Monomer-Induced Customization of UV-Cured Atelocollagen Hydrogel Networks

Chemistry matters: A side-by-side comparison of two chemically distinct methacryloylated dECM bioresins for vat photopolymerization

Decellularized extracellular matrix (dECM) is an excellent natural source for 3D bioprinting materials due to its inherent cell compatibility. In vat photopolymerization, the use of dECM-based bioresins is just emerging, and extensive research is needed to fully exploit their potential. In this study, two distinct methacryloyl-functionalized, photocrosslinkable dECM-based bioresins were prepared from digested porcine liver dECM through functionalization with glycidyl methacrylate (GMA) or conventional methacrylic anhydride (MA) under mild conditions for systematic comparison. Although the chemical modifications did not significantly affect the structural integrity of the dECM proteins, mammalian cells encapsulated in the respective hydrogels performed differently in long-term culture. In either case, photocrosslinking during 3D (bio)printing resulted in transparent, highly swollen, and soft hydrogels with good shape fidelity, excellent biomimetic properties and tunable mechanical properties (~ 0.2-2.5 kPa). Interestingly, at a similar degree of functionalization (DOF ~ 81.5-83.5 %), the dECM-GMA resin showed faster photocrosslinking kinetics in photorheology resulting in lower final stiffness and faster enzymatic biodegradation compared to the dECM-MA gels, yet comparable network homogeneity as assessed via Brillouin imaging. While human hepatic HepaRG cells exhibited comparable cell viability directly after 3D bioprinting within both materials, cell proliferation and spreading were clearly enhanced in the softer dECM-GMA hydrogels at a comparable degree of crosslinking. These differences were attributed to the additional hydrophilicity introduced to dECM via methacryloylation through GMA compared to MA. Due to its excellent printability and cytocompatibility, the functional porcine liver dECM-GMA biomaterial enables the advanced biofabrication of soft 3D tissue analogs using vat photopolymerization-based bioprinting.

A collagen-based theranostic wound dressing with visual, long-lasting infection detection capability

Continuous wound monitoring is one strategy to minimise infection severity and inform prompt variations in therapeutic care following infection diagnosis. However, integration of this functionality in therapeutic wound dressings is still challenging. We hypothesised that a theranostic dressing could be realised by integrating a collagen-based wound contact layer with previously demonstrated wound healing capability, and a halochromic dye, i.e. bromothymol blue (BTB), undergoing colour change following infection-associated pH changes (pH: 5-6 ➔ >7). Two different BTB integration strategies, i.e. electrospinning and drop-casting, were pursued to introduce long-lasting visual infection detection capability through retention of BTB within the dressing. Both systems had an average BTB loading efficiency of 99 wt% and displayed a colour change within 1 min of contact with simulated wound fluid. Drop-cast samples retained up to 85 wt% of BTB after 96 h in a near-infected wound environment, in contrast to the fibre-bearing prototypes, which released over 80 wt% of BTB over the same time period. An increase in collagen denaturation temperature (DSC) and red shifts (ATR-FTIR) suggest the formation of secondary interactions between the collagen-based hydrogel and the BTB, which are attributed to count for the long-lasting dye confinement and durable dressing colour change. Given the high L929 fibroblast viability in drop-cast sample extracts (92 %, 7 days), the presented multiscale design is simple, cell- and regulatory-friendly, and compliant with industrial scale-up. This design, therefore, offers a new platform for the development of theranostic dressings enabling accelerated wound healing and prompt infection diagnosis.

A Biomimetic Nonwoven-Reinforced Hydrogel for Spinal Cord Injury Repair

In clinical trials, new scaffolds for regeneration after spinal cord injury (SCI) should reflect the importance of a mechanically optimised, hydrated environment. Composite scaffolds of nonwovens, self-assembling peptides (SAPs) and hydrogels offer the ability to mimic native spinal cord tissue, promote aligned tissue regeneration and tailor mechanical properties. This work studies the effects of an aligned electrospun nonwoven of P11-8—enriched poly(ε-caprolactone) (PCL) fibres, integrated with a photo-crosslinked hydrogel of glycidylmethacrylated collagen (collagen-GMA), on neurite extension. Mechanical properties of collagen-GMA hydrogel in compression and shear were recorded, along with cell viability. Collagen-GMA hydrogels showed J-shaped stress–strain curves in compression, mimicking native spinal cord tissue. For hydrogels prepared with a 0.8-1.1 wt.% collagen-GMA concentration, strain at break values were 68 ± 1–81 ± 1% (±SE); maximum stress values were 128 ± 9–311 ± 18 kPa (±SE); and maximum force values were 1.0 ± 0.1–2.5 ± 0.1 N (±SE). These values closely mimicked the compression values for feline and porcine tissue in the literature, especially those for 0.8 wt.%. Complex shear modulus values fell in the range 345–2588 Pa, with the lower modulus hydrogels in the range optimal for neural cell survival and growth. Collagen-GMA hydrogel provided an environment for homogenous and three-dimensional cell encapsulation, and high cell viability of 84 ± 2%. Combination of the aligned PCL/P11-8 electrospun nonwoven and collagen-GMA hydrogel retained fibre alignment and pore structure, respectively, and promoted aligned neurite extension of PC12 cells. Thus, it is possible to conclude that scaffolds with mechanical properties that both closely mimic native spinal cord tissue and are optimal for neural cells can be produced, which also promote aligned tissue regeneration when the benefits of hydrogels and electrospun nonwovens are combined.

A long-lasting guided bone regeneration membrane from sequentially functionalised photoactive atelocollagen

The fast degradation of collagen-based membranes in the biological environment remains a critical challenge, resulting in underperforming Guided Bone Regeneration (GBR) therapy leading to compromised clinical results. Photoactive atelocollagen (AC) systems functionalised with ethylenically unsaturated monomers, such as 4-vinylbenzyl chloride (4VBC), have been shown to generate mechanically competent materials for wound healing, inflammation control and drug delivery, whereby control of the molecular architecture of the AC network is key. Building on this platform, the sequential functionalisation with 4VBC and methacrylic anhydride (MA) was hypothesised to generate UV-cured AC hydrogels with reduced swelling ratio, increased proteolytic stability and barrier functionality for GBR therapy. The sequentially functionalised atelocollagen precursor (SAP) was characterised via TNBS and ninhydrin colourimetric assays, circular dichroism and UV-curing rheometry, which confirmed nearly complete consumption of collagen's primary amino groups, preserved triple helices and fast (< 180 s) gelation kinetics, respectively. Hydrogel's swelling ratio and compression modulus were adjusted depending on the aqueous environment used for UV-curing, whilst the sequential functionalisation of AC successfully generated hydrogels with superior proteolytic stability in vitro compared to both 4VBC-functionalised control and the commercial dental membrane Bio-Gide®. These in vitro results were confirmed in vivo via both subcutaneous implantation and a proof-of-concept study in a GBR calvarial model, indicating integrity of the hydrogel and barrier defect, as well as tissue formation following 1-month implantation in rats. STATEMENT OF SIGNIFICANCE: Collagen-based membranes remain a key component in Guided Bone Regeneration (GBR) therapy, but their properties, e.g. proteolytic stability and soft tissue barrier functionality, are still far from optimal. This is largely attributed to the complex molecular configuration of collagen, which makes chemical accessibility and structure-function relations challenging. Here, we fabricated a UV-cured hydrogel network of atelocollagen, whereby triple helices were sequentially functionalised with two distinct ethylenically unsaturated monomers. The effects of the sequential functionalisation and UV-curing on the macroscopic properties, degradation behaviour and GBR capability were investigated in vitro and in vivo. The results highlight the key role of the sequential functionalisation and provide important insights for the design of future, longer-lasting resorbable membranes for GBR therapy.

Effect of Mammalian Tissue Source on the Molecular and Macroscopic Characteristics of UV-Cured Type I Collagen Hydrogel Networks

The tissue source of type I collagen is critical to ensure scalability and regulation-friendly clinical translation of new medical device prototypes. However, the selection of a commercial source of collagen that fulfils both aforementioned requirements and is compliant with new manufacturing routes is challenging. This study investigates the effect that type I collagen extracted from three different mammalian tissues has on the molecular and macroscopic characteristics of a new UV-cured collagen hydrogel. Pepsin-solubilised bovine atelocollagen (BA) and pepsin-solubilised porcine atelocollagen (PA) were selected as commercially available raw materials associated with varying safety risks and compared with in-house acid-extracted type I collagen from rat tails (CRT). All raw materials displayed the typical dichroic and electrophoretic characteristics of type I collagen, while significantly decreased lysine content was measured on samples of PA. Following covalent functionalisation with 4-vinylbenzyl chloride (4VBC), BA and CRT products generated comparable UV-cured hydrogels with significantly increased averaged gel content (G ≥ 97 wt.%), while the porcine variants revealed the highest swelling ratio (SR = 2224 ± 242 wt.%) and an order of magnitude reduction in compression modulus (Ec = 6 ± 2 kPa). Collectively, these results support the use of bovine tissues as a chemically viable source of type I collagen for the realisation of UV-cured hydrogels with competitive mechanical properties and covalent network architectures.

Insights into enzymatic degradation of physically crosslinked hydrogels anchored by functionalized carbon nanofillers

Immobilization of hydrophobic enzymes on the surface of nanofillers disturbs the non-covalent interactions of polymer–filler networks and destabilizes physically crosslinked hydrogels.

Hierarchically Assembled Type I Collagen Fibres as Biomimetic Building Blocks of Biomedical Membranes

Wet spinning is an established fibre manufacturing route to realise collagen fibres with preserved triple helix architecture and cell acceptability for applications in biomedical membranes. However, resulting fibres still need to be chemically modified post-spinning to ensure material integrity in physiological media, with inherent risks of alteration of fibre morphology and with limited opportunities to induce fibrillogenesis following collagen fixation in the crosslinked state. To overcome this challenge, we hypothesised that a photoactive type I collagen precursor bearing either single or multiple monomers could be employed to accomplish hierarchically assembled fibres with improved processability, macroscopic properties and nanoscale organisation via sequential wet spinning and UV-curing. In-house-extracted type I rat tail collagen functionalised with both 4-vinylbenzyl chloride (4VBC) and methacrylate residues generated a full hydrogel network following solubilisation in a photoactive aqueous solution and UV exposure, whereby ~85 wt.% of material was retained following 75-day hydrolytic incubation. Wide-angle X-ray diffraction confirmed the presence of typical collagen patterns, whilst an averaged compression modulus and swelling ratio of more than 290 kPa and 1500 wt.% was recorded in the UV-cured hydrogel networks. Photoactive type I collagen precursors were subsequently wet spun into fibres, displaying the typical dichroic features of collagen and regular fibre morphology. Varying tensile modulus (E = 5 ± 1 - 11 ± 4 MPa) and swelling ratio (SR = 1880 ± 200 - 3350 ± 500 wt.%) were measured following post-spinning UV curing and equilibration with phosphate-buffered saline (PBS). Most importantly, 72-h incubation of the wet spun fibres in PBS successfully induced renaturation of collagen-like fibrils, which were fixed following UV-induced network formation. The whole process proved to be well tolerated by cells, as indicated by a spread-like cell morphology following a 48-h culture of L929 mouse fibroblasts on the extracts of UV-cured fibres.

Bioprinting of Collagen: Considerations, Potentials, and Applications

Collagen is the most abundant extracellular matrix protein that is widely used in tissue engineering (TE). There is little research done on printing pure collagen. To understand the bottlenecks in printing pure collagen, it is imperative to understand collagen from a bottom-up approach. Here it is aimed to provide a comprehensive overview of collagen printing, where collagen assembly in vivo and the various sources of collagen available for TE application are first understood. Next, the current printing technologies and strategy for printing collagen-based materials are highlighted. Considerations and key challenges faced in collagen printing are identified. Finally, the key research areas that would enhance the functionality of printed collagen are presented.

Rose Bengal Crosslinking to Stabilize Collagen Sheets and Generate Modulated Collagen Laminates

For medical application, easily accessible biomaterials with tailored properties are desirable. Collagen type I represents a biomaterial of choice for regenerative medicine and tissue engineering. Here, we present a simple method to modify the properties of collagen and to generate collagen laminates. We selected three commercially available collagen sheets with different thicknesses and densities and examined the effect of rose bengal and green light collagen crosslinking (RGX) on properties such as microstructure, swelling degree, mechanical stability, cell compatibility and drug release. The highest impact of RGX was measured for Atelocollagen, for which the swelling degree was reduced from 630% (w/w) to 520% (w/w) and thickness measured under force application increased from 0.014 mm to 0.455 mm, indicating a significant increase in mechanical stability. Microstructural analysis revealed that the sponge-like structure was replaced by a fibrous structure. While the initial burst effect during vancomycin release was not influenced by crosslinking, RGX increased cell proliferation on sheets of Atelocollagen and on Collagen Solutions. We furthermore demonstrate that RGX can be used to covalently attach different sheets to create materials with combined properties, making the modification and combination of readily available sheets with RGX an attractive approach for clinical application.

A redox-responsive hyaluronic acid-based hydrogel for chronic wound management

Polymer-based hydrogels have been widely applied for chronic wound therapeutics, due to their well-acclaimed wound exudate management capability. At the same time, there is still an unmet clinical need for simple wound diagnostic tools to assist clinical decision-making at the point of care and deliver on the vision of patient-personalised wound management. To explore this challenge, we present a one-step synthetic strategy to realise a redox-responsive, hyaluronic acid (HA)-based hydrogel that is sensitive to wound environment-related variations in glutathione (GSH) concentration. By selecting aminoethyl disulfide (AED) as a GSH-sensitive crosslinker and considering GSH concentration variations in active and non-self-healing wounds, we investigated the impact of GSH-induced AED cleavage on hydrogel dimensions, aiming to build GSH-size relationships for potential point-of-care wound diagnosis. The hydrogel was also found to be non-cytotoxic and aided L929 fibroblast growth and proliferation over seven days in vitro. Such a material offers a very low-cost tool for the visual detection of a target analyte that varies dependent on the status of the cells and tissues (wound detection), and may be further exploited as an implant for fibroblast growth and tissue regeneration (wound repair).