Fiber-reinforced Hydrogels Research Articles

Vaginal Tissue Engineering via Gelatin-Elastin Fiber-Reinforced Hydrogels.

The vagina is a fibromuscular tube-shaped organ spanning from the hymenal ring to the cervix that plays critical roles in menstruation, pregnancy, and female sexual health. Vaginal tissue constituents, including cells and extracellular matrix components, contribute to tissue structure, function, and prevention of injury. However, much microstructural function remains unknown, including how the fiber-cell and cell-cell interactions influence macromechanical properties. A deeper understanding of these interactions will provide critical information needed to reduce and prevent vaginal injuries. Our objectives for this work herein are to first engineer a suite of biomaterials for vaginal tissue engineering and second to characterize the performance of these biomaterials in the vaginal microenvironment. We successfully created fiber-reinforced hydrogels of gelatin-elastin electrospun fibers infiltrated with gelatin methacryloyl hydrogels. These composites recapitulate vaginal material properties, including stiffness, and are compatible with the vaginal microenvironment: biocompatible with primary vaginal epithelial cells and in acidic conditions. This work significantly advances progress in vaginal tissue engineering by developing novel materials and developing a state-of-the-art tissue engineered vagina.

Augmentation of Tendon and Ligament Repair with Fiber-Reinforced Hydrogel Composites.

This review highlights the promise of fiber-reinforced hydrogel composites (FRHCs) for augmenting tendon and ligamentrepair and regeneration. Composed of reinforcing fibers embedded in a hydrogel, these scaffolds provide both mechanical strength and a conducive microenvironment for biological processes required for connective tissue regeneration. Typical properties of FRHCs are discussed, highlighting their ability to simultaneously fulfill essential mechanical and biological design criteria for a regenerative scaffold. Furthermore, features of FRHCs are described that improve specific biological aspects of tendon healing including mesenchymal progenitor cell recruitment, early polarization to a pro-regenerative immune response, tenogenic differentiation of recruited progenitor cells, and subsequent production of a mature, aligned collagenous matrix. Finally, the review offers a perspective on clinical translation of tendon FRHCs and outlines key directions for future work.

3D printing of hydrogel composite systems: Recent advances in technology for tissue engineering.

Three-dimensional (3D) printing of hydrogels is now an attractive area of research due to its capability to fabricate intricate, complex and highly customizable scaffold structures that can support cell adhesion and promote cell infiltration for tissue engineering. However, pure hydrogels alone lack the necessary mechanical stability and are too easily degraded to be used as printing ink. To overcome this problem, significant progress has been made in the 3D printing of hydrogel composites with improved mechanical performance and biofunctionality. Herein, we provide a brief overview of existing hydrogel composite 3D printing techniques including laser based-3D printing, nozzle based-3D printing, and inkjet printer based-3D printing systems. Based on the type of additives, we will discuss four main hydrogel composite systems in this review: polymer- or hydrogel-hydrogel composites, particle-reinforced hydrogel composites, fiber-reinforced hydrogel composites, and anisotropic filler-reinforced hydrogel composites. Additionally, several emerging potential applications of hydrogel composites in the field of tissue engineering and their accompanying challenges are discussed in parallel.

Bioactive fiber-reinforced hydrogel to tailor cell microenvironment for structural and functional regeneration of myotendinous junction.

Myotendinous junction (MTJ) injuries are prevalent in clinical practice, yet the treatment approaches are limited to surgical suturing and conservative therapy, exhibiting a high recurrence rate. Current research on MTJ tissue engineering is scarce and lacks in vivo evaluation of repair efficacy. Here, we developed a three-dimensional-printed bioactive fiber-reinforced hydrogel containing mesenchymal stem cells (MSCs) and Klotho for structural and functional MTJ regeneration. In a rat MTJ defect model, the bioactive fiber-reinforced hydrogel promoted the structural restoration of muscle, tendon, and muscle-tendon interface and enhanced the functional recovery of injured MTJ. In vivo proteomics and in vitro cell cultures elucidated the regenerative mechanisms of the bioactive fiber-reinforced hydrogel by modulating oxidative stress and inflammation, thus engineering an optimized microenvironment to support the survival and differentiation of transplanted MSCs and maintain the functional phenotype of resident cells within MTJ tissues, including tendon/muscle cells and macrophages. This strategy provides a promising treatment for MTJ injuries.

Engineered Microenvironmental Cues from Fiber-Reinforced Hydrogel Composites Drive Tenogenesis and Aligned Collagen Deposition.

Effective tendon regeneration following injury is contingent on appropriate differentiation of recruited cells and deposition of mature, aligned, collagenous extracellular matrix that can withstand the extreme mechanical demands placed on the tissue. As such, myriad biomaterial approaches have been explored to provide biochemical and physical cues that encourage tenogenesis and template aligned matrix deposition in lieu of dysfunctional scar tissue formation. Fiber-reinforced hydrogels present an ideal biomaterial system toward this end given their transdermal injectability, tunable stiffness over a range amenable to tenogenic differentiation of progenitors, and capacity for modular inclusion of biochemical cues. Here, tunable and modular, fiber-reinforced, synthetic hydrogels are employed to elucidate salient microenvironmental determinants of tenogenesis and aligned collagen deposition by tendon progenitor cells. Transforming growth factor β3 drives a cell fate switch toward pro-regenerative or pro-fibrotic phenotypes, which can be biased toward the former by culture in softer microenvironments or inhibition of the RhoA/ROCK activity. Furthermore, studies demonstrate that topographical anisotropy in fiber-reinforced hydrogels critically mediates the alignment of de novo collagen fibrils, reflecting native tendon architecture. These findings inform the design of cell-free, injectable, synthetic hydrogels for tendon tissue regeneration and, likely, that of a range of load-bearing connective tissues.

Mechanical Field Guiding Structure Design Strategy for Meta-Fiber Reinforced Hydrogel Composites by Deep Learning.

Fiber-reinforced hydrogel composites are widely employed in many engineering applications, such as drug release, and flexible electronics, with more flexible mechanical properties than pure hydrogel materials. Comparing to the hydrogel strengthened by continuous fiber, the meta-fiber reinforced hydrogel provides stronger individualized design ability of deformation patterns and tunable stiffness, especially for the elaborate applications in joint, cartilage, and organ. In this paper, a novel structure design strategy based on deep learning algorithm is proposed for hydrogel reinforced by meta-fiber to achieve targeted mechanical properties, such as stress and displacement fields. A solid mechanic model for meta-fiber reinforced hydrogel is first developed to construct the dataset of fiber distribution and the corresponding mechanical properties of the composite. Generative adversarial network (GAN) is then trained to characterize the relationship between stress or displacement field, and meta-fiber distribution. The well-trained GAN is implemented to design meta-fiber reinforced hydrogel composite structure under specific operation conditions. The results show that the deep learning method may efficiently predict the structure of the hydrogel composite with satisfied confidence, and has great potential for applications in drug delivery and flexible electronics.

Modeling the Stimulus-Responsive Behaviors of Fiber-Reinforced Soft Materials

Hydrogels can change their size upon swelling. The swelling ratio is the same for all directions in the stress-free state. Dielectric elastomers can reduce their thickness and expand the area upon an electric field. Similarly, the expansion ratio in the plane is also the same for different directions. This isotropic shape change effect limits the function of these soft materials in certain circumstances. To address this issue, recent works have shown that the incorporation of fibers into the polymer matrix can induce an anisotropic response upon external stimulus. In this work, we develop multi-field coupling models for both fiber-reinforced hydrogels and dielectric elastomers. For the former, the change in free energy is caused by the stretching of polymer chains and fibers and the mixing of solvents and polymer networks. The Fickian-type law is adopted for the solvent diffusion. The free energy density for the latter consists of a mechanical part, considering the deformation of both polymer matrix and fibers, and an electric polarization component. Gauss’s law is adopted to obtain the distribution of the electric field. The multi-field models are then implemented for finite element analysis. We consider the stimulus-responsiveness of bilayer strips with an active layer and a passive layer. Without fibers, the strips bend upon the external stimulus. In contrast, the shape changes to helix shapes, which can be further tuned by changing the distribution of fibers. The work provides an efficient design tool for self-folding structures based on stimulus-responsive polymers.

Plant Biomimetic Principles of Multifunctional Soft Composite Development: A Synergistic Approach Enabling Shape Morphing and Mechanical Robustness.

Plant tissues are constructed as composite material systems of stiff cellulose microfibers reinforcing a soft matrix. Thus, they comprise smart and multifunctional structures that can change shape in response to external stimuli due to asymmetrical fiber alignment and possess robust mechanical properties. Herein, we demonstrate the biomimetics of the plant material system using silk fiber-reinforced alginate hydrogel matrix biocomposites. We fabricate single and bilamellar biocomposites with different fiber orientations. The mechanical behavior of the biocomposites is nonlinear, with large deformations, as in plant tissues. In general, the bilamellar system shows increased modulus, strain UTS, and toughness compared to the single-lamellar system for most of the tested orientations. Overall, the biocomposites present a wide range of elastic modulus values (3.0 ± 0.6-104.7 ± 11.3 MPa) and UTS values (0.23 ± 0.04-12.5 ± 2.0 MPa). The bilamellar biocomposites demonstrated shape-transforming abilities with diverse morphing modes, emulating different plant tissues and creating complex shape-morphing structures. These multifunctional biocomposites possess tunable and robust mechanical properties, controllable shape-morphing deformations, and the ability to self-controlled encapsulation, grip, and release objects. By harnessing biomimetic principles, these soft, smart, and multifunctional materials hold potential applications spanning from soft robotics, medicine, and tissue engineering to sensing and drug delivery.

Reinforcing melt electrowritten elements with entangled multifibrillar strands for thin hydrogels with potential in bone resurfacing

Osteonecrosis of the femoral head (ONFH) is a disease that affects young adults in their thirties to fifties, representing the progressive destruction of the hip bone caused by deficient vascularization. As the condition slowly leads to complete collapse of the femoral head, the conventional solution is total hip arthroplasty. Thin scaffolds consisting of fiber-reinforced hydrogels could be used to regenerate the affected bone surface, coupled with hip resurfacing for less invasive approaches. Melt electrowriting (MEW) was used to produce polycaprolactone (PCL) reinforcing elements for thin scaffolds, with four mesh densities and highly tunable mechanical properties. The influence of the MEW process parameters on the PCL filaments’ morphology was investigated using Design of Experiments to optimize their fabrication and obtain tailorable structures with entangled fiber morphology for increased contact surface with the hydrogel component. Uniaxial tensile tests were performed to investigate the difference in tensile properties of the entangled design versus the aligned counterparts, including the exploration of the mesh size effect. The microstructure and microstructural changes of the entangled meshes at uniaxial tensile deformation were explored using micro-computed tomography. Plasma treated meshes were embedded in gelatin methacryloyl/alginate hydrogels, developing reinforced composite scaffolds with potential for bone surface reconstruction.

GelMA-catechol coated FeHAp nanorods functionalized nanofibrous reinforced bio-instructive and mechanically robust composite hydrogel scaffold for bone tissue engineering

Critical bone defects complicate tissue graft-based surgeries, raising healthcare expenditures and underscoring scaffold-based tissue-engineering strategies to support bone reconstruction. Our study highlighted that the phase-compatible combination of inorganic nanorods, nanofibers, and hydrogels is promising for developing biomimetic and cell-instructive scaffolds since the bone matrix is a porous organic/inorganic composite. In brief, methacrylated gelatin (GelMA) was reacted with dopamine to form catechol-modified GeLMA (GelMA-C). The GelMA-C was nanocoated onto an iron-doped hydroxyapatite (FeHAp) nanorod via metal-catechol network coordination. The modified nanorod (FeHAp@GelMA-C) was loaded onto GelMA-based nanofibers. The nanorods loaded pre-fibers were electrospun onto GelMA solution and photochemically crosslinked to fabricate a fiber-reinforced hydrogel. The structural, mechanical, physicochemical, biocompatibility, swelling properties, osteogenic potential, and bone remodelling potential (using rat femoral defect model) of modified nanorods, simple hydrogel, and nanorod-loaded fiber-reinforced hydrogel were studied. The results supported that the interface interaction between GelMA-C/nanorods, nanorods/nanofibers, nanorods/hydrogels, and nanofiber/hydrogels significantly improved the microstructural and mechanical properties of the scaffold. Compared to pristine hydrogel, the nanorod-loaded fiber-reinforced scaffold better supported cellular responses, osteogenic differentiation, matrix mineralization, and accelerated bone regeneration. The nanorod-loaded fiber-reinforced hydrogel proved more biomimetic and cell-instructive for guided bone reconstruction.

Microfiber-reinforced hydrogels prolong the release of human induced pluripotent stem cell-derived extracellular vesicles to promote endothelial migration

Extracellular vesicle (EV)-based approaches for promoting angiogenesis have shown promising results. Yet, further development is needed in vehicles that prolong EV exposure to target organs. Here, we hypothesized that microfiber-reinforced gelatin methacryloyl (GelMA) hydrogels could serve as sustained delivery platforms for human induced pluripotent stem cell (hiPSC)-derived EV. EV with 50–200 nm size and typical morphology were isolated from hiPSC-conditioned culture media and tested negative for common co-isolated contaminants. hiPSC-EV were then incorporated into GelMA hydrogels with or without a melt electrowritten reinforcing mesh. EV release was found to increase with GelMA concentration, as 12 % (w/v) GelMA hydrogels provided higher release rate and total release over 14 days in vitro, compared to lower hydrogel concentrations. Release profile modelling identified diffusion as a predominant release mechanism based on a Peppas-Sahlin model. To study the effect of reinforcement-dependent hydrogel mechanics on EV release, stress relaxation was assessed. Reinforcement with highly porous microfiber meshes delayed EV release by prolonging hydrogel stress relaxation and reducing the swelling ratio, thus decreasing the initial burst and overall extent of release. After release from photocrosslinked reinforced hydrogels, EV remained internalizable by human umbilical vein endothelial cells (HUVEC) over 14 days, and increased migration was observed in the first 4 h. EV and RNA cargo stability was investigated at physiological temperature in vitro, showing a sharp decrease in total RNA levels, but a stable level of endothelial migration-associated small noncoding RNAs over 14 days. Our data show that hydrogel formulation and microfiber reinforcement are superimposable approaches to modulate EV release from hydrogels, thus depicting fiber-reinforced GelMA hydrogels as tunable hiPSC-EV vehicles for controlled release systems that promote endothelial cell migration.

Versatile fiber-reinforced hydrogels to mimic the microstructure and mechanics of human vocal-fold upper layers

Human vocal folds are remarkable soft laryngeal structures that enable phonation due to their unique vibro-mechanical performances. These properties are tied to their specific fibrous architecture, especially in the upper layers, which comprise a gel-like composite called lamina propria. The lamina propria can withstand large and reversible deformations under various multiaxial loadings. Despite their importance, the relationships between the microstructure of vocal folds and their resulting macroscopic properties remain poorly understood. There is a need for versatile models that encompass their structural complexity while mimicking their mechanical features. In this study, we present a candidate model inspired by histological measurements of the upper layers of human vocal folds. Bi-photonic observations were used to quantify the distribution, orientation, width, and volume fraction of collagen and elastin fibers between histological layers. Using established biomaterials, polymer fiber-reinforced hydrogels were developed to replicate the fibrillar network and ground substance of native vocal fold tissue. To achieve this, jet-sprayed poly(ε-caprolactone) fibrillar mats were successfully impregnated with poly(L-lysine) dendrimers/polyethylene glycol hydrogels. The resulting composites exhibited versatile structural, physical and mechanical properties that could be customized through variations in the chemical formulation of their hydrogel matrix, the microstructural architecture of their fibrous networks (i.e., fiber diameter, orientation and volume fraction) and their assembly process. By mimicking the collagen network of the lamina propria with polymer fibers and the elastin/ground substance with the hydrogel composition, we successfully replicated the non-linear, anisotropic, and viscoelastic mechanical behavior of the vocal-fold upper layers, accounting for inter/intra-individual variations. The development of this mimetic model offers promising avenues for a better understanding of the complex mechanisms involved in voice production. Statement of significanceHuman vocal folds are outstanding vibrating soft living tissues allowing phonation. Simple physical models that take into account the histological structure of the vocal fold and recapitulate its mechanical features are scarce. As a result, the relations between tissue components, organisation and vibro-mechanical performances still remain an open question. We describe here the development and the characterization of fiber-reinforced hydrogels inspired from the vocal-fold microstructure. These systems are able to reproduce the mechanics of vocal-fold tissues upon realistic cyclic and large strains under various multi-axial loadings, thus providing a mimetic model to further understand the impact of the fibrous network microstructure in phonation.

Dual-responsive fiber-reinforced hydrogel actuators by direct ion patterning

Dual-responsive fiber-reinforced hydrogel actuators by direct ion patterning

Biomimetic Nanofibrillar Hydrogel with Cell-Adaptable Network for Enhancing Cellular Mechanotransduction, Metabolic Energetics, and Bone Regeneration.

The natural extracellular matrix, with its heterogeneous structure, provides a stable and dynamic biophysical framework and biochemical signals to guide cellular behaviors. It is challenging but highly desirable to develop a synthetic matrix that emulates the heterogeneous fibrous structure with macroscopic stability and microscopical dynamics and contains inductive biochemical signals. Herein, we introduce a peptide fiber-reinforced hydrogel in which the stiff ß-sheet fiber functions as a multivalent cross-linker to enhance the hydrogel's macroscopic stability. The dynamic imine cross-link between the peptide fiber and polymer network endows the hydrogel with a microscopically dynamic network. The obtained fibrillar nanocomposite hydrogel, with its cell-adaptable dynamic network, enhances cell-matrix and cell-cell interactions and therefore significantly promotes the mechanotransduction, metabolic energetics, and osteogenesis of encapsulated stem cells. Furthermore, the hydrogel can codeliver a fiber-attached inductive drug to further enhance osteogenesis and bone regeneration. We believe that our work provides valuable guidance for the design of cell-adaptive and bioactive biomaterials for therapeutic applications.

Constitutive modeling for hydrogel composites with arbitrary fiber distribution

In this work, a constitutive model is developed to consider the effect of arbitrary fiber distribution on the swelling and tensile deformations of fiber-reinforced hydrogels. In the model, the free energy of total system is divided into two parts, i.e., the hydrogel contribution and fiber phase one. For hydrogel subsystem, we adopt the hydrogel free energy function of Flory-Rehner so that the swelling characteristic of hydrogel can be well captured; for the fiber, a statistical method is used to construct its free energy function. To effectively model the effect of fiber distribution, a principal invariant and dispersion coefficient are introduced so that the quantitative relationship between fiber distribution and anisotropy of material can be well described. Furthermore, through the analytical method or finite element implementation, both the swelling and tensile deformations of hydrogel composites with different fiber distributions are investigated. The modeling results show that the model can not only quantitatively predict the swelling and tensile deformations, but also can capture the effect of fiber distribution on the mechanical response of hydrogel composites. The new model may be helpful to the design and optimization of hydrogel composites.

Fiber density and matrix stiffness modulate distinct cell migration modes in a 3D stroma mimetic composite hydrogel.

The peritumoral stroma is a complex 3D tissue that provides cells with myriad biophysical and biochemical cues. Histologic observations suggest that during metastatic spread of carcinomas, these cues influence transformed epithelial cells, prompting a diversity of migration modes spanning single cell and multicellular phenotypes. Purported consequences of these variations in tumor escape strategies include differential metastatic capability and therapy resistance. Therefore, understanding how cues from the peritumoral stromal microenvironment regulate migration mode has both prognostic and therapeutic value. Here, we utilize a synthetic stromal mimetic in which matrix fiber density and bulk hydrogel mechanics can be orthogonally tuned to investigate the contribution of these two key matrix attributes on MCF10A migration mode phenotypes, epithelial-mesenchymal transition (EMT), and invasive potential. We develop an automated computational image analysis framework to extract migratory phenotypes from fluorescent images and determine 3D migration metrics relevant to metastatic spread. Using this analysis, we find that matrix fiber density and bulk hydrogel mechanics distinctly contribute to a variety of MCF10A migration modes including amoeboid, single mesenchymal, clusters, and strands. We identify combinations of physical and soluble cues that induce a variety of migration modes originating from the same MCF10A spheroid and use these settings to examine a functional consequence of migration mode -resistance to apoptosis. We find that cells migrating as strands are more resistant to staurosporine-induced apoptosis than either disconnected clusters or individual invading cells. Improved models of the peritumoral stromal microenvironment and understanding of the relationships between matrix attributes and cell migration mode can aid ongoing efforts to identify effective cancer therapeutics that address cell plasticity-based therapy resistances. STATEMENT OF SIGNIFICANCE: Stromal extracellular matrix structure dictates both cell homeostasis and activation towards migratory phenotypes. However decoupling the effects of myriad biophysical cues has been difficult to achieve. Here, we encapsulate electrospun fiber segments within an amorphous hydrogel to create a fiber-reinforced hydrogel composite in which fiber density and hydrogel stiffness can be orthogonally tuned. Quantification of 3D cell migration reveal these two parameters uniquely contribute to a diversity of migration phenotypes spanning amoeboid, single mesenchymal, multicellular cluster, and collective strand. By tuning biophysical and biochemical cues to elicit heterogeneous migration phenotypes, we find that collective strands best resist apoptosis. This work establishes a composite approach to modulate fibrous topography and bulk hydrogel mechanics and identified biomaterial parameters to direct distinct 3D cell migration phenotypes.

Synthesis and characterization of natural fibers reinforced alginate hydrogel fibers loaded with diclofenac sodium for wound dressings

Hydrogels which become increasingly important in the biomedical field are composed of a three-dimensional hydrophilic network. Pure hydrogels are usually weak and brittle; therefore, reinforcements are assimilated into the hydrogel structure to improve the mechanical strength of the hydrogels. However, even if mechanical properties are enhanced, drapability remains an issue. In that regard, natural fiber-reinforced composite hydrogel fibers for wound dressing application are investigated in this study. Kapok and hemp fibers were used as reinforcement to improve the strength of hydrogel fibers. The properties of the prepared composite hydrogel fibers were studied with Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and differential scanning calorimeter (DSC). The effect of alginate concentration and fiber weight percent on the mechanical characteristics and water absorbency was studied. Diclofenac sodium drug was loaded in the hydrogel fibers and investigated the drug release as well as antibacterial characteristics. Both fibers' reinforcement enhanced the strength of the alginate hydrogel fiber, but hemp reinforcement showed better mechanical properties. Kapok reinforcement resulted in a maximum tensile strength of 174 cN (1.24 % elongation) and 432 % exudate absorbency, while hemp reinforcement resulted in 185 cN (1.48 % elongation) and 435 % exudate absorbency. Statistical analysis revealed significant effects of sodium alginate concentration on tensile strength (p-value 0.042) and exudate absorbency (p-value 0.020) and of reinforcement (wt%) on exudate absorbency (p-value 0.043). Therefore, these composite hydrogel fibers with improved mechanical properties are capable of drug release and exhibit antibacterial effectiveness, making them a promising option for use as wound dressings.

Naturally-derived thermal barrier based on fiber-reinforced hydrogel for the prevention of thermal runaway propagation in high-energetic lithium-ion battery packs

Due to the constant increase in energy density of lithium-ion batteries (LIBs), safety concerns in case of a thermal runaway event become more critical. Thermal runaway propagation (TP) can destroy an entire LIB pack, rapidly releasing energy and material through heat and fire. A solution for TP that minimally affects the LIB pack remains one of the main unsolved safety-related issues in battery systems. This paper presents an eco-friendly thermal barrier made of a high water-content hydrogel derived from natural polymers reinforced by a flame-retardant fiber material. The 2-mm-thick barrier, when placed between 50 Ah LIB cells, was able to prevent thermal propagation in 2-cell modules. Once the barrier temperature reaches around 100 °C, it can absorb a large amount of energy due to water vaporization. Afterward, the remaining low thermally conductive fiber material provides thermal insulation between cells. In addition, the mechanical properties of the barrier, such as compressive strength and flexibility, make it a suitable material to be placed between LIB cells. Furthermore, its ease of manufacturing and biodegradability enable the barrier to provide higher safety for LIB packs while maintaining the cost and environmental impact to a minimum.

The Construction of Biologically Relevant Fiber-Reinforced Hydrogel Geometries Using Air-Assisted Dual-Polarity Electrospinning.

Fiber-reinforced hydrogels are a class of soft composite materials that have seen increased use across a wide variety of biomedical applications. However, existing fabrication techniques for these hydrogels are unable to realize biologically relevant macro/mesoscale geometries. To address this limitation, this paper presents a novel air-assisted, dual-polarity electrospinning printhead that converges high-strength electric fields, with low velocity air flow to remove the collector dependency seen with traditional far-field electrospinning setups. The use of this printhead in conjunction with different configurations of deformable collection templates has resulted in the production of three classes of fiber-reinforced hydrogel prototype geometries, viz., (i) tubular geometries with bifurcations and mesoscale texturing; (ii) hollow, nontubular geometries with single and dual-entrances; and (iii) three-dimensional (3D) printed flat geometries with varying fiber density. All three classes of prototype geometries were mechanically characterized to have properties that were in line with those observed in living soft tissues. With the realization of this printhead, biologically relevant macro/mesoscale geometries can be realized using fiber-reinforced hydrogels to aid a wide array of biomedical applications.

Effect of Fiber Geometry on Fracture and Fatigue of Composite Hydrogels

Abstract Hydrogel-based biomedical applications are under rapid development. These applications usually demand hydrogels to have high toughness and high fatigue threshold. Recently, various fiber-reinforced composite hydrogels have been developed to meet this challenge. However, the effect of fiber geometry on the fracture and fatigue of composite hydrogels is still elusive. Here, we use a model composite hydrogel to study the influence of fiber width, fiber spacing, and fiber configuration on these properties. It is found that the toughness of the composite hydrogel does not increase monotonically with the fiber width or fiber spacing, but presents a peak. This is because the variation of fiber width and fiber spacing not only affects the volume of fiber in the fracture process zone but also influences the dissipated elastic energy density in that volume, which is affected by the stress concentration. The peak is a consequence of the trade-off between these two factors. Our study further shows that the shape of the fiber network affects the stress concentration in the fiber dramatically, thereby leading to a huge difference in the toughness and fatigue threshold of the composite hydrogels. This work highlights the importance of fiber size as well as the shape of fiber networks on the mechanical properties of composite hydrogels. It may help the design of tough and fatigue-resistant stretchable composite materials.