorable transparency, refractive, and mechanical properties of a native lens. The dipping/coating method we developed to covalently tether the HA wetting agent is simple, readily scalable, and a highly efficient route for contact lens modification.

Tuning the properties of injectable poly(oligoethylene glycol methacrylate) hydrogels by controlling precursor polymer molecular weight

The properties of POEGMA hydrogels are tuned in a chemistry-independent manner via manipulation of the molecular weight of precursor polymers.