Reinforcement of Mono- and Bi-layer Poly(Ethylene Glycol) Hydrogels with a Fibrous Collagen Scaffold

Controlled Mechanical Property Gradients Within a Digital Light Processing Printed Hydrogel-Composite Osteochondral Scaffold

Tissue engineered scaffolds are needed to support physiological loads and emulate the micrometer-scale strain gradients within tissues that guide cell mechanobiological responses. We designed and fabricated micro-truss structures to possess spatially varying geometry and controlled stiffness gradients. Using a custom projection microstereolithography (μSLA) system, using digital light projection (DLP), and photopolymerizable poly(ethylene glycol) diacrylate (PEGDA) hydrogel monomers, three designs with feature sizes < 200 μm were formed: (1) uniform structure with 1 MPa structural modulus ( E ) designed to match equilibrium modulus of healthy articular cartilage, (2) E  = 1 MPa gradient structure designed to vary strain with depth, and (3) osteochondral bilayer with distinct cartilage ( E  = 1 MPa) and bone ( E  = 7 MPa) layers. Finite element models (FEM) guided design and predicted the local mechanical environment. Empty trusses and poly(ethylene glycol) norbornene hydrogel-infilled composite trusses were compressed during X-ray microscopy (XRM) imaging to evaluate regional stiffnesses. Our designs achieved target moduli for cartilage and bone while maintaining 68-81% porosity. Combined XRM imaging and compression of empty and hydrogel-infilled micro-truss structures revealed regional stiffnesses that were accurately predicted by FEM. In the infilling hydrogel, FEM demonstrated the stress-shielding effect of reinforcing structures while predicting strain distributions. Composite scaffolds made from stiff μSLA-printed polymers support physiological load levels and enable controlled mechanical property gradients which may improve in vivo outcomes for osteochondral defect tissue regeneration. Advanced 3D imaging and FE analysis provide insights into the local mechanical environment surrounding cells in composite scaffolds.

Multilayer functional bionic fabricated polycaprolactone based fibrous membranes for osteochondral integrated repair

Osteochondral defect repair is one of the challenging problems in orthopedics. In this study, a multilayer polycaprolactone (PCL) based fibrous membrane for osteochondral defect repair was biomimetically fabricated by combining self-induced crystallization, biomimetic mineralization and layer-by-layer electrospinning techniques. The multilayer functional bionic fibrous membrane consisted of cartilage repair layer, intermediate transition repair layer and subchondral bone repair layer. Glucosamine hydrochloride (GAH) encapsulated in core-shell structured PCL fibrous membrane (MGPCL) was suitable for cartilage repair. Shish-kebab (SK) structured PCL fibrous membrane with calcium phosphate coating (MSKPCL) was designed for subchondral bone repair. SK structured MGPCL fibrous membrane (SKMGPCL) was used as intermediate transition repair. The tensile modulus of MG/SKMG/MSKPCL fibrous membrane was 34.24 ± 2.39 MPa which met the requirements of cartilage and subchondral bone repair scaffolds, and in vitro culture results showed that MG/SKMG/MSKPCL fibrous membrane had good biological activity and osteogenic ability. These results showed that MG/SKMG/MSKPCL fibrous membrane provides a promising material basis for osteochondral integrated repair scaffold.

Physical Gold Nanoparticle-Decorated Polyethylene Glycol-Hydroxyapatite Composites Guide Osteogenesis and Angiogenesis of Mesenchymal Stem Cells

In this study, polyethylene glycol (PEG) with hydroxyapatite (HA), with the incorporation of physical gold nanoparticles (AuNPs), was created and equipped through a surface coating technique in order to form PEG-HA-AuNP nanocomposites. The surface morphology and chemical composition were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), UV–Vis spectroscopy (UV–Vis), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and contact angle assessment. The effects of PEG-HA-AuNP nanocomposites on the biocompatibility and biological activity of MC3T3-E1 osteoblast cells, endothelial cells (EC), macrophages (RAW 264.7), and human mesenchymal stem cells (MSCs), as well as the guiding of osteogenic differentiation, were estimated through the use of an in vitro assay. Moreover, the anti-inflammatory, biocompatibility, and endothelialization capacities were further assessed through in vivo evaluation. The PEG-HA-AuNP nanocomposites showed superior biological properties and biocompatibility capacity for cell behavior in both MC3T3-E1 cells and MSCs. These biological events surrounding the cells could be associated with the activation of adhesion, proliferation, migration, and differentiation processes on the PEG-HA-AuNP nanocomposites. Indeed, the induction of the osteogenic differentiation of MSCs by PEG-HA-AuNP nanocomposites and enhanced mineralization activity were also evidenced in this study. Moreover, from the in vivo assay, we further found that PEG-HA-AuNP nanocomposites not only facilitate the anti-immune response, as well as reducing CD86 expression, but also facilitate the endothelialization ability, as well as promoting CD31 expression, when implanted into rats subcutaneously for a period of 1 month. The current research illustrates the potential of PEG-HA-AuNP nanocomposites when used in combination with MSCs for the regeneration of bone tissue, with their nanotopography being employed as an applicable surface modification approach for the fabrication of biomaterials.

3D Silk Fiber Construct Embedded Dual-Layer PEG Hydrogel for Articular Cartilage Repair - In vitro Assessment

Since articular cartilage does not regenerate itself, researches are underway to heal damaged articular cartilage by applying biomaterials such as a hydrogel. In this study, we have constructed a dual-layer composite hydrogel mimicking the layered structure of articular cartilage. The top layer consists of a high-density PEG hydrogel prepared with 8-arm PEG and PEG diacrylate using thiol-norbornene photo-click chemistry. The compressive modulus of the top layer was 700.1 kPa. The bottom layer consists of a low-density PEG hydrogel reinforced with a 3D silk fiber construct. The low-density PEG hydrogel was prepared with 4-arm PEG using the same cross-linking chemistry, and the compressive modulus was 13.2 kPa. Silk fiber was chosen based on the strong interfacial bonding with the low-density PEG hydrogel. The 3D silk fiber construct was fabricated by moving the silk fiber around the piles using a pile frame, and the compressive modulus of the 3D silk fiber construct was 567 kPa. The two layers were joined through a covalent bond which endowed sufficient stability against repeated torsions. The final 3D silk fiber construct embedded dual-layer PEG hydrogel had a compressive modulus of 744 kPa. Chondrogenic markers confirmed the chondrogenic differentiation of human mesenchymal stem cells encapsulated in the bottom layer.

A Review on the Adaption of Alginate-Gelatin Hydrogels for 3D Cultures and Bioprinting

Sustaining the vital functions of cells outside the organism requires strictly defined parameters. In order to ensure their optimal growth and development, it is necessary to provide a range of nutrients and regulators. Hydrogels are excellent materials for 3D in vitro cell cultures. Their ability to retain large amounts of liquid, as well as their biocompatibility, soft structures, and mechanical properties similar to these of living tissues, provide appropriate microenvironments that mimic extracellular matrix functions. The wide range of natural and synthetic polymeric materials, as well as the simplicity of their physico-chemical modification, allow the mechanical properties to be adjusted for different requirements. Sodium alginate-based hydrogel is a frequently used material for cell culture. The lack of cell-interactive properties makes this polysaccharide the most often applied in combination with other materials, including gelatin. The combination of both materials increases their biological activity and improves their material properties, making this combination a frequently used material in 3D printing technology. The use of hydrogels as inks in 3D printing allows the accurate manufacturing of scaffolds with complex shapes and geometries. The aim of this paper is to provide an overview of the materials used for 3D cell cultures, which are mainly alginate-gelatin hydrogels, including their properties and potential applications.

The effects of dynamic compressive loading on human mesenchymal stem cell osteogenesis in the stiff layer of a bilayer hydrogel

Bilayer hydrogels with a soft cartilage-like layer and a stiff bone-like layer embedded with human mesenchymal stem cells (hMSCs) are promising for osteochondral tissue engineering. The goals of this work were to evaluate the effects of dynamic compressive loading (2.5% applied strain, 1 Hz) on osteogenesis in the stiff layer and spatially map local mechanical responses (strain, stress, hydrostatic pressure, and fluid velocity). A bilayer hydrogel was fabricated from soft (24 kPa) and stiff (124 kPa) poly (ethylene glycol) hydrogels. With hMSCs embedded in the stiff layer, osteogenesis was delayed under loading evident by lower OSX and OPN expressions, alkaline phosphatase activity, and collagen content. At Day 28, mineral deposits were present throughout the stiff layer without loading but localized centrally and near the interface under loading. Local strains mapped by particle tracking showed substantial equivalent strain (~1.5%) transferring to the stiff layer. When hMSCs were cultured in stiff single-layer hydrogels subjected to similar strains, mineralization was inhibited. Finite element analysis revealed that hydrostatic pressures ≥~600 Pa correlated to regions lacking mineralization in both hydrogels. Fluid velocities were low (~1-10 nm/s) in the hydrogels with no apparent correlation to mineralization. Mineralization was recovered by inhibiting ERK1/2, indicating cell-mediated inhibition. These findings suggest that high strains (~1.5%) combined with higher hydrostatic pressures negatively impact osteogenesis, but in a manner that depends on the magnitude of each mechanical response. This work highlights the importance of local mechanical responses in mediating osteogenesis of hMSCs in bilayer hydrogels being studied for osteochondral tissue engineering.

Nanofibers and Microfibers for Osteochondral Tissue Engineering

The use of fibers into scaffolds is a way to mimic natural tissues, in which fibrils are embedded in a matrix. The use of fibers can improve the mechanical properties of the scaffolds and may act as structural support for cell growth. Also, as the morphology of fibrous scaffolds is similar to the natural extracellular matrix, cells cultured on these scaffolds tend to maintain their phenotypic shape. Different materials and techniques can be used to produce micrfibers- and nanofibers for scaffolds manufacturing; cells, in general, adhere and proliferate very well on PCL, chitosan, silk fibroin, and other nanofibers. One of the most important techniques to produce microfibers/nanofibers is electrospinning. Nanofibrous scaffolds are receiving increasing attention in bone tissue engineering, because they are able to offer a favorable microenvironment for cell attachment and growth. Different polymers can be electrospun, i.e., polyester, polyurethane, PLA, PCL, collagen, and silk. Other materials such as bioglass fibers, nanocellulose, and even carbon fiber and fabrics have been used to help increase bioactivity, mechanical properties of the scaffold, and cell proliferation. A compilation of mechanical properties and most common biological tests performed on fibrous scaffolds is included in this chapter.

HIGHLIGHTS

The use of microfibers and nanofibers allows for tailoring the scaffold properties. Electrospinning is one of the most important techniques nowadays to produce fibrous scaffolds. Microfibers and nanofibers use in scaffolds is a promising field to improve the behavior of scaffolds in osteochondral applications.

A Stereolithography-Based 3D Printed Hybrid Scaffold for In Situ Cartilage Defect Repair

Damage to articular cartilage can over time cause degeneration to the tissue surrounding the injury. To address this problem, scaffolds that prevent degeneration and promote neotissue growth are needed. A new hybrid scaffold that combines a stereolithography-based 3D printed support structure with an injectable and photopolymerizable hydrogel for delivering cells to treat focal chondral defects is introduced. In this proof of concept study, the ability to a) infill the support structure with an injectable hydrogel precursor solution, b) incorporate cartilage cells during infilling using a degradable hydrogel that promotes neotissue deposition, and c) minimize damage to the surrounding cartilage when the hybrid scaffold is placed in situ in a focal chondral defect in an osteochondral plug that is cultured under mechanical loading is demonstrated. With the ability to independently control the properties of the structure and the injectable hydrogel, this hybrid scaffold approach holds promise for treating chondral defects.

Injectable scaffolds: Preparation and application in dental and craniofacial regeneration

Injectable scaffolds are appealing for tissue regeneration because they offer many advantages over pre-formed scaffolds. This article provides a comprehensive review of the injectable scaffolds currently being investigated for dental and craniofacial tissue regeneration. First, we provide an overview of injectable scaffolding materials, including natural, synthetic, and composite biomaterials. Next, we discuss a variety of characteristic parameters and gelation mechanisms of the injectable scaffolds. The advanced injectable scaffolding systems developed in recent years are then illustrated. Furthermore, we summarize the applications of the injectable scaffolds for the regeneration of dental and craniofacial tissues that include pulp, dentin, periodontal ligament, temporomandibular joint, and alveolar bone. Finally, our perspectives on the injectable scaffolds for dental and craniofacial tissue regeneration are offered as signposts for the future advancement of this field.

Mechanical characterization of sequentially layered photo-clickable thiol-ene hydrogels

Multi-layer hydrogels are promising for tissue engineering due to the ability to control the local properties within each layer. However, the interface that forms between each layer has the potential to affect the performance of the hydrogel. The goals of this study were to characterize how the interface forms via its thickness and mechanical properties, identify its impact on the overall hydrogel properties, and provide new insights into how to control the interface. A photo-clickable poly(ethylene glycol) hydrogel was used to form bilayer hydrogels that were sequentially polymerized in a step-and-repeat process. Different processing conditions were studied: the time (0-20min) before initiating polymerization of the second layer (soak time, t_s) and the hydrogel crosslink density (the same, less crosslinked, or more crosslinked) of the first layer as compared to the second layer. Interface thickness was characterized by confocal microscopy, monomer transport by Fickian diffusion, single and bilayer hydrogel mechanics by bulk moduli measurements, and interface moduli measurements using AFM, nanoindentation, and strain mapping. The interface thickness ranged from ~70 to 600μm (1-10% of total height) depending on processing conditions, but did not affect the bulk hydrogel modulus. Analysis of monomer transport revealed that convection, due to changes in hydrogel swelling, and diffusion contribute to interface thickness. Nanomechanical analysis of bilayer hydrogels formed from soft (75kPa) and stiff (250kPa) layers showed a gradient in elastic modulus across the interface, which corresponded to strain maps. In summary, this work identifies that diffusive and convective transport of monomers across the interface controls its thickness and that a mechanically robust interface forms, which does not affect the hydrogel modulus. By controlling the processing conditions, the thickness of the interface can be tuned without affecting the mechanical properties of the bulk hydrogel.