Thiolene- and Polycaprolactone Methacrylate-Based Polymerized High Internal Phase Emulsion (PolyHIPE) Scaffolds for Tissue Engineering

Emulsion template fabricated heterogeneous bilayer gelatin-based scaffolds with sustained-delivery of lycium barbarum glycopeptide for periodontitis treatment

Periodontitis is a chronic inflammatory disease raising the risks of tooth-supporting structures destruction and even tooth loss. The way to reconstruct periodontal bone tissues in inflammatory microenvironment has been long in demand for periodontitis treatment. In this study, the lycium barbarum glycopeptide (LbGP) loaded gelatin-based scaffolds were fabricated for periodontitis treatment. Gelatin microspheres with suitable size were prepared by emulsification and gathered by oxidized sodium alginate to prepare heterogeneous bilayer gelatin-based scaffolds, and then they were loaded with LbGP. The prepared scaffolds possessed interconnected porous microstructures, good degradation properties, sufficient mechanical properties, sustained release behavior and well biocompatibility. In vitro experiments suggested that the LbGP loaded gelatin-based scaffolds could inhibit the expression of inflammatory factors (IL-1β, IL-6, and TNF-α), promote the expression of anti-inflammatory factor (IL-10), and the expression of osteogenic markers (BMP2, Runx2, ALP, and OCN) in PDLSCs under the LPS-stimulated inflammatory microenvironment. Moreover, in rat periodontitis models, the LbGP gelatin-based scaffolds would reduce the alveolar bone resorption of rats, increase the collagen fiber content of periodontal membrane, alleviate local inflammation and improve the expression of osteogenesis-related factors. Therefore, the LbGP loaded gelatin-based scaffolds in this study will provide a potential therapeutic strategy for periodontitis treatment.

A double-network porous hydrogel based on high internal phase emulsions as a vehicle for potassium sucrose octasulfate delivery accelerates diabetic wound healing

Diabetic wounds are a difficult medical challenge. Excessive secretion of matrix metalloproteinase-9 (MMP-9) in diabetic wounds further degrades the extracellular matrix and growth factors and causes severe vascular damage, which seriously hinders diabetic wound healing. To solve these issues, a double-network porous hydrogel composed of poly (methyl methacrylate-co-acrylamide) (p(MMA-co-AM)) and polyvinyl alcohol (PVA) was constructed by the high internal phase emulsion (HIPE) technique for the delivery of potassium sucrose octasulfate (PSO), a drug that can inhibit MMPs, increase angiogenesis and improve microcirculation. The hydrogel possessed a typical polyHIPE hierarchical microstructure with interconnected porous morphologies, high porosity, high specific surface area, excellent mechanical properties and suitable swelling properties. Meanwhile, the p(MMA-co-AM)/PVA@PSO hydrogel showed high drug-loading performance and effective PSO release. In addition, both in vitro and in vivo studies showed that the p(MMA-co-AM)/PVA@PSO hydrogel had good biocompatibility and significantly accelerated diabetic wound healing by inhibiting excessive MMP-9 in diabetic wounds, increasing growth factor secretion, improving vascularization, increasing collagen deposition and promoting re-epithelialization. Therefore, this study provided a reliable therapeutic strategy for diabetic wound healing, some theoretical basis and new insights for the rational design and preparation of wound hydrogel dressings with high porosity, high drug-loading performance and excellent mechanical properties.

Composite Bioprinted Hydrogels Containing Porous Polymer Microparticles Provide Tailorable Mechanical Properties for 3D Cell Culture

The mechanical and architectural properties of the three-dimensional (3D) tissue microenvironment can have large impacts on cellular behavior and phenotype, providing cells with specialized functions dependent on their location. This is especially apparent in macrophage biology where the function of tissue resident macrophages is highly specialized to their location. 3D bioprinting provides a convenient method of fabricating biomaterials that mimic specific tissue architectures. If these printable materials also possess tunable mechanical properties, they would be highly attractive for the study of macrophage behavior in different tissues. Currently, it is difficult to achieve mechanical tunability without sacrificing printability, scaffold porosity, and a loss in cell viability. Here, we have designed composite printable biomaterials composed of traditional hydrogels [nanofibrillar cellulose (cellulose) or methacrylated gelatin (gelMA)] mixed with porous polymeric high internal phase emulsion (polyHIPE) microparticles. By varying the ratio of polyHIPEs to hydrogel, we fabricate composite hydrogels that mimic the mechanical properties of the neural tissue (0.1-0.5 kPa), liver (1 kPa), lungs (5 kPa), and skin (10 kPa) while maintaining good levels of biocompatibility to a macrophage cell line.

Electroactive 4D Porous Scaffold Based on Conducting Polymer as a Responsive and Dynamic In Vitro Cell Culture Platform

In vivo, cells reside in a 3D porous and dynamic microenvironment. It provides biochemical and biophysical cues that regulate cell behavior in physiological and pathological processes. In the context of fundamental cell biology research, tissue engineering, and cell-based drug screening systems, a challenge is to develop relevant in vitro models that could integrate the dynamic properties of the cell microenvironment. Taking advantage of the promising high internal phase emulsion templating, we here designed a polyHIPE scaffold with a wide interconnected porosity and functionalized its internal 3D surface with a thin layer of electroactive conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) to turn it into a 4D electroresponsive scaffold. The resulting scaffold was cytocompatible with fibroblasts, supported cellular infiltration, and hosted cells, which display a 3D spreading morphology. It demonstrated robust actuation in ion- and protein-rich complex culture media, and its electroresponsiveness was not altered by fibroblast colonization. Thanks to customized electrochemical stimulation setups, the electromechanical response of the polyHIPE/PEDOT scaffolds was characterized in situ under a confocal microscope and showed 10% reversible volume variations. Finally, the setups were used to monitor in real time and in situ fibroblasts cultured into the polyHIPE/PEDOT scaffold during several cycles of electromechanical stimuli. Thus, we demonstrated the proof of concept of this tunable scaffold as a tool for future 4D cell culture and mechanobiology studies.

Development of tissue-engineered vascular grafts from decellularized parsley stems

Cardiovascular diseases are mostly associated with narrowing or blockage of blood vessels, and it is the most common cause of death worldwide. The use of vascular grafts is a promising approach to bypass or replace the blocked vessels for long-term treatment. Although autologous arteries or veins are the most preferred tissue sources for vascular bypass, the limited presence and poor quality of autologous vessels necessitate seeking alternative biomaterials. Recently, synthetic grafts have gained attention as an alternative to autologous grafts. However, the high failure rate of synthetic grafts has been reported primarily due to thrombosis, atherosclerosis, intimal hyperplasia, or infection. Thrombosis, the main reason for failure upon implantation, is associated with damage or absence of endothelial cell lining in the vascular graft's luminal surface. To overcome this, tissue-engineered vascular grafts (TEVGs) have come into prominence. Alongside the well-established scaffold manufacturing techniques, decellularized plant-based constructs have recently gained significant importance and are an emerging field in tissue engineering and regenerative medicine. Accordingly, in this study, we demonstrated the fabrication of tubular scaffolds from decellularized parsley stems and recellularized them with human endothelial cells to be used as a potential TEVG. Our results suggested that the native plant DNA was successfully removed, and soft tubular biomaterials were successfully manufactured via the chemical decellularization of the parsley stems. The decellularized parsley stems showed suitable mechanical and biological properties to be used as a TEVG material, and they provided a suitable environment for the culture of human endothelial cells to attach and create a pseudo endothelium prior to implantation. This study is the first one to demonstrate the potential of the parsley stems to be used as a potential TEVG biomaterial.

Ultrastable High Internal Phase Pickering Emulsions: Forming Mechanism, Processability, and Application in 3D Printing

High internal phase Pickering emulsions (HIPPEs) are versatile platforms for various applications owing to their low-density, solid-like structure, and large specific surface area. Here, naturally occurring polysaccharide-protein hybrid nanoparticles (PPH NPs) were used to stabilize HIPPEs with an internal phase fraction of 80% at a PPH NP concentration of 1.5%. The obtained HIPPEs displayed a gel-like behavior with excellent stability against centrifugation (10000g, 10 min), temperature (4-121 °C), pH (1.0-11.0), and ionic strength (0-500 mM). Confocal laser scanning microscope and cryo-scanning electron microscopy results showed that PPH NPs contributed to the stability of HIPPEs by effectively adsorbing and anchoring on the surface of the emulsion droplets layer by layer to form a dense 3D network barrier to inhibit droplet coalescence. The rheological analysis showed that the HIPPEs possessed a higher viscosity and lower frequency dependence with increasing PPH NP concentration, suggesting the potential application of such HIPPEs in three-dimensional (3D) printing, which was subsequently confirmed by a 3D printing experiment. This work provides highly stable and processable HIPPEs, which can be developed as facile and reusable materials for numerous applications. They can also be directly used for future food manufacturing, drug and nutrient delivery, and tissue reconstruction.

Hydroxyapatite nanoparticle-modified porous bone grafts with improved cell attachment

Emulsion-templated foams have displayed promise as injectable bone grafts; however, the use of a surfactant as an emulsifier resulted in relatively small pores and impedes cell attachment. Hydroxyapatite nanoparticles were explored as an alternative stabilizer to address these limitations. To this end, hydroxyapatite nanoparticles were first modified with myristic acid to generate the appropriate balance of hydrophobicity to stabilize a water-in-oil emulsion of neopentyl glycol diacrylate and 1,4-butanedithiol. In situ surface modification of the resulting foam with hydroxyapatite was confirmed with elemental mapping and transmission electron microscopy. Nanoparticle-stabilized foams displayed improved human mesenchymal stem cell viability (91 ± 5%) over surfactant-stabilized foams (23 ± 11%). Although the pore size was appropriate for bone grafting applications (115 ± 71 μm), the foams lacked the interconnected architecture necessary for cell infiltration. We hypothesized that a co-stabilization approach with both surfactant and nanoparticles could be used to achieve interconnected pores while maintaining improved cell attachment and larger pore sizes. A range of hydroxyapatite nanoparticle and surfactant concentrations were investigated to determine the effects on microarchitecture and cell behavior. By balancing these interactions, a co-stabilized foam was identified that possessed large, interconnected pores (108 ± 67 μm) and improved cell viability and attachment. The co-stabilized foam was then evaluated as an injectable bone graft including network formation, microscale integration with bone, push out strength, and compressive properties. Overall, this work demonstrated that in situ surface modification with nHA improved cell attachment while retaining desirable bone grafting features and injectability.

Hydroxyapatite/Poly (Butylene Succinate)/Metoprolol Tartrate Composites with Controllable Drug Release and a Porous Structure for Bone Scaffold Application

Nowadays, it is a challenge for a bone scaffold to achieve controllable drug release and a porous structure at the same time. Herein, we fabricated hydroxyapatite/poly (butylene succinate)/metoprolol tartrate (HA/PBS/MPT) composites via melt blending, aiming to provide the option of an in situ pore-forming strategy. The introduction of HA not only significantly improved the hydrophilicity of the PBS matrix by reducing the hydrophilic contact angle by approximately 36% at a 10% content, but also damaged the integrity of the PBS crystal. Both were beneficial for the penetration of phosphate-buffered saline solution into matrix and the acceleration of MPT release. Accompanied with MPT release, porous structures were formed in situ, and the HA inside the matrix was exposed. With the increase in HA content, the MPT release rate accelerated and the pore size became larger. The in vitro cytocompatibility evaluation indicated that HA/PBS/MPT composites were conductive to the adhesion, growth, and proliferation of MC3T3-E1 cells due to the HA being exposed around the pores. Thus, the MPT release rate, pore size, and cell induction ability of the HA/PBS/MPT composites were flexibly and effectively adjusted by the composition at the same time. By introducing HA, we innovatively achieved the construction of porous structures during the drug release process, without the addition of pore-forming agents. This approach allows the drug delivery system to combine controllable drug release and biocompatibility effectively, offering a novel method for bone repair material preparation. This work might provide a convenient and robust strategy for the fabrication of bone scaffolds with controllable drug release and porous structures.

PEG-Based Photo-Cross-Linked Networks with Adjustable Topologies and Mechanical Properties

We report the synthesis of networks having adjustable topologies and mechanical properties. Our approach consists of photopolymerizing poly(ethylene glycol) diacrylates (PEG-DA) in the presence of mixtures of mono- and multifunctional thiols. We show that the introduction of monothiols as non-cross-linking transfer agents provides a simple way to tune the topology of the networks and produce soft extensible networks. In a systematic study with model short PEG-DA (Mn = 700 g·mol-1), we explored how the gel point and network properties, such as the swelling ratio, the soluble fraction, the viscoelastic moduli, and the ultimate stress and strain, can be adjusted by varying the ratio of thiol to acrylate functions and the average functionality of the thiol mixture. We applied this strategy to longer chains of PEG-DA (Mn = 2300 and 3200 g·mol-1) and varied the viscoelastic and tensile responses of these networks to optimize their adhesive performance. This simple and robust approach further enriches the toolbox of thiol-acrylate polymerization and expands the application scope of PEG-based hydrogels.

Surfactant-free gelatin-stabilised biodegradable polymerised high internal phase emulsions with macroporous structures

High internal phase emulsion (HIPE) templating is a well-established method for the generation of polymeric materials with high porosity (>74%) and degree of interconnectivity. The porosity and pore size can be altered by adjusting parameters during emulsification, which affects the properties of the resulting porous structure. However, there remain challenges for the fabrication of polyHIPEs, including typically small pore sizes (∼20-50 μm) and the use of surfactants, which can limit their use in biological applications. Here, we present the use of gelatin, a natural polymer, during the formation of polyHIPE structures, through the use of two biodegradable polymers, polycaprolactone-methacrylate (PCL-M) and polyglycerol sebacate-methacrylate (PGS-M). When gelatin is used as the internal phase, it is capable of stabilising emulsions without the need for an additional surfactant. Furthermore, by changing the concentration of gelatin within the internal phase, the pore size of the resulting polyHIPE can be tuned. 5% gelatin solution resulted in the largest mean pore size, increasing from 53 μm to 80 μm and 28 μm to 94 µm for PCL-M and PGS-M respectively. In addition, the inclusion of gelatin further increased the mechanical properties of the polyHIPEs and increased the period an emulsion could be stored before polymerisation. Our results demonstrate the potential to use gelatin for the fabrication of surfactant-free polyHIPEs with macroporous structures, with potential applications in tissue engineering, environmental and agricultural industries.

Synthesis and Applications of Elastomeric Polymerized High Internal Phase Emulsions (PolyHIPEs)

Polymer foams (PFs) are among the most industrially produced polymeric materials, and they are found in applications including aerospace, packaging, textiles, and biomaterials. PFs are predominantly prepared using gas-blowing techniques, but PFs can also be prepared from templating techniques such as polymerized high internal phase emulsions (polyHIPEs). PolyHIPEs have many experimental design variables which control the physical, mechanical, and chemical properties of the resulting PFs. Both rigid and elastic polyHIPEs can be prepared, but while elastomeric polyHIPEs are less commonly reported than hard polyHIPEs, elastomeric polyHIPEs are instrumental in the realization of new materials in applications including flexible separation membranes, energy storage in soft robotics, and 3D-printed soft tissue engineering scaffolds. Furthermore, there are few limitations to the types of polymers and polymerization methods that have been used to prepare elastic polyHIPEs due to the wide range of polymerization conditions that are compatible with the polyHIPE method. In this review, an overview of the chemistry used to prepare elastic polyHIPEs from early reports to modern polymerization methods is provided, focusing on the applications that flexible polyHIPEs are used in. The review consists of four sections organized around polymer classes used in the preparation of polyHIPEs: (meth)acrylics and (meth)acrylamides, silicones, polyesters and polyurethanes, and naturally occurring polymers. Within each section, the common properties, current challenges, and an outlook is suggested on where elastomeric polyHIPEs can be expected to continue to make broad, positive impacts on materials and technology for the future.

Recreating the Trabecular Outflow Tissue on Implantable, Micropatterned, Ultrathin, Porous Polycaprolactone Scaffolds

Glaucoma, where increased intraocular pressure (IOP) leads to damage to the optic nerve and loss of sight, is amongst the foremost causes of irreversible blindness worldwide. In primary open angle glaucoma, the increased IOP is a result of the malfunctioning human trabecular meshwork (HTM) cells' inability to properly regulate the outflow of aqueous humor from the eye. A potential future treatment for glaucoma is to replace damaged HTM cells with a tissue-engineered substitute, thus restoring proper fluid outflow. Polycaprolactone (PCL) is a versatile, biodegradable, and implantable material that is widely used for cell culture and tissue engineering. In this work, PCL scaffolds were lithographically fabricated using a sacrificial process to produce submicron-thick scaffolds with openings of specific sizes and shapes (e.g., grid, hexagonal pattern). The HTM cell growth on gelatin-coated PCL scaffolds was assessed by scanning electron microscopy, tetrazolium metabolic activity assay, and cytoskeletal organization of F-actin. Expression of HTM-specific markers and ECM deposition were assessed by immunocytochemistry and qPCR analysis. Gelatin-coated, micropatterned, ultrathin, porous PCL scaffolds with a grid pattern supported proper HTM cell growth, cytoskeleton organization, HTM-marker expression, and ECM deposition, demonstrating the feasibility of using these PCL scaffolds to tissue-engineer implantable, healthy ocular outflow tissue.

Synthesis of patterned polyHIPE-hydrogel composite materials using thiol-ene chemistry

Elastomeric materials combining multiple properties within a single composite are highly desired in applications including biomaterials interfaces, actuators, and soft robotics. High spatial resolution is required to impart different properties across the composite for the intended application, but many techniques used to prepare these composites rely on multistep and complex methods. There is a need for the development of simple and efficient platforms to design layered composite materials. Here, we report the synthesis of horizontally- and vertically-patterned composites consisting of PDMS-based polymerized high internal phase emulsion (polyHIPE) porous elastomers and PDMS/PEG hydrogels. Composites with defined interfaces that were mechanically robust were prepared, and rheological analysis of the polyHIPE and hydrogel layers showed storage moduli values of ∼ 35 kPa and 45 kPa respectively. The compressive Young's Modulus and maximum strain of the polyHIPEs were dependent on the thiol to ene ratio in the formulation and obtained values ranging from 6 to 25 kPa and 50-65% respectively. The mechanical properties, total porosity of the polyHIPE, and swelling ratio of the hydrogel were unaffected by the patterning technique compared to non-patterned controls. PolyHIPE-hydrogel composite materials having up to 7-different horizontally pattered layers could be prepared that could expand and contract up hydration and drying.

3D Printed Hierarchical Porous Poly(ε-caprolactone) Scaffolds from Pickering High Internal Phase Emulsion Templating

In the realm of biomaterials, particularly bone tissue engineering, there has been a great increase in interest in scaffolds with hierarchical porosity and customizable multifunctionality. Recently, the three-dimensional (3D) printing of biopolymer-based inks (solutions or emulsions) has gained high popularity for fabricating tissue engineering scaffolds, which optimally satisfies the desired properties and performances. Herein, therefore, we explore the fabrication of 3D printed hierarchical porous scaffolds of poly(ε-caprolactone) (PCL) using the water-in-oil (w/o) Pickering PCL high internal phase emulsions (HIPEs) as the ink in 3D printer. The Pickering PCL HIPEs stabilized using hydrophobically modified nanoclay comprised of aqueous poly(vinyl alcohol) (PVA) as the dispersed phase. Rheological measurements suggested the shear thinning behavior of Pickering HIPEs having a dispersed droplet diameter of 3-25 μm. The pore morphology resembling the natural extracellular matrix and the mechanical properties of scaffolds were customized by tuning the emulsion composition and 3D printing parameters. In vitro biomineralization and drug release studies proved the scaffolds' potential in developing the apatite-rich bioactive interphase and controlled drug delivery, respectively. During in vitro osteoblast (MG63) growth experiments for up to 7 days, good adhesion and proliferation on PCL scaffolds confirmed their cytocompatibility, assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) analysis. This study suggests that the assembly of HIPE templates and 3D printing is a promising approach to creating hierarchical porous scaffolds potentially suitable for bone tissue engineering and can be stretched to other biopolymers as well.

Ascorbic Acid Enhances the Metabolic Activity, Growth and Collagen Production of Human Dermal Fibroblasts Growing in Three-dimensional (3D) Culture

Tissue engineering (TE) enables developing functional synthetic substitutes to be replaced with damaged tissues and organs instead of the use of auto or allografts. A wide range of biomaterials is currently in use as TE scaffolds. Among these materials, naturally-sourced ones are favourable due to being highly biocompatible and supporting cell growth and function whereas synthetic ones are advantageous because of the high tunability on mechanical and physical properties as well as being easy to process. Alongside the advantages of synthetic polymers, they mostly show hydrophobic behaviour that limits biomaterial-cell interaction and consequently the functioning of the developed TE constructs. In this study, we assessed the impact of L-Ascorbic acid 2-phosphate (AA2P) on improving the culture of human dermal fibroblasts (HDFs) growing on a three-dimensional (3D) scaffold made of polycaprolactone by emulsion templating technique. Our results demonstrated that AA2P enhances the metabolic activity, growth, and collagen production of HDFs when supplemented to their growth medium at 50 µg/mL concentration. It showed a great potential to be used as a growth medium supplement to circumvent the disadvantages of culturing human cells on a synthetic biomaterial that is not favoured in default. AA2P's potential to improve cell growth and collagen deposition may prove an effective way to culture human cells on 3D PCL PolyHIPE scaffolds for various TE applications.

Tough Photo-Cross-Linked PCL-Hydroxyapatite Composites for Bone Tissue Engineering

Acrylate-based photo-cross-linked poly(ε-caprolactone) (PCL) tends to show low elongation and strength. Incorporation of osteo-inductive hydroxyapatite (HAp) further enhances this effect, which limits its applicability in bone tissue engineering. To overcome this, the thiol-ene click reaction is introduced for the first time in order to photo-cross-link PCL composites with 0, 10, 20, and 30 wt % HAp nanoparticles. It is demonstrated that the elongation at break and ultimate strength increase 10- and 2-fold, respectively, when the photopolymerization mechanism is shifted from a radical chain-growth (i.e., acrylate cross-linking) toward a radical step-growth polymerization (i.e., thiol-ene cross-linking). Additionally, it is illustrated that osteoblasts can attach to and proliferate on the surface of the photo-cross-linked PCL-HAp composites. Finally, the incorporation of HAp nanoparticles is shown to reduce the ALP activity of osteoblasts. Overall, thiol-ene cross-linked PCL-HAp composites can be considered as promising potential materials for bone tissue engineering.

Emulsion templated scaffolds of poly(ε-caprolactone) - a review

The role of poly(ε-caprolactone) (PCL) and its 3D scaffolds in tissue engineering has already been established due to its ease of processing into long-term degradable implants and approval from the FDA. This review presents the role of high internal phase emulsion (HIPE) templating in the fabrication of PCL scaffolds, and the versatility of the technique along with challenges associated with it. Considering the huge potential of HIPE templating, which so far has mainly been focused on free radical polymerization of aqueous HIPEs, we provide a summary of how the technique has been expanded to non-aqueous HIPEs and other modes of polymerization such as ring-opening. The scope of coupling of HIPE templating with some of the advanced fabrication methods such as 3D printing or electrospinning is also explored.