Photo-crosslinkable polyester microneedles as sustained drug release systems toward hypertrophic scar treatment

Burn injuries can result in a significant inflammatory response, often leading to hypertrophic scarring (HTS). Local drug therapies e.g. corticoid injections are advised to treat HTS, although they are invasive, operator-dependent, extremely painful and do not permit extended drug release. Polymer-based microneedle (MN) arrays can offer a viable alternative to standard care, while allowing for direct, painless dermal drug delivery with tailorable drug release profile. In the current study, we synthesized photo-crosslinkable, acrylate-endcapped urethane-based poly(ε-caprolactone) (AUP-PCL) toward the fabrication of MNs. Physico-chemical characterization (1H-NMR, evaluation of swelling, gel fraction) of the developed polymer was performed and confirmed successful acrylation of PCL-diol. Subsequently, AUP-PCL, and commercially available PCL-based microneedle arrays were fabricated for comparative evaluation of the constructs. Hydrocortisone was chosen as model drug. To enhance the drug release efficiency of the MNs, Brij®35, a nonionic surfactant was exploited. The thermal properties of the MNs were evaluated via differential scanning calorimetry. Compression testing of the arrays confirmed that the MNs stay intact upon applying a load of 7 N, which correlates to the standard dermal insertion force of MNs. The drug release profile of the arrays was evaluated, suggesting that the developed PCL arrays can offer efficient drug delivery for up to two days, while the AUP-PCL arrays can provide a release up to three weeks. Finally, the insertion of MN arrays into skin samples was performed, followed by histological analysis demonstrating the AUP-PCL MNs outperforming the PCL arrays upon providing pyramidical-shaped perforations through the epidermal layer of the skin.

The importance of mechanical and biological cues of tympanic membrane grafts to ensure optimal regeneration

Chronic suppurative otitis media (CSOM) is often associated with permanent tympanic membrane (TM) perforation and conductive hearing loss. The current clinical gold standard, using autografts and allografts, suffers from several drawbacks. Artificial replacement materials can help to overcome these drawbacks. Therefore, scaffolds fabricated through digital light processing (DLP) were herein created to support TM regeneration. Various UV-curable printing inks, including gelatin methacryloyl (GelMA), gelatin-norbornene-norbornene (GelNBNB) (crosslinked with thiolated gelatin (GelSH)) and alkene-functionalized poly-ε-caprolactone (E-PCL) (crosslinked with pentaerythritol tetrakis(3-mercaptopropionate) (PETA4SH)) were optimized regarding photo-initiator (PI) and photo-absorber (PA) concentrations through viscosity characterization, photo-rheology and the establishment of working curves for DLP. Our material platform enabled the development of constructs with a range of mechanical properties (plateau storage modulus varying between 15 and 119 kPa). Excellent network connectivity for the GelNBNB and E-PCL constructs was demonstrated (gel fractions >95 %) whereas a post-crosslinking step was required for the GelMA constructs. All samples showed excellent biocompatibility (viability >93 % and metabolic activity >88 %). Finally, in vivo and ex vivo assessments, including histology, vibration and deformation responses measured through laser doppler vibrometry and digital image correlation respectively, were performed to investigate the effects of the scaffolds on the anatomical and physiological regeneration of acute TM perforations in rabbits. The data showed that the most efficient healing with the best functional quality was obtained when both mechanical (obtained with the PCL-based resin) and biological (obtained with the gelatin-based resins) material properties were taken into account.

In vitro and in vivo evaluation of periosteum-derived cells and iPSC-derived chondrocytes encapsulated in GelMA for osteochondral tissue engineering

Osteochondral defects are deep joint surface lesions that affect the articular cartilage and the underlying subchondral bone. In the current study, a tissue engineering approach encompassing individual cells encapsulated in a biocompatible hydrogel is explored in vitro and in vivo. Cell-laden hydrogels containing either human periosteum-derived progenitor cells (PDCs) or human induced pluripotent stem cell (iPSC)-derived chondrocytes encapsulated in gelatin methacryloyl (GelMA) were evaluated for their potential to regenerate the subchondral mineralized bone and the articular cartilage on the joint surface, respectively. PDCs are easily isolated and expanded progenitor cells that are capable of generating mineralized cartilage and bone tissue in vivo via endochondral ossification. iPSC-derived chondrocytes are an unlimited source of stable and highly metabolically active chondrocytes. Cell-laden hydrogel constructs were cultured for up to 28 days in a serum-free chemically defined chondrogenic medium. On day 1 and day 21 of the differentiation period, the cell-laden constructs were implanted subcutaneously in nude mice to evaluate ectopic tissue formation 4 weeks post-implantation. Taken together, the data suggest that iPSC-derived chondrocytes encapsulated in GelMA can generate hyaline cartilage-like tissue constructs with different levels of maturity, while using periosteum-derived cells in the same construct type generates mineralized tissue and cortical bone in vivo. Therefore, the aforementioned cell-laden hydrogels can be an important part of a multi-component strategy for the manufacturing of an osteochondral implant.

Effect of Porosity on the Colonization of Digital Light-Processed 3D Hydrogel Constructs toward the Development of a Functional Intestinal Model

With the rapid increase of the number of patients with gastrointestinal diseases in modern society, the need for the development of physiologically relevant in vitro intestinal models is key to improve the understanding of intestinal dysfunctions. This involves the development of a scaffold material exhibiting physiological stiffness and anatomical mimicry of the intestinal architecture. The current work focuses on evaluating the scaffold micromorphology of gelatin-methacryloyl-aminoethyl-methacrylate-based nonporous and porous intestinal 3D, intestine-like constructs, fabricated via digital light processing, on the cellular response. To this end, Caco-2 intestinal cells were utilized in combination with the constructs. Both porous and nonporous constructs promoted cell growth and differentiation toward enterocyte-like cells (VIL1, ALPI, SI, and OCLD expression showed via qPCR, ZO-1 via immunostaining). The porous constructs outperformed the nonporous ones regarding cell seeding efficiency and growth rate, confirmed by MTS assay, live/dead staining, and TEER measurements, due to the presence of surface roughness.

Bottom-Up Extrusion-Based Biofabrication of the Osteoid Niche

Bone regeneration remains a clinical challenge given the transplantation incidence rate and the associated economic burden. Bottom-up osteoid tissue engineering has the potential to offer an alternative approach to current clinical solutions that suffer from various drawbacks. In this paper, deposition-based bioprinting is exploited while the effect is explored of both the crosslinking mechanism (gelatin methacryloyl (GelMA) versus gelatin norbornene (DS 91) crosslinked with thiolated gelatin (GelNBSH)) and the degree of substitution (GelNBSH versus norbornene-norbornene-modified gelatin (DS 169) crosslinked with thiolated gelatin (GelNBNBSH)) on the presented biophysical cues as well as on the osteogenic differentiation. The incorporation of tris(2-carboxyethyl)phosphine (TCEP) to the step-growth inks allows the production of reproducible and biocompatible scaffolds based on thiol-ene chemistry. Dental pulp stem cell encapsulation in GelNBNBSH biofabricated constructs shows a favorable response due to the combination of its stress relaxation and substrate rigidity (bulk compressive modulus of 11-30 kPa) as reflected by a sevenfold increase in calcium production compared to the tissue engineering standard GelMA. This work is the first to exploit a controlled biocompatible and cell-interactive thiolated macromolecular crosslinker (GelSH + TCEP) allowing the extrusion-based biofabrication of low concentration (5 w/v%) modified osteogenic gelatin-based inks (GelNBNBSH + TCEP).

Design of 3D-Photoprintable, Bio-, and Hemocompatible Nonisocyanate Polyurethane Elastomers for Biomedical Implants

Polyurethanes (PUs) have adjustable mechanical properties, making them suitable for a wide range of applications, including in the biomedical field. Historically, these PUs have been synthesized from isocyanates, which are toxic compounds to handle. This has encouraged the search for safer and more environmentally friendly synthetic routes, leading today to the production of nonisocyanate polyurethanes (NIPUs). Among these NIPUs, polyhydroxyurethanes (PHUs) bear additional hydroxyl groups, which are particularly attractive for derivatizing and adjusting their physicochemical properties. In this paper, polyether-based NIPU elastomers with variable stiffness are designed by functionalizing the hydroxyl groups of a poly(propylene glycol)-PHU by a cyclic carbonate carrying a pendant unsaturation, enabling them to be post-photo-cross-linked with polythiols (thiol-ene). Elastomers with remarkable mechanical properties whose stiffness can be adjusted are obtained. Thanks to the unique viscous properties of these PHU derivatives and their short gel times observed by rheology experiments, formulations for light-based three-dimensional (3D) printing have been developed. Objects were 3D-printed by digital light processing with a resolution down to the micrometer scale, demonstrating their ability to target various designs of prime importance for personalized medicine. In vitro biocompatibility tests have confirmed the noncytotoxicity of these materials for human fibroblasts. In vitro hemocompatibility tests have revealed that they do not induce hemolytic effects, they do not increase platelet adhesion, nor activate coagulation, demonstrating their potential for future applications in the cardiovascular field.

Effect of Poly(Vinyl Pyrrolidone) on Iodine Release from Acrylate-Endcapped Urethane-Based Poly(Ethylene Glycol) Hydrogels as Antibacterial Wound Dressing

Infections are still a major cause of morbidity in burn wounds. Although silver has been used strongly in past centuries as an anti-bacterial, it can lead to allergic reactions, bacterial resistance, and delayed wound healing. Iodine-based antibacterials are becoming an interesting alternative. In this work, the effect of complexation with poly(vinyl pyrrolidone) (PVP) and poly(ethylene oxide) (PEO)-based polymers is explored by using different acrylate-endcapped urethane-based poly(ethylene glycol) (AUP) polymers, varying the molar mass (MM) of the poly(ethylene glycol) (PEG) backbone, with possible addition of PVP. The higher MM AUP outperforms the swelling potential of commercial wound dressings such as Kaltostat, Aquacel Ag, and Hydrosorb and all MM show superior mechanical properties. The addition of iodine to the polymers is compared to Iso-Betadine Tulle (IBT). Interestingly, the addition of PVP does not lead to increased iodine complexation compared to the blank AUP polymers, while all have a prolonged iodine release compared to the IBT, which leads to a burst release. The observed prolonged release also leads to larger inhibition zones during antibacterial tests. Complexing iodine in AUP polymers with or without PVP leads to antimicrobial wound dressings which may hold potential for future application to treat infected wounds.

Powdered Cross-Linked Gelatin Methacryloyl as an Injectable Hydrogel for Adipose Tissue Engineering

The tissue engineering field is currently advancing towards minimally invasive procedures to reconstruct soft tissue defects. In this regard, injectable hydrogels are viewed as excellent scaffold candidates to support and promote the growth of encapsulated cells. Cross-linked gelatin methacryloyl (GelMA) gels have received substantial attention due to their extracellular matrix-mimicking properties. In particular, GelMA microgels were recently identified as interesting scaffold materials since the pores in between the microgel particles allow good cell movement and nutrient diffusion. The current work reports on a novel microgel preparation procedure in which a bulk GelMA hydrogel is ground into powder particles. These particles can be easily transformed into a microgel by swelling them in a suitable solvent. The rheological properties of the microgel are independent of the particle size and remain stable at body temperature, with only a minor reversible reduction in elastic modulus correlated to the unfolding of physical cross-links at elevated temperatures. Salts reduce the elastic modulus of the microgel network due to a deswelling of the particles, in addition to triple helix denaturation. The microgels are suited for clinical use, as proven by their excellent cytocompatibility. The latter is confirmed by the superior proliferation of encapsulated adipose tissue-derived stem cells in the microgel compared to the bulk hydrogel. Moreover, microgels made from the smallest particles are easily injected through a 20G needle, allowing a minimally invasive delivery. Hence, the current work reveals that powdered cross-linked GelMA is an excellent candidate to serve as an injectable hydrogel for adipose tissue engineering.

Gelatin-Based Hybrid Hydrogels as Matrices for Organoid Culture

The application of liver organoids is very promising in the field of liver tissue engineering; however, it is still facing some limitations. One of the current major limitations is the matrix in which they are cultured. The mainly undefined and murine-originated tumor matrices derived from Engelbreth-Holm-Swarm (EHS) sarcoma, such as Matrigel, are still the standard culturing matrices for expansion and differentiation of organoids toward hepatocyte-like cells, which will obstruct its future clinical application potential. In this study, we exploited the use of newly developed highly defined hydrogels as potential matrices for the culture of liver organoids and compared them to Matrigel and two hydrogels that were already researched in the field of organoid research [i.e., polyisocyanopeptides, enriched with laminin-entactin complex (PIC-LEC) and gelatin methacryloyl (GelMA)]. The newly developed hydrogels are materials that have a physicochemical resemblance with native liver tissue. Norbornene-modified dextran cross-linked with thiolated gelatin (DexNB-GelSH) has a swelling ratio and macro- and microscale properties that highly mimic liver tissue. Norbornene-modified chondroitin sulfate cross-linked with thiolated gelatin (CSNB-GelSH) contains chondroitin sulfate, which is a glycosaminoglycan (GAG) that is present in the liver ECM. Furthermore, CSNB-GelSH hydrogels with different mechanical properties were evaluated. Bipotent intrahepatic cholangiocyte organoids (ICOs) were applied in this work and encapsulated in these materials. This research revealed that the newly developed materials outperformed Matrigel, PIC-LEC, and GelMA in the differentiation of ICOs toward hepatocyte-like cells. Furthermore, some trends indicate that an interplay of both the chemical composition and the mechanical properties has an influence on the relative expression of certain hepatocyte markers. Both DexNB-GelSH and CSNB-GelSH showed promising results for the expansion and differentiation of intrahepatic cholangiocyte organoids. The stiffest CSNB-GelSH hydrogel even significantly outperformed Matrigel based on ALB, BSEP, and CYP3A4 gene expression, being three important hepatocyte markers.

3D-Printed Shape Memory Poly(alkylene terephthalate) Scaffolds as Cardiovascular Stents Revealing Enhanced Endothelialization

Cardiovascular diseases are the leading cause of death and current treatments such as stents still suffer from disadvantages. Balloon expansion causes damage to the arterial wall and limited and delayed endothelialization gives rise to restenosis and thrombosis. New more performing materials that circumvent these disadvantages are required to improve the success rate of interventions. To this end, the use of a novel polymer, poly(hexamethylene terephthalate), is investigated for this application. The synthesis to obtain polymers with high molar masses up to 126.5 kg mol-1 is optimized and a thorough chemical and thermal analysis is performed. The polymers are 3D-printed into personalized cardiovascular stents using the state-of-the-art solvent-cast direct-writing technique, the potential of these stents to expand using their shape memory behavior is established, and it is shown that the stents are more resistant to compression than the poly(l-lactide) benchmark. Furthermore, the polymer's hydrolytic stability is demonstrated in an accelerated degradation study of 6 months. Finally, the stents are subjected to an in vitro biological evaluation, revealing that the polymer is non-hemolytic and supports significant endothelialization after only 7 days, demonstrating the enormous potential of these polymers to serve cardiovascular applications.

Optimization of hybrid gelatin-polysaccharide bioinks exploiting thiol-norbornene chemistry using a reducing additive

Thiol-norbornene chemistry offers great potential in the field of hydrogel development, given its step growth crosslinking mechanism. However, limitations exist with regard to deposition-based bioprinting of thiol-containing hydrogels, associated with premature crosslinking of thiolated (bio)polymers resulting from disulfide formation in the presence of oxygen. More specifically, disulfide formation can result in an increase in viscosity thereby impeding the printing process. In the present work, hydrogels constituting norbornene-modified dextran (DexNB) combined with thiolated gelatin (GelSH) are selected as case study to explore the potential of incorporating the reducing agent tris(2-carboxyethyl)phosphine (TCEP), to prevent the formation of disulfides. We observed that, in addition to preventing disulfide formation, TCEP also contributed to premature, spontaneous thiol-norbornene crosslinking without the use of UV light as evidenced via1H-NMR spectroscopy. Herein, an optimal concentration of 25 mol% TCEP with respect to the amount of thiols was found, thereby limiting auto-gelation by both minimizing disulfide formation and spontaneous thiol-norbornene reaction. This concentration results in a constant viscosity during at least 24 h, a more homogeneous network being formed as evidenced using atomic force microscopy while retaining bioink biocompatibility as evidenced by a cell viability of human foreskin fibroblasts exceeding 70% according to ISO 10993-6:2016.

2D fibrillar osteoid niche mimicry through inclusion of visco-elastic and topographical cues in gelatin-based networks

Given the clinical need for osteoregenerative materials incorporating controlled biomimetic and biophysical cues, a novel highly-substituted norbornene-modified gelatin was developed enabling thiol-ene crosslinking exploiting thiolated gelatin as cell-interactive crosslinker. Comparing the number of physical crosslinks, the degree of hydrolytic degradation upon modification, the network density and the chemical crosslinking type, the osteogenic effect of visco-elastic and topographical properties was evaluated. This novel network outperformed conventional gelatin-based networks in terms of osteogenesis induction, as evidenced in 2D dental pulp stem cell seeding assays, resulting from the presentation of both a local (substrate elasticity, 25-40 kPa) and a bulk (compressive modulus, 25-45 kPa) osteogenic substrate modulus in combination with adequate fibrillar cell adhesion spacing to optimally transfer traction forces from the fibrillar ECM (as evidenced by mesh size determination with the rubber elasticity theory) and resulting in a 1.7-fold increase in calcium production (compared to the gold standard gelatin methacryloyl (GelMA)).

Polymeric reinforcements for cellularized collagen-based vascular wall models: influence of the scaffold architecture on the mechanical and biological properties

A previously developed cellularized collagen-based vascular wall model showed promising results in mimicking the biological properties of a native vessel but lacked appropriate mechanical properties. In this work, we aim to improve this collagen-based model by reinforcing it using a tubular polymeric (reinforcement) scaffold. The polymeric reinforcements were fabricated exploiting commercial poly (ε-caprolactone) (PCL), a polymer already used to fabricate other FDA-approved and commercially available devices serving medical applications, through 1) solution electrospinning (SES), 2) 3D printing (3DP) and 3) melt electrowriting (MEW). The non-reinforced cellularized collagen-based model was used as a reference (COL). The effect of the scaffold's architecture on the resulting mechanical and biological properties of the reinforced collagen-based model were evaluated. SEM imaging showed the differences in scaffolds' architecture (fiber alignment, fiber diameter and pore size) at both the micro- and the macrolevel. The polymeric scaffold led to significantly improved mechanical properties for the reinforced collagen-based model (initial elastic moduli of 382.05 ± 132.01 kPa, 100.59 ± 31.15 kPa and 245.78 ± 33.54 kPa, respectively for SES, 3DP and MEW at day 7 of maturation) compared to the non-reinforced collagen-based model (16.63 ± 5.69 kPa). Moreover, on day 7, the developed collagen gels showed stresses (for strains between 20% and 55%) in the range of [5-15] kPa for COL, [80-350] kPa for SES, [20-70] kPa for 3DP and [100-190] kPa for MEW. In addition to the effect on the resulting mechanical properties, the polymeric tubes' architecture influenced cell behavior, in terms of proliferation and attachment, along with collagen gel compaction and extracellular matrix protein expression. The MEW reinforcement resulted in a collagen gel compaction similar to the COL reference, whereas 3DP and SES led to thinner and longer collagen gels. Overall, it can be concluded that 1) the selected processing technique influences the scaffolds' architecture, which in turn influences the resulting mechanical and biological properties, and 2) the incorporation of a polymeric reinforcement leads to mechanical properties closely matching those of native arteries.

Effect of molar mass and alkyl chain length on the surface properties and biocompatibility of poly(alkylene terephthalate)s for potential cardiovascular applications

Cardiovascular diseases are the leading cause of death worldwide. Treatments for occluded arteries include balloon angioplasty with or without stenting and bypass grafting surgery. Poly(ethylene terephthalate) is frequently used as a vascular graft material, but its high stiffness leads to compliance mismatch with the human blood vessels, resulting in altered hemodynamics, thrombus formation and graft failure. Poly(alkylene terephthalate)s (PATs) with longer alkyl chain lengths hold great potential for improving the compliance. In this work, the effect of the polymer molar mass and the alkyl chain length on the surface roughness and wettability of spin-coated PAT films was investigated, as well as the endothelial cell adhesion and proliferation on these samples. We found that surface roughness generally increases with increasing molar mass and alkyl chain length, while no trend for the wettability could be observed. All investigated PATs are non-cytotoxic and support endothelial cell adhesion and growth. For some PATs, the endothelial cells even reorganized into a tubular-like structure, suggesting angiogenic maturation. In conclusion, this research demonstrates the biocompatibility of PATs and their potential to be applied as materials serving cardiovascular applications.

A preclinical platform for assessing long-term drug efficacy exploiting mechanically tunable scaffolds colonized by a three-dimensional tumor microenvironment

BACKGROUND

Long-term drug evaluation heavily relies upon rodent models. Drug discovery methods to reduce animal models in oncology may include three-dimensional (3D) cellular systems that take into account tumor microenvironment (TME) cell types and biomechanical properties.

METHODS

In this study we reconstructed a 3D tumor using an elastic polymer (acrylate-endcapped urethane-based poly(ethylene glycol) (AUPPEG)) with clinical relevant stiffness. Single cell suspensions from low-grade serous ovarian cancer (LGSOC) patient-derived early passage cultures of cancer cells and cancer-associated fibroblasts (CAF) embedded in a collagen gel were introduced to the AUPPEG scaffold. After self-organization in to a 3D tumor, this model was evaluated by a long-term (> 40 days) exposure to a drug combination of MEK and HSP90 inhibitors. The drug-response results from this long-term in vitro model are compared with drug responses in an orthotopic LGSOC xenograft mouse model.

RESULTS

The in vitro 3D scaffold LGSOC model mimics the growth ratio and spatial organization of the LGSOC. The AUPPEG scaffold approach allows to test new targeted treatments and monitor long-term drug responses. The results correlate with those of the orthotopic LGSOC xenograft mouse model.

CONCLUSIONS

The mechanically-tunable scaffolds colonized by a three-dimensional LGSOC allow long-term drug evaluation and can be considered as a valid alternative to reduce, replace and refine animal models in drug discovery.

Gelatin-Based Hybrid Hydrogel Scaffolds: Toward Physicochemical Liver Mimicry

There exists a clear need to develop novel materials that could serve liver tissue engineering purposes. Those materials need to be researched for the development of bioengineered liver tissue as an alternative to donor livers, as well as for materials that could be applied for scaffolds to develop an in vitro model for drug-induced liver injury (DILI) detection . In this paper, the hydrogels oxidized dextran-gelatin (Dexox-Gel) and norbornene-modified dextran-thiolated gelatin (DexNB-GelSH) were developed, and their feasibility toward processing via indirect 3D-printing was investigated with the aim to develop hydrogel scaffolds that physicochemically mimic native liver tissue. Furthermore, their in vitro biocompatibility was assessed using preliminary biological tests using HepG2 cells. Both materials were thoroughly physicochemically characterized and benchmarked to the methacrylated gelatin (GelMA) reference material. Due to inferior properties, Dexox-gel was not further processed into 3D-hydrogel scaffolds. This research revealed that DexNB-GelSH exhibited physicochemical properties that were in excellent agreement with the properties of natural liver tissue in contrast to GelMA. In combination with an equally good biological evaluation of DexNB-GelSH in comparison with GelMA based on an MTS proliferation assay and an albumin quantification assay, DexNB-GelSH can be considered promising in the field of liver tissue engineering.

Light-Based 3D Printing of Gelatin-Based Biomaterial Inks to Create a Physiologically Relevant In Vitro Fish Intestinal Model

To provide prominent accessibility of fishmeal to the European population, the currently available, time- and cost-extensive feeding trials, which evaluate fish feed, should be replaced. The current paper reports on the development of a novel 3D culture platform, mimicking the microenvironment of the intestinal mucosa in vitro. The key requirements of the model include sufficient permeability for nutrients and medium-size marker molecules (equilibrium within 24 h), suitable mechanical properties (G' < 10 kPa), and close morphological similarity to the intestinal architecture. To enable processability with light-based 3D printing, a gelatin-methacryloyl-aminoethyl-methacrylate-based biomaterial ink is developed and combined with Tween 20 as porogen to ensure sufficient permeability. To assess the permeability properties of the hydrogels, a static diffusion setup is utilized, indicating that the hydrogel constructs are permeable for a medium size marker molecule (FITC-dextran 4 kg mol-1 ). Moreover, the mechanical evaluation through rheology evidence a physiologically relevant scaffold stiffness (G' = 4.83 ± 0.78 kPa). Digital light processing-based 3D printing of porogen-containing hydrogels results in the creation of constructs exhibiting a physiologically relevant microarchitecture as evidenced through cryo-scanning electron microscopy. Finally, the combination of the scaffolds with a novel rainbow trout (Oncorhynchus mykiss) intestinal epithelial cell line (RTdi-MI) evidence scaffold biocompatibility.

Cell Guiding Multicomponent Nanoyarn Tendon Scaffolds with Tunable Morphology and Flexibility

Nanofibrous scaffolds are widely investigated for tendon tissue engineering due to their porous structure, high flexibility, and the ability to guide cells in a preferred direction. Previous research has shown that providing a microenvironment similar to in vivo settings improves tissue regeneration. Therefore, in this work, ingenious multicomponent nanoyarn scaffolds that mimic the fibrillar and tubular structures of tendons are developed for the first time through electrospinning and bundling nanoyarns followed by electrospinning of a nanofibrous shell around the bundle. Multicomponent nanoyarn scaffolds out of poly(ε-caprolactone) with varying porosity, density, and diameter were successfully produced by coelectrospinning with water-soluble poly(2-ethyl-2-oxazoline) as a sacrificial component. The diameter and fiber orientation of the nanoyarns were successfully tuned based on parameter-morphology models obtained by the design of experiments. Cyclic bending tests were performed, indicating that the flexibility of the multicomponent nanoyarn scaffolds depends on the morphology and can be tuned through controlling the number of nanoyarns in the bundle and the porosity. Indirect and direct cell culture tests using mouse and equine tendon cells revealed excellent cytocompatibility of the nanofibrous products and demonstrated the potential of the nanoyarns to guide the growing cells along the nanofiber direction, which is crucial for tendon tissue engineering.

Ionically Modified Gelatin Hydrogels Maintain Murine Myogenic Cell Viability and Fusion Capacity

For tissue engineering of skeletal muscles, there is a need for biomaterials which do not only allow cell attachment, proliferation, and differentiation, but also support the physiological conditions of the tissue. Next to the chemical nature and structure of the biomaterial, its response to the application of biophysical stimuli, such as mechanical deformation or application of electrical pulses, can impact in vitro tissue culture. In this study, gelatin methacryloyl (GelMA) is modified with hydrophilic 2-acryloxyethyltrimethylammonium chloride (AETA) and 3-sulfopropyl acrylate potassium (SPA) ionic comonomers to obtain a piezoionic hydrogel. Rheology, mass swelling, gel fraction, and mechanical characteristics are determined. The piezoionic properties of the SPA and AETA-modified GelMA are confirmed by a significant increase in ionic conductivity and an electrical response as a function of mechanical stress. Murine myoblasts display a viability of >95% after 1 week on the piezoionic hydrogels, confirming their biocompatibility. The GelMA modifications do not influence the fusion capacity of the seeded myoblasts or myotube width after myotube formation. These results describe a novel functionalization providing new possibilities to exploit piezo-effects in the tissue engineering field.

Vascularized adipose tissue engineering: moving towards soft tissue reconstruction

Soft tissue defects are a common clinical challenge mostly caused by trauma, congenital anomalies and oncological surgery. Current soft tissue reconstruction (STR) options include synthetic materials (fillers and implants) and autologous adipose tissue transplantation through flap surgery and/or lipotransfer. Both reconstructive options hold important disadvantages to which vascularized adipose tissue engineering (VATE) strategies could offer solutions. In this review, we first summarized pivotal characteristics of functional adipose tissue (FAT) such as the structure, function, cell types, development and extracellular matrix (ECM). Next, we discussed relevant cell sources and how they are applied in different state-of-the-art VATE techniques. Herein, biomaterial scaffolds and hydrogels, ECMs, spheroids, organoids, cell sheets, 3D bioprinting and microfluidics are overviewed. Also, we included extracellular vesicles and emphasized their potential role in VATE. Lastly, current challenges and future perspectives in VATE are pointed out to help to pave the road towards clinical applications.

Volumetric Printing of Thiol-Ene Photo-Cross-Linkable Poly(ε-caprolactone): A Tunable Material Platform Serving Biomedical Applications

Current thoroughly described biodegradable and cross-linkable polymers mainly rely on acrylate cross-linking. However, despite the swift cross-linking kinetics of acrylates, the concomitant brittleness of the resulting materials limits their applicability. Here, photo-cross-linkable poly(ε-caprolactone) networks through orthogonal thiol-ene chemistry are introduced. The step-growth polymerized networks are tunable, predictable by means of the rubber elasticity theory and it is shown that their mechanical properties are significantly improved over their acrylate cross-linked counterparts. Tunability is introduced to the materials, by altering M_c (or the molar mass between cross-links), and its effect on the thermal properties, mechanical strength and degradability of the materials is evaluated. Moreover, excellent volumetric printability is illustrated and the smallest features obtained via volumetric 3D-printing to date are reported, for thiol-ene systems. Finally, by means of in vitro and in vivo characterization of 3D-printed constructs, it is illustrated that the volumetrically 3D-printed materials are biocompatible. This combination of mechanical stability, tunability, biocompatibility, and rapid fabrication by volumetric 3D-printing charts a new path toward bedside manufacturing of biodegradable patient-specific implants.

Surface Functionalization of 4D Printed Substrates Using Polymeric and Metallic Wrinkles

Wrinkle topographies have been studied as simple, versatile, and in some cases biomimetic surface functionalization strategies. To fabricate surface wrinkles, one material phenomenon employed is the mechanical-instability-driven wrinkling of thin films, which occurs when a deforming substrate produces sufficient compressive strain to buckle a surface thin film. Although thin-film wrinkling has been studied on shape-changing functional materials, including shape-memory polymers (SMPs), work to date has been primarily limited to simple geometries, such as flat, uniaxially-contracting substrates. Thus, there is a need for a strategy that would allow deformation of complex substrates or 3D parts to generate wrinkles on surfaces throughout that complex substrate or part. Here, 4D printing of SMPs is combined with polymeric and metallic thin films to develop and study an approach for fiber-level topographic functionalization suitable for use in printing of arbitrarily complex shape-changing substrates or parts. The effect of nozzle temperature, substrate architecture, and film thickness on wrinkles has been characterized, as well as wrinkle topography on nuclear alignment using scanning electron microscopy, atomic force microscopy, and fluorescent imaging. As nozzle temperature increased, wrinkle wavelength increased while strain trapping and nuclear alignment decreased. Moreover, with increasing film thickness, the wavelength increased as well.

Development of a Microfluidic Chip Powered by EWOD for In Vitro Manipulation of Bovine Embryos

Digital microfluidics (DMF) holds great potential for the alleviation of laboratory procedures in assisted reproductive technologies (ARTs). The electrowetting on dielectric (EWOD) technology provides dynamic culture conditions in vitro that may better mimic the natural embryo microenvironment. Thus far, EWOD microdevices have been proposed for in vitro gamete and embryo handling in mice and for analyzing the human embryo secretome. This article presents the development of the first microfluidic chip utilizing EWOD technology designed for the manipulation of bovine embryos in vitro. The prototype sustains the cell cycles of embryos manipulated individually on the chips during in vitro culture (IVC). Challenges related to the chip fabrication as well as to its application during bovine embryo IVC in accordance with the adapted on-chip protocol are thoroughly discussed, and future directions for DMF in ARTs are indicated.

Nature-Inspired Dual Purpose Strategy toward Cell-Adhesive PCL Networks: C(-linker-)RGD Incorporation via Thiol-ene Crosslinking

In an attempt to mimic nature's ability to adhere cells, PCL is often coated with nature-derived polymers or its surface is functionalized with a cell-binding motif. However, said surface modifications are limited to the material's surface, include multiple steps, and are mediated by harsh conditions. Here, we introduce a single-step strategy toward cell-adhesive polymer networks where thiol-ene chemistry serves a dual purpose. First, alkene-functionalized PCL is crosslinked by means of a multifunctional thiol. Second, by means of a cysteine coupling site, the cell-binding motif C(-linker-)RGD is covalently bound throughout the PCL networks during crosslinking. Moreover, the influence of various linkers (type and length), between the cysteine coupling site and the cell-binding motif RGD, is investigated and the functionalization is assessed by means of static contact angle measurements and X-ray photoelectron spectroscopy. Finally, successful introduction of cell adhesiveness is illustrated for the networks by seeding fibroblasts onto the functionalized PCL networks.

Fine-Tuning the Endcap Chemistry of Acrylated Poly(Ethylene Glycol)-Based Hydrogels for Efficient Burn Wound Exudate Management

Most commercial dressings with moderate to high exudate uptake capacities are mechanically weaker and/or require a secondary dressing. The current research article focuses on the development of hydrogel-based wound dressings combining mechanical strength with high exudate absorption capacities using acrylate-endcapped urethane-based precursors (AUPs). AUPs with varying poly(ethylene glycol) backbone molar masses (10 and 20 kg mol^-1 ) and endcap chemistries are successfully synthesized in toluene, subsequently processed into UV-cured hydrogel sheets and are benchmarked against several commercial wound dressings (Hydrosorb, Kaltostat, and Mepilex Ag). The AUP materials show high gel fractions (>90%) together with strong swelling degrees in water, phosphate buffered saline and simulated wound fluid (12.7-19.6 g g^-1 ), as well as tunable mechanical properties (e.g., Young's modulus: 0.026-0.061 MPa). The AUPs have significantly (p < 0.05) higher swelling degrees than the tested commercial dressings, while also being mechanically resistant. The elasticity of the synthesized materials leads to an increased resistance against fatigue. The di- and hexa-acrylated AUPs show excellent in vitro biocompatibility against human foreskin fibroblasts, as evidenced by indirect MTS assays and live/dead cell assays. In conclusion, the processed AUP materials demonstrate high potential for wound healing application and can even compete with commercially available dressings.

The native liver as inspiration to create superior in vitro hepatic models

Drug induced liver injury (DILI) is one of the major reasons of drug withdrawal during the different phases of drug development. The later in the drug development a drug is discovered to be toxic, the higher the economical as well as the ethical impact will be. In vitro models for early detection of drug liver toxicity are under constant development, however to date a superior model of the liver is still lacking. Ideally, a highly reliable model should be established to maintain the different hepatic cell functionalities to the greatest extent possible, during a period of time long enough to allow for tracking of the toxicity of compounds. In the case of DILI, toxicity can appear even after months of exposure. To reach this goal, an in vitro model should be developed that mimics the in vivo liver environment, function and response to external stimuli. The different approaches for the development of liver models currently used in the field of tissue engineering will be described in this review. Combining different technologies, leading to optimal materials, cells and 3D-constructs will ultimately lead to an ideal superior model that fully recapitulates the liver.

Solid-State Crosslinkable, Shape-Memory Polyesters Serving Tissue Engineering

Acrylate-endcapped urethane-based precursors constituting a poly(D,L-lactide)/poly(ε-caprolactone) (PDLLA/PCL) random copolymer backbone are synthesized with linear and star-shaped architectures and various molar masses. It is shown that the glass transition and thus the actuation temperature could be tuned by varying the monomer content (0-8 wt% ε-caprolactone, T_{g,crosslinked} = 10-42 °C) in the polymers. The resulting polymers are analyzed for their physico-chemical properties and viscoelastic behavior (G'_max = 9.6-750 kPa). The obtained polymers are subsequently crosslinked and their shape-memory properties are found to be excellent (R_r = 88-100%, R_f = 78-99.5%). Moreover, their potential toward processing via various additive manufacturing techniques (digital light processing, two-photon polymerization and direct powder extrusion) is evidenced with retention of their shape-memory effect. Additionally, all polymers are found to be biocompatible in direct contact in vitro cell assays using primary human foreskin fibroblasts (HFFs) through MTS assay (up to ≈100% metabolic activity relative to TCP) and live/dead staining (>70% viability).

Treatment of Hypertrophic Scars with Corticoid-Embedded Dissolving Microneedles

Hypertrophic scarring (HTS) is frequently observed after deep dermal and full-thickness skin defects. Local drug delivery in HTS has been shown more effective compared to other (minimally) invasive treatments. Disadvantages being operator-dependency and non-uniform drug distribution. Moreover, injections are painful and difficult when confronted with extensive scars or HTS in children. Corticoid-embedded dissolving microneedles (CEDMN) were developed that provide painless skin penetration and direct dermal drug delivery. Hyaluronic acid-based DMN and CEDMN patches were utilized. Structural analysis was performed via nuclear magnetic resonance (NMR) spectroscopy while gel permeation chromatography (GPC) was applied to determine chain length (molar mass) and dispersity of hyaluronic acid. Mechanical properties were evaluated by compression testing. Five burn victims with HTS were included. For each individual, three comparable scars were chosen. One control scar was left untreated. Two scars were treated with either 600 or 800 µm CEDMN patches. Patients were treated monthly for 4 months. Treatment with 800 µm CEDMN was initiated after 8 weeks. Assessor-blinded POSAS was registered. Hydration, evaporation, color and elasticity were recorded. The physico-chemical characterization suggests that the mechanical properties enable skin penetration and adequate drug delivery. Patients experienced the therapy as painless. According to the POSAS, all scars improved over time. However, the scars that were treated with CEDMN patches improved faster and with increased increment. The 800 µm CEDMN ensured the fastest POSAS-decrease. Hyaluronic acid-based CEDMN patches are valuable alternatives to intracicatrical injections, as they offer a painless and effective method for administering corticosteroids in HTS.

Pharmacokinetics of Intramuscularly Administered Thermoresponsive Polymers

Aqueous solutions of some polymers exhibit a lower critical solution temperature (LCST); that is, they form phase-separated aggregates when heated above a threshold temperature. Such polymers found many promising (bio)medical applications, including in situ thermogelling with controlled drug release, polymer-supported radiotherapy (brachytherapy), immunotherapy, and wound dressing, among others. Yet, despite the extensive research on medicinal applications of thermoresponsive polymers, their biodistribution and fate after administration remained unknown. Thus, herein, they studied the pharmacokinetics of four different thermoresponsive polyacrylamides after intramuscular administration in mice. In vivo, these thermoresponsive polymers formed depots that subsequently dissolved with a two-phase kinetics (depot maturation, slow redissolution) with half-lives 2 weeks to 5 months, as depot vitrification prolonged their half-lives. Additionally, the decrease of T_CP of a polymer solution increased the density of the intramuscular depot. Moreover, they detected secondary polymer depots in the kidneys and liver; these secondary depots also followed two-phase kinetics (depot maturation and slow dissolution), with half-lives 8 to 38 days (kidneys) and 15 to 22 days (liver). Overall, these findings may be used to tailor the properties of thermoresponsive polymers to meet the demands of their medicinal applications. Their methods may become a benchmark for future studies of polymer biodistribution.

In Vitro and In Vivo Evaluation of 3D Constructs Engineered with Human iPSC-Derived Chondrocytes in Gelatin-Methacryloyl Hydrogel

Articular cartilage defects have limited healing potential and, when left untreated, can lead to osteoarthritis. Tissue engineering focuses on regenerating the damaged joint surface, preferably in an early stage. Here we investigate the regenerative potential of 3D constructs consisting of human iPSC-derived chondrocytes in gelatin-methacryloyl (GelMA) hydrogel for stable hyaline cartilage production. iPSC-derived chondrocytes are encapsulated in GelMA hydrogel at low (1x10⁷ mL^-1 ) and high (2x10⁷ mL^-1 ) density. In conventional medium, GelMA hydrogel supports the chondrocyte phenotype, as opposed to cells cultured in 3D in absence of hydrogel. Moreover, encapsulated iPSC-derived chondrocytes preserve their in vivo matrix formation capacity after 21 days in vitro. In differentiation medium, hyaline cartilage-like tissue forms after 21 days, demonstrated by highly sulfated glycosaminoglycans and collagen type II. Matrix deposition is delayed at low encapsulation density, corroborating with lower transcript levels of COL2A1. An ectopic assay in nude mice demonstrates further maturation of the matrix deposited in vitro. Direct ectopic implantation of iPSC-derived chondrocyte-laden GelMA, without in vitro priming, also generates hyaline cartilage-like tissue, albeit less mature. Since it is unclear what maturity upon implantation is desired for joint surface regeneration, this is an attractive technology to generate immature and more mature hyaline cartilage-like tissue. This article is protected by copyright. All rights reserved.

Commercial wound dressings for the treatment of exuding wounds: an in-depth physico-chemical comparative study

Background

Nowadays, a wide range of wound dressings is already commercially available. The selection of the dressing is of paramount importance as inappropriate wound management and dressing selection can delay the wound healing process. Not only can this be distressing for the patient, but it can also contribute to complications such as maceration and subsequent infection. Many researchers are targeting the design of dressings with superior properties over existing commercial dressings. However, reported results in the state-of-the-art are rarely benchmarked against commercial dressings. The aim of this study was to determine several characteristics of a large variety of the most frequently used commercial wound dressings, providing an overview for both practitioners and researchers.

Methods

For this comparative study, 11 frequently used commercial wound dressings were selected, representing the different types. The morphology was studied using scanning electron microscopy. The dressings were characterized in terms of swelling capacity (water, phosphate buffered saline and simulated wound fluid), moisture vapour transmission rate (MVTR) and moisture uptake capacity (via dynamic vapour sorption) as well as mechanical properties using tensile testing and texturometry.

Results

The selected dressings showed distinctive morphological differences (fibrous, porous and/or gel) which was reflected in the different properties. Indeed, the swelling capacities ranged between 1.5 and 23.2 g/g (water), 2.1 and 17.6 g/g (phosphate buffered saline) or 2.9 and 20.8 g/g (simulated wound fluid). The swelling capacity of the dressings in water increased even further upon freeze-drying, due to the formation of pores. The MVTR values varied between 40 and 930 g/m²/24 h. The maximal moisture uptake capacity varied between 5.8% and 105.7% at 95% relative humidity. Some commercial dressings exhibited a superior mechanical strength, due to either being hydrophobic or multi-layered.

Conclusions

The present work not only offers insight into a valuable toolbox of suitable wound dressing characterization techniques, but also provides an extensive landscaping of commercial dressings along with their physico-chemical properties, obtained through reproducible experimental protocols. Furthermore, it ensures appropriate benchmark values for commercial dressings in all forthcoming studies and could aid researchers with the development of novel modern wound dressings. The tested dressings either exhibited a high strength or a high swelling capacity, suggesting that there is still a strong potential in the wound dressings market for dressings that possess both.

Enhancing Myoblast Fusion and Myotube Diameter in Human 3D Skeletal Muscle Constructs by Electromagnetic Stimulation

Skeletal muscle tissue engineering (SMTE) aims at the in vitro generation of 3D skeletal muscle engineered constructs which mimic the native muscle structure and function. Although native skeletal muscle is a highly dynamic tissue, most research approaches still focus on static cell culture methods, while research on stimulation protocols indicates a positive effect, especially on myogenesis. A more mature muscle construct may be needed especially for the potential applications for regenerative medicine purposes, disease or drug disposition models. Most efforts towards dynamic cell or tissue culture methods have been geared towards mechanical or electrical stimulation or a combination of those. In the context of dynamic methods, pulsed electromagnetic field (PEMF) stimulation has been extensively used in bone tissue engineering, but the impact of PEMF on skeletal muscle development is poorly explored. Here, we evaluated the effects of PEMF stimulation on human skeletal muscle cells both in 2D and 3D experiments. First, PEMF was applied on 2D cultures of human myoblasts during differentiation. In 2D, enhanced myogenesis was observed, as evidenced by an increased myotube diameter and fusion index. Second, 2D results were translated towards 3D bioartificial muscles (BAMs). BAMs were subjected to PEMF for varying exposure times, where a 2-h daily stimulation was found to be effective in enhancing 3D myotube formation. Third, applying this protocol for the entire 16-days culture period was compared to a stimulation starting at day 8, once the myotubes were formed. The latter was found to result in significantly higher myotube diameter, fusion index, and increased myosin heavy chain 1 expression. This work shows the potential of electromagnetic stimulation for enhancing myotube formation both in 2D and 3D, warranting its further consideration in dynamic culturing techniques.

Guiding cell migration in 3D with high-resolution photografting

Multi-photon lithography (MPL) has proven to be a suitable tool to precisely control the microenvironment of cells in terms of the biochemical and biophysical properties of the hydrogel matrix. In this work, we present a novel method, based on multi-photon photografting of 4,4′-diazido-2,2′-stilbenedisulfonic acid (DSSA), and its capabilities to induce cell alignment, directional cell migration and endothelial sprouting in a gelatin-based hydrogel matrix. DSSA-photografting allows for the fabrication of complex patterns at a high-resolution and is a biocompatible, universally applicable and straightforward process that is comparably fast. We have demonstrated the preferential orientation of human adipose-derived stem cells (hASCs) in response to a photografted pattern. Co-culture spheroids of hASCs and human umbilical vein endothelial cells (HUVECs) have been utilized to study the directional migration of hASCs into the modified regions. Subsequently, we have highlighted the dependence of endothelial sprouting on the presence of hASCs and demonstrated the potential of photografting to control the direction of the sprouts. MPL-induced DSSA-photografting has been established as a promising method to selectively alter the microenvironment of cells.

Electrically-driven handling of gametes and embryos: taking a step towards the future of ARTs

Electrical stimulation of gametes and embryos and on-chip manipulation of microdroplets of culture medium serve as promising tools for assisted reproductive technologies (ARTs). Thus far, dielectrophoresis (DEP), electrorotation (ER) and electrowetting on dielectric (EWOD) proved compatible with most laboratory procedures offered by ARTs. Positioning, entrapment and selection of reproductive cells can be achieved with DEP and ER, while EWOD provides the dynamic microenvironment of a developing embryo to better mimic the functions of the oviduct. Furthermore, these techniques are applicable for the assessment of the developmental competence of a mammalian embryo in vitro. Such research paves the way towards the amelioration and full automation of the assisted reproduction methods. This article aims to provide a summary on the recent developments regarding electrically stimulated lab-on-chip devices and their application for the manipulation of gametes and embryos in vitro.

Tissue engineered scaffolds for corneal endothelial regeneration: a material's perspective

Currently, the treatment of corneal diseases caused by damage to the corneal endothelium requires a donor cornea. Because of their limited availability (1 donor cornea for 70 patients in need), researchers are investigating alternative approaches that are independent of donor tissue. One of them includes the development of a tissue engineered scaffold onto which corneal endothelial cells are seeded. In order to function as a suitable substrate, some of its essential properties including thickness, permeability, transparency and mechanical strength should meet certain demands. Additionally, the membrane should be biocompatible and allow the formation of a functional endothelium on the surface. Many materials have already been investigated in this regard including natural, semi-synthetic and synthetic polymers. In the current review, we present an overview of their characteristics and provide a critical view on the methods exploited for material characterization. Next, also the suitability of scaffolds to serve their purpose is discussed along with an overview of natural tissues (e.g. amniotic membrane and lens capsule) previously investigated for this application. Eventually, we propose a consistent approach to be exploited ideally for membrane characterization in future research. This will allow a scientifically sound comparison of materials and membranes investigated by different research groups, hence benefitting research towards the creation of a suitable/optimal tissue engineered endothelial graft.

Flexor tendon repair using a reinforced tubular, medicated electrospun construct

A reinforced tubular, medicated electrospun construct was developed for deep flexor tendon repair. This construct combines mechanical strength with the release of anti-inflammatory and anti-adhesion drugs. In this study, the reinforced construct was evaluated using a rabbit model. It was compared to its components (a tubular, medicated electrospun polymer without reinforcement and a tubular braid as such) on the one hand to a modified Kessler suture as a control group. Forty New Zealand rabbits were randomly divided into two groups. Surgery was performed in the second and fourth deep flexor tendons of one hind paw of the rabbits in the two groups using four repair techniques. Biomechanical tensile testing and macroscopic and histological evaluations were performed at 3 and 8 weeks postoperatively. A two-way analysis of variance with pairwise comparisons revealed that the three experimental surgical techniques (a reinforced tubular medicated electrospun construct, tubular-medicated construct, and tubular braid as such) showed similar strength as that of a modified Kessler suture repair, which was characterized by a mean load at ultimate failure of 19.85 N (standard deviation [SD] 5.29 N) at 3 weeks and 18.15 N (SD 8.01 N) at 8 weeks. Macroscopically, a significantly different adhesion pattern was observed at the suture knots, either centrally or peripherally, depending on the technique. Histologically, a qualitative assessment showed good to excellent repair at the tendon repair site, irrespective of the applied technique. This study demonstrates that mechanical and biological repair strategies for flexor tendon repair can be successfully combined.

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.

Design, preparation and in vitro characterization of biomimetic and bioactive chitosan/polyethylene oxide based nanofibers as wound dressings

Chitosan-based nanofibers (CS-NFs) are excellent artificial extracellular matrices (ECMs) due to the resemblance of CS with the glycosaminoglycans of the natural ECMs. Despite this excellent feature, the poor electrospinnability and mechanical properties of CS are responsible for important limitations in respect to its biomedical applications. To improve the CS's physico-chemical properties, new bioactive and biomimetic CS-NFs were formulated with polyethylene oxide (PEO), having incorporated different active components (ACs) with important beneficial effects for healing. Manuka honey (trophic and antimicrobial effects), propolis (antimicrobial effects), Calendula officinalis infusion (antioxidant effect, reepithelialization stimulating agent), insulin (trophic effect), and L-arginine (angiogenic effect) were selected as ACs. SEM morphology analysis revealed well-alignment, unidirectional arrays, with small diameters, no beads, and smooth surfaces for developed CS_PEO-ACs NFs. The developed NFs showed good biodegradability (NFs mats lost up to 60% of their initial weight in PBS), increased hemocompatibility (hemolytic index less than 4%), and a reduced cytotoxicity degree (cell viability degree more than 90%). In addition, significant antioxidant and antimicrobial effects were noted for the developed NFs which make them suitable for chronic wounds, due to the role of oxidative stress and infection risk in delaying normal wound healing. The most suitable for wound healing applications seems to be CS_PEO@P_C which showed an improved hemolysis index (2.92 ± 0.16%), is non-toxic (cell viability degree more than 97%), and has also significant radical scavenging effect (DPPH inhibition more than 65%). In addition, CS_PEO@P_C presents increased antimicrobial effects, more noticeably for Staphylococcus aureus strain, which is a key feature in preventing wound infection and delaying the healing process. It can be concluded that the developed CS/PEO-ACs NFs are very promising biomaterials for wound care, especially CS_PEO@P_C.

Gelatin methacryloyl as environment for chondrocytes and cell delivery to superficial cartilage defects

Cartilage damage typically starts at its surface, either due to wear or trauma. Treatment of these superficial defects is important in preventing degradation and osteoarthritis. Biomaterials currently used for deep cartilage defects lack appropriate properties for this application. Therefore, we investigated photo‐crosslinked gelatin methacryloyl (gelMA) as a candidate for treatment of surface defects. It allows for liquid application, filling of surface defects and forming a protective layer after UV‐crosslinking, thereby keeping therapeutic cells in place. gelMA and photo‐initiator lithium phenyl‐2,4,6‐trimethyl‐benzoylphosphinate (Li‐TPO) concentration were optimized for application as a carrier to create a favorable environment for human articular chondrocytes (hAC). Primary hAC were used in passages 3 and 5, encapsulated into two different gelMA concentrations (7.5 wt% (soft) and 10 wt% (stiff)) and cultivated for 3 weeks with TGF‐β3 (0, 1 and 10 ng/mL). Higher TGF‐β3 concentrations induced spherical cell morphology independent of gelMA stiffness, while low TGF‐β3 concentrations only induced rounded morphology in stiff gelMA. Gene expression did not vary across gel stiffnesses. As a functional model gelMA was loaded with two different cell types (hAC and/or human adipose‐derived stem cells [ASC/TERT1]) and applied to human osteochondral osteoarthritic plugs. GelMA attached to the cartilage, smoothened the surface and retained cells in place. Resistance against shear forces was tested using a tribometer, simulating normal human gait and revealing maintained cell viability. In conclusion gelMA is a versatile, biocompatible material with good bonding capabilities to cartilage matrix, allowing sealing and smoothening of superficial cartilage defects while simultaneously delivering therapeutic cells for tissue regeneration.

Increasing the Microfabrication Performance of Synthetic Hydrogel Precursors through Molecular Design

Implementation of hydrogel precursors in two-photon polymerization (2PP) technology provides promising opportunities in the tissue engineering field thanks to their soft characteristics and similarity to extracellular matrix. Most of the hydrogels, however, are prone to post-fabrication deformations, leading to a mismatch between the computer-aided design and the printed structure. In the present work, we have developed novel synthetic hydrogel precursors to overcome the limitations associated with 2PP processing of conventional hydrogel precursors such as post-processing deformations and a narrow processing window. The precursors are based on a poly(ethylene glycol) backbone containing urethane linkers and are, on average, functionalized with six acrylate terminal groups (three on each terminal group). As a benchmark material, we exploited a precursor with an identical backbone and urethane linkers, albeit functionalized with two acrylate groups, that were reported as state-of-the-art. An in-depth characterization of the hexafunctional precursors revealed a reduced swelling ratio (<0.7) and higher stiffness (>36 MPa Young's modulus) compared to their difunctional analogs. The superior physical properties of the newly developed hydrogels lead to 2PP-based fabrication of stable microstructures with excellent shape fidelity at laser scanning speeds up to at least 90 mm s^-1, in contrast with the distorted structures of conventional difunctional precursors. The hydrogel films and microscaffolds revealed a good cell interactivity after functionalization of their surface with a gelatin methacrylamide-based coating. The proposed synthesis strategy provides a one-pot and scalable synthesis of hydrogel building blocks that can overcome the current limitations associated with 2PP fabrication of hydrogel microstructures.

Acrylate-endcapped urethane-based hydrogels: An in vivo study on wound healing potential

Improving wound healing by developing innovative dressing materials has been an important focus over the past few years in the biomedical field. In this regard, the current study focuses on developing new dressings based on acrylate-endcapped urethane-based polymers (AUPs). The materials have been processed into films and electrospun mats. Exudate uptake capacity, mechanical properties and fiber morphology were evaluated herein. The results showed superior uptake capacity of both films and mats when compared to Aquacel®Ag, Exufiber® and Help®. Addition of a high molar mass poly(ethylene glycol) to the AUP polymers benefits both the film and electrospun dressings in terms of flexibility and elongation. An in vivo study was conducted to assess the wound healing properties of these dressings on an acute wound model induced to rats. A macroscopic evaluation indicated that wound contraction and wound fraction percentages were improved significantly in case of the AUP-materials when compared to both the positive (Aquacel®Ag) and negative (Exufiber® and Help®) controls. A histopathological assay, to underline the changes noticed on a macroscopical level, was also performed. The data obtained proved that the developed dressings are beneficial towards tissue regeneration and accelerated wound healing. These findings offer a practical yet adequate strategy for the fabrication of acrylate-endcapped urethane-based materials for wound healing applications.

Development of photo-crosslinkable collagen hydrogel building blocks for vascular tissue engineering applications: A superior alternative to methacrylated gelatin?

The present work targets the development of collagen-based hydrogel precursors, functionalized with photo-crosslinkable methacrylamide moieties (COL-MA), for vascular tissue engineering (vTE) applications. The developed materials were physico-chemically characterized in terms of crosslinking kinetics, degree of modification/conversion, swelling behavior, mechanical properties and in vitro cytocompatibility. The collagen derivatives were benchmarked to methacrylamide-modified gelatin (GEL-MA), due to its proven track record in the field of tissue engineering. To the best of our knowledge, this is the first paper in its kind comparing these two methacrylated biopolymers for vTE applications. For both gelatin and collagen, two derivatives with varying degrees of substitutions (DS) were developed by altering the added amount of methacrylic anhydride (MeAnH). This led to photo-crosslinkable derivatives with a DS of 74 and 96% for collagen, and a DS of 73 and 99% for gelatin. The developed derivatives showed high gel fractions (i.e. 74% and 84%, for the gelatin derivatives; 87 and 83%, for the collagen derivatives) and an excellent crosslinking efficiency. Furthermore, the results indicated that the functionalization of collagen led to hydrogels with tunable mechanical properties (i.e. storage moduli of [4.8-9.4 kPa] for the developed COL-MAs versus [3.9-8.4 kPa] for the developed GEL-MAs) along with superior cell-biomaterial interactions when compared to GEL-MA. Moreover, the developed photo-crosslinkable collagens showed superior mechanical properties compared to extracted native collagen. Therefore, the developed photo-crosslinkable collagens demonstrate great potential as biomaterials for vTE applications.

Potential of poly(alkylene terephthalate)s to control endothelial cell adhesion and viability

Poly(ethylene terephthalate) (PET) is known for its various useful characteristics, including its applicability in cardiovascular applications, more precisely as synthetic bypass grafts for large diameter (≥ 6 mm) blood vessels. Although it is widely used, PET is not an optimal material as it is not interactive with endothelial cells, which is required for bypasses to form a complete endothelium. Therefore, in this study, poly(alkylene terephthalate)s (PATs) have been studied. They were synthesized via a single-step solution polycondensation reaction, which requires mild reaction conditions and avoids the use of a catalyst or additives like heat stabilizers. A homologous series was realized in which the alkyl chain length varied from 5 to 12 methylene groups (n = 5-12). Molar masses up to 28,000 g/mol were obtained, while various odd-even trends were observed with modulated differential scanning calorimetry (mDSC) and rapid heat-cool calorimetry (RHC) to access the thermal properties within the homologous series. The synthesized PATs have been subjected to in vitro cell viability assays using Human Umbilical Vein Endothelial Cells (HUVECs) and Human Dermal Microvascular Endothelial Cells (HDMECs). The results showed that HUVECs adhere and proliferate most pronounced onto PAT_(n=9) surfaces, which could be attributed to the surface roughness and morphology as determined by atomic force microscopy (AFM) (i.e. R_q = 204.7 nm). HDMECs were investigated in the context of small diameter vessels and showed superior adhesion and proliferation after seeding onto PAT_(n=6) substrates. These preliminary results already pave the way towards the use of PAT materials as substrates to support endothelial cell adhesion and growth. Indeed, as superior endothelial cell interactivity compared to PET was observed, time-consuming and costly surface modifications of PET grafts could be avoided by exploiting this novel material class.

Atomic Layer Deposition on Polymer Thin Films: On the Role of Precursor Infiltration and Reactivity

Inorganic barriers grown by atomic layer deposition (ALD) can overcome the stability issues originating from the permeation of foreign species (water and oxygen) into polymer thin films. Alternatively, infiltration of ALD species into the bulk of the polymer can be used to modify its characteristic properties. In this study, the feasibility of growing an inorganic barrier with ALD on polystyrene, poly(methyl methacrylate), and poly(ethylene terephthalate glycol) thin films is evaluated. The nucleation and growth of the ALD layer, including the infiltration into the polymer thin film, are monitored in situ using spectroscopic ellipsometry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy for Al₂O₃-ALD with trimethylaluminum as the Al precursor and H₂O as the reactant. The results show that the deposition temperature and the presence and location of functional groups in the polymer chain exert the strongest influence on the infiltration behavior and as such allow us to manipulate (i.e. to prevent or expedite) the infiltration into the polymer thin film.

Photo-Crosslinked Gelatin-Based Hydrogel Films to Support Wound Healing

Gelatin is used widely in the biomedical field, among other for wound healing. Given its upper critical solution temperature, crosslinking is required. To this end, gelatin is chemically modified with different photo-crosslinkable moieties with low (32-34%) and high (63-65%) degree of substitution (DS): gelatin-methacrylamide (gel-MA) and gelatin-acrylamide (gel-AA) and gelatin-pentenamide (gel-PE). Next to the more researched gel-MA, it is especially interesting and novel to compare with other gelatin-derived compounds for the application of wound healing. An additional comparison is made with commercial dressings. The DS is directly proportional to the mechanical characteristics and inversely proportional to the swelling capacity. Gel-PE shows weaker mechanical properties (G' < 15 kPa) than gel-AA and gel-MA (G' < 39 and 45 kPa, respectively). All derivatives are predominantly elastic (recovery indices of 89-94%). Gel-AA and gel-MA show excellent biocompatibility, whereas gel-PE shows a significantly lower initial biocompatibility, evolving positively toward day 7. Overall, gel-MA shows to have the most potential to be applied as wound dressing. Future blending with gel-AA to improve the curing kinetics can lead to dressings able to compete with current commercial dressings.