Characterization of slow-gelling alginate hydrogels for intervertebral disc tissue-engineering applications

Innovative method for producing plant-based meat analogs: Acid/calcium-induced internal gelation of potato protein/alginate composites

When creating plant-based meat analogs, it is often challenging to mimic the structural and textural attributes of real meat products during the cooking process. In this study, we investigated the potential of using potato protein/calcium alginate composite gels to formulate plant-based meat analogs. These gels provide a semi-solid texture at ambient temperature that remains intact during cooking because the electrostatic crosslinks are resistant to heat. Composite gels consisting of potato protein (10 wt%) and alginate (0-2 wt%) were prepared using the internal gelation method. This method involves dispersing an insoluble form of calcium (CaHPO4) throughout the protein-polysaccharide matrix and then using glucono-delta-lactone (GDL) to slowly lower the pH, thereby releasing the Ca2+ ions evenly throughout the system. The calcium alginate increased the strength of the potato protein gels and provided structural resistance to heat. Appreciable water loss occurred during cooking for simple calcium alginate gels, but this was prevented when potato proteins were present. Increasing the alginate concentration from 0 to 1.5 % increased the strength of the composite gels but higher levels promoted phase separation and network disruption, which reduced the gel strength. Heating did not appreciably alter the microstructure of the composite gels, but it did alter that of the pure potato protein gels. Finally, the potential of the composite gels as plant-based meat analogs was assessed by comparing their thermal denaturation and textural properties to those of real chicken breast. The potato protein/alginate composites were shown to simulate the thermal denaturation and textural changes of real chicken during the cooking process. Overall, our results suggest that calcium alginate gels may be useful in the formulation of plant-based meat products with improved cooking properties.

Exploiting light-based 3D-printing for the fabrication of mechanically enhanced, patient-specific aortic grafts

Despite polyester vascular grafts being routinely used in life-saving aortic aneurysm surgeries, they are less compliant than the healthy, native human aorta. This mismatch in mechanical behaviour has been associated with disruption of haemodynamics contributing to several long-term cardiovascular complications. Moreover, current fabrication approaches mean that opportunities to personalise grafts to the individual anatomical features are limited. Various modifications to graft design have been investigated to overcome such limitations; yet optimal graft functionality remains to be achieved. This study reports on the development and characterisation of an alternative vascular graft material. An alginate:PEGDA (AL:PE) interpenetrating polymer network (IPN) hydrogel has been produced with uniaxial tensile tests revealing similar strength and stiffness (0.39 ± 0.05 MPa and 1.61 ± 0.19 MPa, respectively) to the human aorta. Moreover, AL:PE tubular conduits of similar geometrical dimensions to segments of the aorta were produced, either via conventional moulding methods or stereolithography (SLA) 3D-printing. While both fabrication methods successfully demonstrated AL:PE hydrogel production, SLA 3D-printing was more easily adaptable to the fabrication of complex structures without the need of specific moulds or further post-processing. Additionally, most 3D-printed AL:PE hydrogel tubular conduits sustained, without failure, compression up to 50% their outer diameter and returned to their original shape upon load removal, thereby exhibiting promising behaviour that could withstand pulsatile pressure in vivo. Overall, these results suggest that this AL:PE IPN hydrogel formulation in combination with 3D-printing, has great potential for accelerating progress towards personalised and mechanically-matched aortic grafts.

Anionic species from multivalent metal salts are differentially retained during aqueous ionic gelation of sodium alginate and could fine-tune the hydrogel properties

The role of anionic counterions of divalent metal salts in alginate gelation and hydrogel properties has been thoroughly investigated. Three anions were selected from the Hofmeister series, namely sulphate, acetate and chloride, paired in all permutations and combinations with divalent metal cations like calcium, zinc and copper. Spectroscopic analysis revealed the presence of anions and their interaction with the respective metal cations in the hydrogel. The data showed that the gelation time and other hydrogel properties were largely controlled by cations. However, subtle yet significant variations in viscoelasticity, water uptake, drug release and cytocompatibility properties were anion dependent in each cationic group. Computational modelling based study showed that metal-anion-alginate configurations were energetically more stable than the metal-alginate models. The in vitro and in silico studies concluded that acetate anions preceded chlorides in the drug release, swelling and cytocompatibility fronts, followed by sulphate anions in each cationic group. Overall, the data confirmed that anions are an integral part of the metal-alginate complex. Furthermore, anions offer a novel option to further fine-tune the properties of alginate hydrogels for myriads of applications. In addition, full exploration of this novel avenue would enhance the usability of alginate polymers in the pharmaceutical, environmental, biomedical and food industries.

Multifunctional alginate/polydeoxyribonucleotide hydrogels for promoting diabetic wound healing

A multifunctional alginate/PDRN hydrogel system by ionic crosslinking and the Schiff base reaction between oxidized alginate (OA) and PDRN was developed in the present study. Biocompatibility assessment of the PDRN-loaded OA hydrogels showed a significant enhancement in cell viability in human dermal fibroblast (HDF) cells. In addition, hydrogels showed migratory, anti-inflammatory, intracellular reactive oxygen species scavenging, and anti-apoptotic activities. In vivo studies using a streptozotocin-induced diabetic Wister rat model indicated that OA-4PDRN had the highest percentage of wound closure (96.1 ± 2.6 %) at day 14 compared to the control (79.0 ± 2.3 %) group. This was accompanied by up-regulation of vascular endothelial growth factor (VEGF), interleukin-10 (IL-10), and transforming growth factor-beta (TGF-β) accompanied by down-regulation of pro-inflammatory markers (IL-6, IL-1β). Following histopathological observations, PDRN-loaded OA hydrogel ensured tissue safety and induced wound healing with granular tissue formation, collagen deposition, re-epithelialization, and regeneration of blood vessels and hair follicles. The downregulation of inflammatory cytokines (CD68) and expression of angiogenesis-related cytokines (CD31) in wound sites revealed the suppression of inflammation and increased angiogenesis, ensuring skin tissue regeneration in diabetic wound healing. In conclusion, the findings suggest that PDRN-loaded OA hydrogel has enormous therapeutic potential as a diabetic wound dressing.

Computational Prediction of Coiled-Coil Protein Gelation Dynamics and Structure

Protein hydrogels represent an important and growing biomaterial for a multitude of applications, including diagnostics and drug delivery. We have previously explored the ability to engineer the thermoresponsive supramolecular assembly of coiled-coil proteins into hydrogels with varying gelation properties, where we have defined important parameters in the coiled-coil hydrogel design. Using Rosetta energy scores and Poisson-Boltzmann electrostatic energies, we iterate a computational design strategy to predict the gelation of coiled-coil proteins while simultaneously exploring five new coiled-coil protein hydrogel sequences. Provided this library, we explore the impact of in silico energies on structure and gelation kinetics, where we also reveal a range of blue autofluorescence that enables hydrogel disassembly and recovery. As a result of this library, we identify the new coiled-coil hydrogel sequence, Q5, capable of gelation within 24 h at 4 °C, a more than 2-fold increase over that of our previous iteration Q2. The fast gelation time of Q5 enables the assessment of structural transition in real time using small-angle X-ray scattering (SAXS) that is correlated to coarse-grained and atomistic molecular dynamics simulations revealing the supramolecular assembling behavior of coiled-coils toward nanofiber assembly and gelation. This work represents the first system of hydrogels with predictable self-assembly, autofluorescent capability, and a molecular model of coiled-coil fiber formation.

Injectable smart stimuli-responsive hydrogels: pioneering advancements in biomedical applications

Hydrogels have established their significance as prominent biomaterials within the realm of biomedical research. However, injectable hydrogels have garnered greater attention compared with their conventional counterparts due to their excellent minimally invasive nature and adaptive behavior post-injection. With the rapid advancement of emerging chemistry and deepened understanding of biological processes, contemporary injectable hydrogels have been endowed with an "intelligent" capacity to respond to various endogenous/exogenous stimuli (such as temperature, pH, light and magnetic field). This innovation has spearheaded revolutionary transformations across fields such as tissue engineering repair, controlled drug delivery, disease-responsive therapies, and beyond. In this review, we comprehensively expound upon the raw materials (including natural and synthetic materials) and injectable principles of these advanced hydrogels, concurrently providing a detailed discussion of the prevalent strategies for conferring stimulus responsiveness. Finally, we elucidate the latest applications of these injectable "smart" stimuli-responsive hydrogels in the biomedical domain, offering insights into their prospects.

GA-coupled ANN model for predicting porosity in alginate gel scaffolds

Alginate gel scaffolds are biocompatible and biodegradable materials that have been used in a variety of tissue engineering applications. The porosity of alginate gel scaffolds is an important property that affects their performance. However, it is difficult to predict the porosity of alginate gel scaffolds accurately. In this study, a GA-coupled ANN model was developed to predict the porosity of alginate gel scaffolds. The model was trained on a dataset of 107 scaffolds with known porosities. The model was able to achieve a mean absolute error of 0.13, which suggests that it is able to accurately predict the porosity of alginate gel scaffolds. The alginate scaffold was fabricated by a microfluidic technique using a syringe pump and a flow device. The crosslinker solution was poured into the Petri dish to crosslink the polymer to the gel structure. The Archimedes method was used to determine the scaffold's apparent porosity. The artificial neural network has been used to model the porosity of the gel scaffold using the input parameters such as alginate-pluronic viscosity, surface tension, and contact angle etc. The maximum porosity was modelled to be 96.4 % using GA whereas the experimental value for the same was measured to be 92.8 ± 2 %. A 3.7% variation in the porosity was found from modelled value. To the best of our knowledge, this study is the first to develop an integrated ANN-coupled GA model to predict the maximum porosity of the gel scaffold. The result indicates that artificial intelligence has great potential for optimizing the parameters to fabricate the gel scaffold that can be used for tissue engineering applications.

Mucoadhesive Alginate/Pectin Films Crosslinked by Calcium Carbonate as Carriers of a Model Antifungal Drug-Posaconazole

The mucosal membrane of the oral cavity, due to its unique structure and availability, constitutes an appropriate site for the delivery of drugs, both with local and systemic effects. Mucoadhesive buccal films are drug dosage forms that due to their convenience of application, flexibility and size, are characterized by patients' compliance. Sodium alginate and pectin are natural polymers from the polysaccharides group, with mucoadhesive properties, that are widely applied to obtain buccal films. However, their hydrophilic nature and poor water resistance limit their application in sustained drug release formulations. Hence, the aim of this investigation was to design alginate/pectin buccal films by a one-step crosslinking technique-with the application of calcium carbonate. This technique was applied to prepare crosslinked alginate and alginate/pectin mucoadhesive films with a model antifungal drug-posaconazole. The obtained formulations were evaluated for the impact of crosslinking and pectin's presence on their pharmaceutical, mucoadhesive, mechanical and physicochemical properties. Additionally, the antifungal activity of the prepared films against Candida spp. was evaluated. It was shown that pectin's presence in the formulations improved flexibility, mucoadhesion and antifungal activity. The crosslinking process reduced mucoadhesiveness and antifungal activity but significantly enhanced the mechanical properties and stability and enabled prolonged drug release.

Aggregating in vitro-grown adipocytes to produce macroscale cell-cultured fat tissue with tunable lipid compositions for food applications

We present a method of producing bulk cell-cultured fat tissue for food applications. Mass transport limitations (nutrients, oxygen, waste diffusion) of macroscale 3D tissue culture are circumvented by initially culturing murine or porcine adipocytes in 2D, after which bulk fat tissue is produced by mechanically harvesting and aggregating the lipid-filled adipocytes into 3D constructs using alginate or transglutaminase binders. The 3D fat tissues were visually similar to fat tissue harvested from animals, with matching textures based on uniaxial compression tests. The mechanical properties of cultured fat tissues were based on binder choice and concentration, and changes in the fatty acid compositions of cellular triacylglyceride and phospholipids were observed after lipid supplementation (soybean oil) during in vitro culture. This approach of aggregating individual adipocytes into a bulk 3D tissue provides a scalable and versatile strategy to produce cultured fat tissue for food-related applications, thereby addressing a key obstacle in cultivated meat production.

Precision medicine strategies for spinal degenerative diseases: Injectable biomaterials with in situ repair and regeneration

As the population ages, spinal degeneration seriously affects quality of life in middle-aged and elderly patients, and prevention and treatment remain challenging for clinical surgeons. In recent years, biomaterials-based injectable therapeutics have attracted much attention for spinal degeneration treatment due to their minimally invasive features and ability to perform precise repair of irregular defects. However, the precise design and functional control of bioactive injectable biomaterials for efficient spinal degeneration treatment remains a challenge. Although many injectable biomaterials have been reported for the treatment of spinal degeneration, there are few reviews on the advances and effects of injectable biomaterials for spinal degeneration treatment. This work reviews the current status of the design and fabrication of injectable biomaterials, including hydrogels, bone cements and scaffolds, microspheres and nanomaterials, and the current progress in applications for treating spinal degeneration. Additionally, registered clinical trials were also summarized and key challenges and clinical translational prospects for injectable materials for the treatment of spinal degenerative diseases are discussed.

Establishing a reproducible approach to study cellular functions of plant cells with 3D bioprinting

Capturing cell-to-cell signals in a three-dimensional (3D) environment is key to studying cellular functions. A major challenge in the current culturing methods is the lack of accurately capturing multicellular 3D environments. In this study, we established a framework for 3D bioprinting plant cells to study cell viability, cell division, and cell identity. We established long-term cell viability for bioprinted Arabidopsis and soybean cells. To analyze the generated large image datasets, we developed a high-throughput image analysis pipeline. Furthermore, we showed the cell cycle reentry of bioprinted cells for which the timing coincides with the induction of core cell cycle genes and regeneration-related genes, ultimately leading to microcallus formation. Last, the identity of bioprinted Arabidopsis root cells expressing endodermal markers was maintained for longer periods. The framework established here paves the way for a general use of 3D bioprinting for studying cellular reprogramming and cell cycle reentry toward tissue regeneration.

Bioinspired in vitro intestinal mucus model for 3D-dynamic culture of bacteria

The intestinal mucus is a biological barrier that supports the intestinal microbiota growth and filters molecules. To perform these functions, mucus possesses optimized microstructure and viscoelastic properties and it is steadily replenished thus flowing along the gut. The available in vitro intestinal mucus models are useful tools in investigating the microbiota-human cells interaction, and are used as matrices for bacterial culture or as static component of microfluidic devices like gut-on-chips. The aim of this work is to engineer an in vitro mucus models (I-Bac3Gel) addressing in a single system physiological viscoelastic properties (i.e., 2-200 Pa), 3D structure and suitability for dynamic bacterial culture. Homogeneously crosslinked alginate hydrogels are optimized in composition to obtain target viscoelastic and microstructural properties. Then, rheological tests are exploited to assess a priori the hydrogels capability to withstand the flow dynamic condition. We experimentally assess the suitability of I-Bac3Gels in the evolving field of microfluidics by applying a dynamic flow to a bacterial-loaded mucus model and by monitoring E. coli growth and survival. The engineered models represent a step forward in the modelling of the mucus, since they can answer to different urgent needs such as a 3D structure, bioinspired properties and compatibility with dynamic system.

L-type Voltage-Gated calcium channels partly mediate Mechanotransduction in the intervertebral disc

Background

Intervertebral disc (IVD) degeneration continues to be a major global health challenge, with strong links to lower back pain, while the pathogenesis of this disease is poorly understood. In cartilage, much more is known about mechanotransduction pathways involving the strain-generated potential (SGP) and function of voltage-gated ion channels (VGICs) in health and disease. This evidence implicates a similar important role for VGICs in IVD matrix turnover. However, the field of VGICs, and to a lesser extent the SGP, remains unexplored in the IVD.

Methods

A two-step process was utilized to investigate the role of VGICs in the IVD. First, immunohistochemical staining was used to identify and localize several different VGICs in bovine and human IVDs. Second, a pilot study was conducted on the function of L-type voltage gated calcium channels (VGCCs) by inhibiting these channels with nifedipine (Nf) and measuring calcium influx in monolayer or gene expression from 3D cell-embedded alginate constructs subject to dynamic compression.

Results

Several VGICs were identified at the protein level, one of which, Cav2.2, appears to be upregulated with the onset of human IVD degeneration. Inhibiting L-type VGCCs with Nf supplementation led to an altered cell calcium influx in response to osmotic loading as well as downregulation of col 1a, aggrecan and ADAMTS-4 during dynamic compression.

Conclusions

This study demonstrates the presence of several VGICs in the IVD, with evidence supporting a role for L-type VGCCs in mechanotransduction. These findings highlight the importance of future detailed studies in this area to fully elucidate IVD mechanotransduction pathways and better inform treatment strategies for IVD degeneration.

Shaping and Patterning Supramolecular Materials─Stem Cell-Compatible Dual-Network Hybrid Gels Loaded with Silver Nanoparticles

Hydrogels with spatio-temporally controlled properties are appealing materials for biological and pharmaceutical applications. We make use of mild acidification protocols to fabricate hybrid gels using calcium alginate in the presence of a preformed thermally triggered gel based on a low-molecular-weight gelator (LMWG) 1,3:2:4-di(4-acylhydrazide)-benzylidene sorbitol (DBS-CONHNH₂). Nonwater-soluble calcium carbonate slowly releases calcium ions over time when exposed to an acidic pH, triggering the assembly of the calcium alginate gel network. We combined the gelators in different ways: (i) the LMWG was used as a template to spatially control slow calcium alginate gelation within preformed gel beads, using glucono-δ-lactone (GdL) to lower the pH; (ii) the LMWG was used as a template to spatially control slow calcium alginate gelation within preformed gel trays, using diphenyliodonium nitrate (DPIN) as a photoacid to lower the pH, and spatial resolution was achieved by masking. The dual-network hybrid gels display highly tunable properties, and the beads are compatible with stem cell growth. Furthermore, they preserve the LMWG function of inducing in situ silver nanoparticle (AgNP) formation, which provides the gels with antibacterial activity. These gels have potential for eventual regenerative medicine applications in (e.g.) bone tissue engineering.

Prevention of collagen hydrogel contraction using polydopamine-coating and alginate outer shell increases cell contractile force

Collagen is the most abundant protein in the extracellular matrix of mammals and has a great effect on various cell behaviors including adhesion, differentiation, and migration. However, it is difficult to utilize collagen gel as a physical scaffold in vitro because of its severe contraction. Decrease in the overall hydrogel volume induces changes in cell distribution, and mass transfer within the gel. Uncontrolled mechanical and physiological factors in the fibrous matrix result in uncontrolled cell behaviors in the surrounding cells. In this study, two strategies were used to minimize the contraction of collagen gel. A disk-shaped frame made of polydopamine-coated polydimethylsiloxane (PDMS) prevented horizontal contraction at the edge of the hydrogel. The sequentially cross-linked collagen gel with alginate outer shell (CA-shell) structure inhibited the vertical gel contraction. The combined method synergistically prevented the hydrogel from shrinkage in long-term 3D cell culture. We observed the shift in balance of differentiation from adipogenesis to osteogenesis in mesenchymal stem cells under the environment where gel contraction was prevented, and confirmed that this phenomenon is closely associated with the mechanotransduction based on Yes-associated protein (YAP) localization. Development of this contraction inhibition platform made it possible to investigate the influence of regulation of cellular microenvironments. The physical properties of the hydrogel fabricated in this study were similar to that of pure collagen gel but completely changed the cell behavior within the gel by inhibition of gel contraction. The platform can be used to broaden our understanding of the fundamental mechanism underlying cell-matrix interactions and reproduce extracellular matrix in vivo.

The role of oxidative stress in intervertebral disc cellular senescence

With the aggravation of social aging and the increase in work intensity, the prevalence of spinal degenerative diseases caused by intervertebral disc degeneration(IDD)has increased yearly, which has driven a heavy economic burden on patients and society. It is well known that IDD is associated with cell damage and degradation of the extracellular matrix. In recent years, it has been found that IDD is induced by various mechanisms (e.g., genetic, mechanical, and exposure). Increasing evidence shows that oxidative stress is a vital activation mechanism of IDD. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) could regulate matrix metabolism, proinflammatory phenotype, apoptosis, autophagy, and aging of intervertebral disc cells. However, up to now, our understanding of a series of pathophysiological mechanisms of oxidative stress involved in the occurrence, development, and treatment of IDD is still limited. In this review, we discussed the oxidative stress through its mechanisms in accelerating IDD and some antioxidant treatment measures for IDD.

Endogenous repair theory enriches construction strategies for orthopaedic biomaterials: a narrative review

The development of tissue engineering has led to new strategies for mitigating clinical problems; however, the design of the tissue engineering materials remains a challenge. The limited sources and inadequate function, potential risk of microbial or pathogen contamination, and high cost of cell expansion impair the efficacy and limit the application of exogenous cells in tissue engineering. However, endogenous cells in native tissues have been reported to be capable of spontaneous repair of the damaged tissue. These cells exhibit remarkable plasticity, and thus can differentiate or be reprogrammed to alter their phenotype and function after stimulation. After a comprehensive review, we found that the plasticity of these cells plays a major role in establishing the cell source in the mechanism involved in tissue regeneration. Tissue engineering materials that focus on assisting and promoting the natural self-repair function of endogenous cells may break through the limitations of exogenous seed cells and further expand the applications of tissue engineering materials in tissue repair. This review discusses the effects of endogenous cells, especially stem cells, on injured tissue repairing, and highlights the potential utilisation of endogenous repair in orthopaedic biomaterial constructions for bone, cartilage, and intervertebral disc regeneration.

Enhanced delivery efficiency and sustained release of biopharmaceuticals by complexation-based gel encapsulated coated microneedles: rhIFNα-1b example

Coated microneedles (MNs) are widely used for delivering biopharmaceuticals. In this study, a novel gel encapsulated coated MNs (GEC-MNs) was developed. The water-soluble drug coating was encapsulated with sodium alginate (SA) in situ complexation gel. The manufacturing process of GEC-MNs was optimized for mass production. Compared to the water-soluble coated MNs (72.02% ± 11.49%), the drug delivery efficiency of the optimized GEC-MNs (88.42% ± 6.72%) was steadily increased, and this improvement was investigated through in vitro drug release. The sustained-release of BSA was observed in vitro permeation through the skin. The rhIFNα-1b GEC-MNs was confirmed to achieve biosafety and 6-month storage stability. Pharmacokinetics of rhIFNα-1b in GEC-MNs showed a linearly dose-dependent relationship. The AUC of rhIFNα-1b in GEC-MNs (4.51 ng/ml·h) was bioequivalent to the intradermal (ID) injection (5.36 ng/ml·h) and significantly higher than water-soluble coated MNs (3.12 ng/ml·h). The rhIFNα-1b elimination half-life of GEC-MNs, soluble coated MNs, and ID injection was 18.16, 1.44, and 2.53 h, respectively. The complexation-based GEC-MNs have proved to be more efficient, stable, and achieve the sustained-release of water-soluble drug in coating MNs, constituting a high value to biopharmaceutical.

Tunable porosity of covalently crosslinked alginate-based hydrogels and its significance in drug release behavior

Task-specific drug release is essential in the development of hydrogels as drug delivery systems. The aim of the study is to report the effect of porosity on alginate hydrogels, which may be controlled by the design of crosslinkers, on drug release behavior. Two alginate-based hydrogels were prepared: alginate-norbornene (Alg-Nb) crosslinked by disulfide-tetrazine (S-Tz; hydrogel A) and alginate-furfuryl amine (Alg-FA) crosslinked by disulfide-maleimide (S-Ma; hydrogel B). Results showed the porosity of hydrogel A was controllable by adjusting the amount of S-Tz. Gel formation was facilitated by a "click" reaction between Alg-Nb and S-Tz, producing nitrogen gas, which, in turn, acted as an in-situ pore generator. Hydrogel B showed a non-porous morphology, as gelation was processed via addition reaction between Alg-FA and S-Ma, which produced no by-product. The study showed that crosslinker proportion and porosity were significant factors influencing drug release behavior of the alginate hydrogels. The presence of a porous structure increased the drug release while non-porous hydrogels led to a very slow release. In addition, the porous alginate hydrogels could sustainably release doxorubicin for 35 days.

Recent advances of hydrogel-based biomaterials for intervertebral disc tissue treatment: A literature review

Low back pain is an increasingly prevalent symptom mainly associated with intervertebral disc (IVD) degeneration. It is highly correlated with aging, as the nucleus pulposus (NP) dehydrates and annulus fibrosus fissure formatting, which finally results in the IVD herniation and related clinical symptoms. Hydrogels have been drawing increasing attention as the ideal candidates for IVD degeneration because of their unique properties such as biocompatibility, highly tunable mechanical properties, and especially the water absorption and retention ability resembling the normal NP tissue. Numerous innovative hydrogel polymers have been generated in the most recent years. This review article will first briefly describe the anatomy and pathophysiology of IVDs and current therapies with their limitations. Following that, the article introduces the hydrogel materials in the classification of their origins. Next, it reviews the recent hydrogel polymers explored for IVD regeneration and analyses what efforts have been made to overcome the existing limitations. Finally, the challenges and prospects of hydrogel-based treatments for IVD tissue are also discussed. We believe that these novel hydrogel-based strategies may shed light on new possibilities in IVD degeneration disease.

A 3D Bioprinted Material That Recapitulates the Perivascular Bone Marrow Structure for Sustained Hematopoietic and Cancer Models

Translational medicine requires facile experimental systems to replicate the dynamic biological systems of diseases. Drug approval continues to lag, partly due to incongruencies in the research pipeline that traditionally involve 2D models, which could be improved with 3D models. The bone marrow (BM) poses challenges to harvest as an intact organ, making it difficult to study disease processes such as breast cancer (BC) survival in BM, and to effective evaluation of drug response in BM. Furthermore, it is a challenge to develop 3D BM structures due to its weak physical properties, and complex hierarchical structure and cellular landscape. To address this, we leveraged 3D bioprinting to create a BM structure with varied methylcellulose (M): alginate (A) ratios. We selected hydrogels containing 4% (w/v) M and 2% (w/v) A, which recapitulates rheological and ultrastructural features of the BM while maintaining stability in culture. This hydrogel sustained the culture of two key primary BM microenvironmental cells found at the perivascular region, mesenchymal stem cells and endothelial cells. More importantly, the scaffold showed evidence of cell autonomous dedifferentiation of BC cells to cancer stem cell properties. This scaffold could be the platform to create BM models for various diseases and also for drug screening.

Engineered biomaterial strategies for controlling growth factors in tissue engineering

Growth factors are multi-functional signaling molecules that coordinate multi-stage process of wound healing. During wound healing, growth factors are transmitted to wound environment in a positive and physiologically related way, therefore, there is a broad prospect for studying the mediated healing process through growth factors. However, growth factors (GFs) themselves have disadvantages of instability, short life, rapid inactivation of physiological conditions, low safety and easy degradation, which hinder the clinical use of GFs. Rapid development of delivery strategies for GFs has been trying to solve the instability and insecurity of GFs. Particularly, in recent years, GFs delivered by scaffolds based on biomaterials have become a hotspot in this filed. This review introduces various delivery strategies for growth factors based on new biodegradable materials, especially polysaccharides, which could provide guidance for the development of the delivery strategies for growth factors in clinic.

Balancing biological and biomechanical performance in intervertebral disc repair: a systematic review of injectable cell delivery biomaterials

Discogenic back pain is a common condition without approved intervertebral disc (IVD) repair therapies. Cell delivery using injectable biomaterial carriers offers promise to restore disc height and biomechanical function, while providing a functional niche for delivered cells to repair degenerated tissues. This systematic review advances the injectable IVD cell delivery biomaterials field by characterising its current state and identifying themes of promising strategies. Preferred Reporting Items for Systematic Reviews and Meta- Analyses (PRISMA) guidelines were used to screen the literature and 183 manuscripts met the inclusion criteria. Cellular and biomaterial inputs, and biological and biomechanical outcomes were extracted from each study. Most identified studies targeted nucleus pulposus (NP) repair. No consensus exists on cell type or biomaterial carrier, yet most common strategies used mesenchymal stem cell (MSC) delivery with interpenetrating network/co-polymeric (IPN/CoP) biomaterials composed of natural biomaterials. All studies reported biological outcomes with about half the studies reporting biomechanical outcomes. Since the IVD is a load-bearing tissue, studies reporting compressive and shear moduli were analysed and two major themes were found. First, a competitive balance, or 'seesaw' effect, between biomechanical and biological performance was observed. Formulations with higher moduli had inferior cellular performance, and vice versa. Second, several low-modulus biomaterials had favourable biological performance and matured throughout culture duration with enhanced extracellular matrix synthesis and biomechanical moduli. Findings identify an opportunity to develop next-generation biomaterials that provide high initial biomechanical competence to stabilise and repair damaged IVDs with a capacity to promote cell function for long-term healing.

Polydeoxyribonucleotide-delivering therapeutic hydrogel for diabetic wound healing

Patients with diabetes experience delayed wound healing because of the uncontrolled glucose level in their bloodstream, which leads to impaired function of white blood cells, poor circulation, decreased production and repair of new blood vessels. Treatment using polydeoxyribonucleotide (PDRN), which is a DNA extracted from the sperm cells of salmon, has been introduced to accelerate the healing process of diabetic wounds. To accelerate the wound-healing process, sustained delivery of PDRN is critical. In this study, taking advantage of the non-invasive gelation property of alginate, PDRN was loaded inside the hydrogel (Alg-PDRN). The release behavior of PDRN was altered by controlling the crosslinking density of the Alg hydrogel. The amount of PDRN was the greatest inside the hydrogel with the highest crosslinking density because of the decreased diffusion. However, there was an optimal degree of crosslinking for the effective release of PDRN. In vitro studies using human dermal fibroblasts and diabetes mellitus fibroblasts and an in ovo chorioallantoic membrane assay confirmed that the Alg-PDRN hydrogel effectively induced cell proliferation and expression of angiogenic growth factors and promoted new blood vessel formation. Its effectiveness for accelerated diabetic wound healing was also confirmed in an in-vivo animal experiment using a diabetic mouse model.

Design of Bio-Conjugated Hydrogels for Regenerative Medicine Applications: From Polymer Scaffold to Biomolecule Choice

Bio-conjugated hydrogels merge the functionality of a synthetic network with the activity of a biomolecule, becoming thus an interesting class of materials for a variety of biomedical applications. This combination allows the fine tuning of their functionality and activity, whilst retaining biocompatibility, responsivity and displaying tunable chemical and mechanical properties. A complex scenario of molecular factors and conditions have to be taken into account to ensure the correct functionality of the bio-hydrogel as a scaffold or a delivery system, including the polymer backbone and biomolecule choice, polymerization conditions, architecture and biocompatibility. In this review, we present these key factors and conditions that have to match together to ensure the correct functionality of the bio-conjugated hydrogel. We then present recent examples of bio-conjugated hydrogel systems paving the way for regenerative medicine applications.

Drug-impregnated, pressurized gas expanded liquid-processed alginate hydrogel scaffolds for accelerated burn wound healing

While the benefits of both hydrogels and drug delivery to enhance wound healing have been demonstrated, the highly hydrophilic nature of hydrogels creates challenges with respect to the effective loading and delivery of hydrophobic drugs beneficial to wound healing. Herein, we utilize pressurized gas expanded liquid (PGX) technology to produce very high surface area (~200 m²/g) alginate scaffolds and describe a method for loading the scaffolds with ibuprofen (via adsorptive precipitation) and crosslinking them (via calcium chelation) to create a hydrogel suitable for wound treatment and hydrophobic drug delivery. The high surface area of the PGX-processed alginate scaffold facilitates >8 wt% loading of ibuprofen into the scaffold and controlled in vitro ibuprofen release over 12-24 h. In vivo burn wound healing assays demonstrate significantly accelerated healing with ibuprofen-loaded PGX-alginate/calcium scaffolds relative to both hydrogel-only and untreated controls, demonstrating the combined benefits of ibuprofen delivery to suppress inflammation as well as the capacity of the PGX-alginate/calcium hydrogel to maintain wound hydration and facilitate continuous calcium release to the wound. The use of PGX technology to produce highly porous scaffolds with increased surface areas, followed by adsorptive precipitation of a hydrophobic drug onto the scaffolds, offers a highly scalable method of creating medicated wound dressings with high drug loadings. STATEMENT OF SIGNIFICANCE: While medicated hydrogel-based wound dressings offer clear advantages in accelerating wound healing, the inherent incompatibility between conventional hydrogels and many poorly water-soluble drugs of relevance in wound healing remains a challenge. Herein, we leveraged supercritical fluids-based strategies to both process and subsequently impregnate alginate, followed by post-crosslinking to form a hydrogel, to create a very high surface area alginate hydrogel scaffold loaded with high hydrophobic drug contents (here, >8 wt% ibuprofen) without the need for any pore-forming additives. The impregnated scaffolds significantly accelerated burn wound healing while also promoting regeneration of the native skin morphology. We anticipate this approach can be leveraged to load clinically-relevant and highly bioavailable dosages of hydrophobic drugs in hydrogels for a broad range of potential applications.

Advanced Strategies for the Regeneration of Lumbar Disc Annulus Fibrosus

Damage to the annulus fibrosus (AF), the outer region of the intervertebral disc (IVD), results in an undesirable condition that may accelerate IVD degeneration causing low back pain. Despite intense research interest, attempts to regenerate the IVD have failed so far and no effective strategy has translated into a successful clinical outcome. Of particular significance, the failure of strategies to repair the AF has been a major drawback in the regeneration of IVD and nucleus replacement. It is unlikely to secure regenerative mediators (cells, genes, and biomolecules) and artificial nucleus materials after injection with an unsealed AF, as IVD is exposed to significant load and large deformation during daily activities. The AF defects strongly change the mechanical properties of the IVD and activate catabolic routes that are responsible for accelerating IVD degeneration. Therefore, there is a strong need to develop effective therapeutic strategies to prevent or reconstruct AF damage to support operational IVD regenerative strategies and nucleus replacement. By the way of this review, repair and regenerative strategies for AF reconstruction, their current status, challenges ahead, and future outlooks were discussed.

Biomaterials-Induced Stem Cells Specific Differentiation Into Intervertebral Disc Lineage Cells

Stem cell therapy, which promotes stem cells differentiation toward specialized cell types, increases the resident population and production of extracellular matrix, and can be used to achieve intervertebral disc (IVD) repair, has drawn great attention for the development of IVD-regenerating materials. Many materials that have been reported in IVD repair have the ability to promote stem cells differentiation. However, due to the limitations of mechanical properties, immunogenicity and uncontrollable deviations in the induction of stem cells differentiation, there are few materials that can currently be translated into clinical applications. In addition to the favorable mechanical properties and biocompatibility of IVD materials, maintaining stem cells activity in the local niche and increasing the ability of stem cells to differentiate into nucleus pulposus (NP) and annulus fibrosus (AF) cells are the basis for promoting the application of IVD-regenerating materials in clinical practice. The purpose of this review is to summarize IVD-regenerating materials that focus on stem cells strategies, analyze the properties of these materials that affect the differentiation of stem cells into IVD-like cells, and then present the limitations of currently used disc materials in the field of stem cell therapy and future research perspectives.

Hydrogel scaffolds for tissue engineering: the importance of polymer choice

We explore the design and synthesis of hydrogel scaffolds for tissue engineering from the perspective of the underlying polymer chemistry. The key polymers, properties and architectures used, and their effect on tissue growth are discussed.

Bio-conjugation of platelet-rich plasma and alginate through carbodiimide chemistry for injectable hydrogel therapies

Alginate is a highly tailorable, biocompatible polymer whose properties can be tuned to mimic the properties of native nucleus pulposus (NP) tissue. Platelet-rich plasma (PRP) is a highly accessible, inexpensive, and readily available mix of pro-regenerative factors. By functionalizing alginate with PRP, a mechanically optimized, bioactive alginate NP analogue may stimulate NP cells to proliferate and accumulate matrix over a longer period of time than if the PRP were solely encapsulated within the hydrogel. In this study, PRP was chemically bound to alginate using carbodiimide chemistry and mechanically, physically, and cytologically compared to plain alginate as well as alginate containing free-floating lyophilized PRP. The alginates were mechanically and physically characterized; PRP-conjugated alginate had similar mechanical properties to controls and had the benefit of retained PRP proteins within the hydrogel. Human nucleus pulposus cells (hNPCs) were seeded within the modified alginates and cultured for 14 days. Quantification data of glycosaminoglycans suggests that PRP-incorporated alginate has the potential to increase ECM production within the characterized alginate constructs, and that PRP-functionalized alginate can retain protein within the hydrogel over time. This is the first study to functionalize the milieu of PRP proteins onto alginate and characterize the mechanical and physical properties of the modified alginates. This study also incorporates hNPCs into the characterized PRP-modified alginates to observe phenotypic maintenance when encapsulated within the in situ gelling constructs.

Smart Hydrogels in Tissue Engineering and Regenerative Medicine

The field of regenerative medicine has tremendous potential for improved treatment outcomes and has been stimulated by advances made in bioengineering over the last few decades. The strategies of engineering tissues and assembling functional constructs that are capable of restoring, retaining, and revitalizing lost tissues and organs have impacted the whole spectrum of medicine and health care. Techniques to combine biomimetic materials, cells, and bioactive molecules play a decisive role in promoting the regeneration of damaged tissues or as therapeutic systems. Hydrogels have been used as one of the most common tissue engineering scaffolds over the past two decades due to their ability to maintain a distinct 3D structure, to provide mechanical support for the cells in the engineered tissues, and to simulate the native extracellular matrix. The high water content of hydrogels can provide an ideal environment for cell survival, and structure which mimics the native tissues. Hydrogel systems have been serving as a supportive matrix for cell immobilization and growth factor delivery. This review outlines a brief description of the properties, structure, synthesis and fabrication methods, applications, and future perspectives of smart hydrogels in tissue engineering.

Mechanical behaviour of alginate film with embedded voids under compression-decompression cycles

Voids of 300 µm diameter were embedded uniformly as monolayer in alginate gel film using a fluidic device. Voids of these dimensions in biopolymer gel film are desired for better transport of bioactive species and cell colonization in engineered tissues. In this article, the role of embedded voids in reducing compressive stress, hysteresis, and time scale of reheal vis-a-vis expulsion of pore fluid and its reabsorption upon reversal of load are reviewed. The cyclic loading was conducted with varying amplitude and frequency. The irreversible changes, if any in the gel structure under extreme compression were analyzed. The rate of expulsion of aqueous phase directly relates to the permeability of the gel film that is estimated here using simplified momentum and volumetric balance equations. The decrease in permeability with deformation is analyzed further, and the contribution of voids in this regard is discussed.

Embedding live bacteria in porous hydrogel/ceramic nanocomposites for bioprocessing applications

In this work, we present a biocompatible one-pot processing route for ceramic/hydrogel nanocomposites in which we embed live bacteria. In our approach, we fabricate a highly stable alginate hydrogel with minimal shrinkage, highly increased structural and mechanical stability, as well as excellent biocompatibility. The hydrogel was produced by ionotropic gelation and reinforced with alumina nanoparticles to form a porous 3D network. In these composite gels, the bacteria Escherichia coli and Bacillus subtilis were embedded. The immobilized bacteria showed high viability and similar metabolic activity as non-embedded cells. Even after repeated glucose consumption cycles, the material maintained high structural stability with stable metabolic activity of the immobilized bacteria. Storing the bionanocomposite for up to 60 days resulted in only minor loss of activity. Accordingly, this approach shows great potential for producing macroscopic bioactive materials for biotechnological processes.

Synthesis, characterization and properties of tropine-based ionic liquids gels

Hydrophilic ionic liquid (IL) gels based-on tropine as a potential separation material.

Are radio-contrast agents commonly used in discography toxic to the intact intervertebral disc tissue cells?

In the literature, there have been no studies showing clear results on how radio-contrast pharmaceuticals would affect intact disc tissue cells. In this context, it was aimed to evaluate the effects of iopromide and gadoxetic acid, frequently used in the discography, on intact lumbar disc tissue in pharmaco-molecular and histopathological level. Primary cell cultures were prepared from the healthy disc tissue of the patients operated in the neurosurgery clinic. Except for the control group, the cultures were incubated with the indicated radio-contrast agents. Cell viability, toxicity and proliferation indices were tested at specific time intervals. The cell viability was quantitatively analysed. It was also visually rechecked under a fluorescence microscope with acridine orange/propidium iodide staining. Simultaneously, cell surface morphology was analysed with an inverted light microscope, while haematoxylin and eosin (H&E) staining methodology was used in the histopathological evaluations. The obtained data were evaluated statistically. Unlike the literature, iopromide or gadoxetic acid did not have any adverse effects on the cell viability, proliferation and toxicity (P < 0.05). Although this study reveals that radio-contrast pharmaceuticals used in the discography, often used in neurosurgical practice, can be safely used, it should be remembered that this study was performed in an in vitro environment.

Pregabalin treatment for neuropathic pain may damage intervertebral disc tissue

The aim of the present study was to determine whether pharmaceutical preparations with pregabalin (PGB) as an active ingredient, which are widely prescribed by clinicians, exert toxic effects on human primary nucleus pulposus (NP) and annulus fibrosis (AF). Primary human cell cultures were obtained from intact (n=6) and degenerated (n=6) tissues resected from the two groups of patients. Different doses of PGB were applied to these cultures and cells were subjected to molecular analyses at 0, 24 and 48 h. Cell vitality, toxicity and proliferation were assessed using a spectrophotometer. The expression of chondroadherin (CHAD), a (member of the NP-specific protein family), hypoxia-inducible factor-1α (HIF-1α) and type II collagen (COL2A1) was measured using reverse transcription-quantitative polymerase chain reaction. The results revealed that cell intensity increased in a time-dependent manner and cell vitality continued in the cultures without pharmaceuticals. Cell proliferation was suppressed in the PGB-treated cultures independent from the dose and duration of application. PGB was demonstrated to suppress the expression of CHAD and HIF-1α. In contrast, COL2A1 gene expression was not revealed in any experimental group. The present study utilized an in vitro model and the PGB active ingredient used herein may not be representative of clinical applications; however, the results demonstrated that PGB has a toxic effect on NP/AF cell cultures containing primary human intervertebral disc tissue. In summary, the use of pharmacological agents containing PGB may suppress the proliferation and differentiation of NP/AF cells and/or tissues, which should be considered when deciding on an appropriate treatment regime.

Fabrication of a novel whole tissue-engineered intervertebral disc for intervertebral disc regeneration in the porcine lumbar spine

Tissue-engineered intervertebral discs (IVDs) have been proposed as a useful therapeutic strategy for the treatment of intervertebral disc degeneration (IDD). However, most studies have focused on fabrication and assessment of tissue-engineered IVDs in small animal models and the mechanical properties of the scaffolds are far below those of native human IVDs. The aim of this study was to produce a novel tissue-engineered IVD for IDD regeneration in the porcine lumbar spine. Firstly, a novel whole tissue-engineered IVD scaffold was fabricated using chitosan hydrogel to simulate the central nucleus pulposus (NP) structure, surrounded with a poly(butylene succinate-co-terephthalate) (PBST) fiber film for inner annulus fibrosus (IAF). And, a poly(ether ether ketone) (PEEK) ring was used to stimulate the outer annulus fibrosus (OAF). Then, the scaffolds were seeded with IVD cells and the cell-scaffold hybrids were transplanted into the porcine damaged spine and harvested at 4 and 8 weeks. In vitro cell experiments showed that IVD cells distributed and grew well in the scaffolds including porous hydrogel and PBST fibers. After implantation into pigs, radiographic and MRI images indicated that the tissue-engineered IVD construct could preserve the disc height in the case of discectomy as the normal disc height and maintain a large extracellular matrix and water content in the NP. Combined with the histological and gene expression results, it was concluded that the tissue-engineered IVD had similar morphological and histological structure to the natural IVD. Moreover, after implantation for 8 weeks, the tissue-engineered IVD showed a good compressive stress and elastic moduli, approaching those of natural porcine IVD. Therefore, the prepared tissue-engineered IVD construct had similar morphological and biofunctional properties to the native tissue. Also, the tissue-engineered IVD construct with excellent biocompatibility and mechanical properties provides a promising candidate for human IDD regeneration.

A novel whole tissue-engineered IVD consisting of a triphasic scaffold demonstrated excellent biocompatibility and mechanical properties in the porcine lumbar spine.

Transforming growth factor-beta1 promotes articular cartilage repair through canonical Smad and Hippo pathways in bone mesenchymal stem cells

AIMS

Transforming growth factor-β1 (TGF-β1) is a chondrogenic factor and has been reported to be able to enhance chondrocyte differentiation from bone marrow mesenchymal stem cells (BMSCs). Here we investigate the molecular mechanism through which TGF-β1 chronically promotes the repair of cartilage defect and inhibit chondrocyte hypertrophy.

MAIN METHODS

Animal models of full thickness cartilage defects were divided into three groups: model group, BMSCs group (treated with BMSCs/calcium alginate gel) and BMSCs+TGF-β1 group (treated with Lentivirus-TGF-β1-EGFP transduced BMSCs/calcium alginate gel). 4 and 8weeks after treatment, macroscopic observation, histopathological study and quantitative reverse transcription-polymerase chain reaction (qRT-PCR) were done to analyze phenotypes of the animals. BMSCs were transduced with Lentivirus-TGF-β1-EGFP in vitro and Western blot analysis was performed.

KEY FINDINGS

We found that TGF-β1-expressiing BMSCs improved the repair of the cartilage defect. The impaired cartilage contained higher amount of GAG and type II collagen and was integrated to the surrounding normal cartilage and higher content of GAG and type II collagen. The major events include increased expression of type II collagen following Smad2/3 phosphorylation, and inhibition of cartilage hypertrophy by increasing Yes-associated protein-1 (YAP-1) and inhibiting Runx2 and Col10 after the completion of chondrogenic differentiation.

SIGNIFICANCE

We conclude that TGF-β1 is beneficial to chondrogenic differentiation of BMSCs via canonical Smad pathway to promote early-repairing of cartilage defect. Furthermore, TGF-β1 inhibits chondrocyte hypertrophy by decreasing hypertrophy marker gene expression via Hippo signaling. Long-term rational use of TGF-β1 may be an alternative approach in clinic for cartilage repair and regeneration.