Heparin-Like Properties of Sulfated Alginates with Defined Sequences and Sulfation Degrees

Regioselective sulfation of alginate at 2-O-position of mannuronic acid unit with Py∙SO3 in DMSO

Alginates are brown algal polysaccharides consisting of β-D-mannuronic (M) and α-l-guluronic acid (G) residues linked with 1→4 glycosidic bonds. To functionalize these natural resources for biomedical use, alginates can be chemically modified, including by sulfation. Here regioselective sulfation of alginates at M-2 in DMSO with Py∙SO3 is described, by either sulfating alginates directly or through using alginates with added protecting groups (PG-s), including TBDMS-ether, Piv-, Bz-esters and intramolecular 3,6-lactone. Highest regioselectivity was found by sulfating TBDMS- and Piv-protected alginates, with over 65 % of M-residues being 2-O-sulfated. However significant reduction in molecular weight was found when alginates were sulfated in DMSO. Results from this work will allow a degree of control over substitution patterns in sulfated alginates. This will allow to more accurately determine structure-property relationships in biomedical research.

Sulfated Alginate for Biomedical Applications

Alginate (Alg) polymers have received much attention due to the mild conditions required for gel formation and their good bio-acceptability. However, due to limited interactions with cells, many drugs, and biomolecules, chemically modified alginates are of great interest. Sulfated alginate (S-Alg) is a promising heparin-mimetic that continues to be investigated both as a drug molecule and as a component of biomaterials. Herein, the S-Alg literature of the past five years (2017-2023) is reviewed. Several methods used to synthesize S-Alg are described, with a focus on new advances in characterization and stereoselectivity. Material fabrication is another focus and spans bulk materials, particles, scaffolds, coatings, and part of multicomponent biomaterials. The new application of S-Alg as an antitumor agent is highlighted together with studies evaluating safety and biodistribution. The high binding affinity of S-Alg for various drugs and heparin-binding proteins is exploited extensively in biomaterial design to tune the encapsulation and release of these agents and this aspect is covered in detail. Recommondations include publishing key material properties to allow reproducibility, careful selection of appropriate sulfation strategies, the use of cross-linking strategies other than ionic cross-linking for material fabrication, and more detailed toxicity and biodistribution studies to inform future work.

Synthesis of sulfated alginate from its tributylammonium salt: Comparing the sulfating agents H2SO4-DCC and SO3·py

Direct comparison of the sulfating agents H2SO4-DCC and SO3·py for the synthesis of sulfated alginate (S-Alg) as well as detailed characterisation of the products that form is lacking. This study involving three researchers used the tributylammonium salt of alginate (T-Alg) as a common substrate for the sulfation reactions. It was found that the use of H2SO4-DCC resulted in poor control of the degree of sulfation (DS) and that the S-Alg polymers contained nitrogen (determined by elemental analysis) as a result of formation of an unwanted N-acylurea adduct. Additionally, a large reduction in chain length was confirmed. In contrast, the use of SO3·py gave reasonable control over DS, resulted in high yields, showed no contamination and no clear change in chain length. Detailed characterisation of such S-Alg polymers by 1H NMR, 13C NMR and 1H,13C-HSQC NMR confirmed sulfation at C2 and C3 with a preference for C2.

Novel Strategies for Spatiotemporal and Controlled BMP-2 Delivery in Bone Tissue Engineering

Bone morphogenetic protein-2 (BMP-2) has been commercially approved by the Food and Drug Administration for use in bone defects and diseases. BMP-2 promotes osteogenic differentiation of mesenchymal stem cells. In bone tissue engineering, BMP-2 incorporated into scaffolds can be used for stimulating bone regeneration in organoid construction, drug testing platforms, and bone transplants. However, the high dosage and uncontrollable release rate of BMP-2 challenge its clinical application, mainly due to the short circulation half-life of BMP-2, microbial contamination in bone extracellular matrix hydrogel, and the delivery method. Moreover, in clinical translation, the requirement of high doses of BMP-2 for efficacy poses challenges in cost and safety. Based on these, novel strategies should ensure that BMP-2 is delivered precisely to the desired location within the body, regulating the timing of BMP-2 release to coincide with the bone healing process, as well as release BMP-2 in a controlled manner to optimize its therapeutic effect and minimize side effects. This review highlights improvements in bone tissue engineering applying spatiotemporal and controlled BMP-2 delivery, including molecular engineering, biomaterial modification, and synergistic therapy, aiming to provide references for future research and clinical trials.

Fabrication of cell-laden microbeads and microcapsules composed of bacterial polyglucuronic acid

In the past decades, the microencapsulation of mammalian cells into microparticles has been extensively studied for various in vitro and in vivo applications. The aim of this study was to demonstrate the viability of bacterial polyglucuronic acid (PGU), an exopolysaccharide derived from bacteria and composed of glucuronic acid units, as an effective material for cell microencapsulation. Using the method of dropping an aqueous solution of PGU-containing cells into a Ca²⁺-loaded solution, we produced spherical PGU microbeads with >93 % viability in the encapsulated human hepatoma HepG2 cells. Hollow-core microcapsules were formed via polyelectrolyte complex layer formation of PGU and poly-l-lysine, after which Ca²⁺, a cross-linker of PGU, was chelated, and this was accomplished by sequential immersion of microbeads in aqueous solutions of poly-l-lysine and sodium citrate. The encapsulated HepG2 cells proliferated and formed cell aggregates within the microparticles over a 14-day culture, with significantly larger aggregates forming within the microcapsules. Our results provide evidence for the viability of PGU for cell microencapsulation for the first time, thereby contributing to advancements in tissue engineering.

Multi-step Strategies Toward Regioselectively Sulfated M-Rich Alginates

Sulfated alginates (ASs), as well as several artificially sulfated polysaccharides, show interesting bioactivities. The key factors for structure-activity relationships studies are the degree of sulfation and the distribution of the sulfate groups along the polysaccharide backbone (sulfation pattern). The former parameter can often be controlled through stoichiometry, while the latter requires the development of suitable chemical or enzymatic, regioselective methods and is still missing for ASs. In this work, a study on the regioselective installation of several different protecting groups on a d-mannuronic acid enriched (M-rich) alginate is reported in order to develop a semi-synthetic access to regioselectively sulfated AS derivatives. A detailed structural characterization of the obtained ASs revealed that the regioselective sulfation could be achieved complementarily at the O-2 or O-3 positions of M units through multi-step sequences relying upon a silylating or benzoylating reagent for the regioselective protection of M-rich alginic acid, followed by sulfation and deprotection.

Assessing cell migration in hydrogels: An overview of relevant materials and methods

Cell migration is essential in numerous living processes, including embryonic development, wound healing, immune responses, and cancer metastasis. From individual cells to collectively migrating epithelial sheets, the locomotion of cells is tightly regulated by multiple structural, chemical, and biological factors. However, the high complexity of this process limits the understanding of the influence of each factor. Recent advances in materials science, tissue engineering, and microtechnology have expanded the toolbox and allowed the development of biomimetic in vitro assays to investigate the mechanisms of cell migration. Particularly, three-dimensional (3D) hydrogels have demonstrated a superior ability to mimic the extracellular environment. They are therefore well suited to studying cell migration in a physiologically relevant and more straightforward manner than in vivo approaches. A myriad of synthetic and naturally derived hydrogels with heterogeneous characteristics and functional properties have been reported. The extensive portfolio of available hydrogels with different mechanical and biological properties can trigger distinct biological responses in cells affecting their locomotion dynamics in 3D. Herein, we describe the most relevant hydrogels and their associated physico-chemical characteristics typically employed to study cell migration, including established cell migration assays and tracking methods. We aim to give the reader insight into existing literature and practical details necessary for performing cell migration studies in 3D environments.

Phenol-Grafted Alginate Sulfate Hydrogel as an Injectable FGF-2 Carrier

In the field of tissue engineering, fibroblast growth factor-2 (FGF-2) effectively regenerates damaged tissue and restores its biological function. However, FGF-2 readily diffuses and degrades under physiological conditions. Therefore, methods for the sustained and localized delivery of FGF-2 are needed. Drug delivery systems using hydrogels as carriers have attracted significant interest. Injectable hydrogels with an affinity for FGF-2 are candidates for FGF-2 delivery systems. In this study, we fabricated a hydrogel from phenol-grafted alginate sulfate (AlgS-Ph) and investigated its application to the delivery of FGF-2. The hydrogel was prepared under mild conditions via horseradish peroxidase (HRP)-mediated cross-linking. Surface plasmon resonance (SPR) measurements show that the AlgS-Ph hydrogel has an affinity for FGF-2 in accordance with its degree of sulfation. Conditions for the preparation of the AlgS-Ph hydrogel, including HRP and H₂O₂ concentrations, are optimized so that the hydrogel can be used as an injectable drug carrier. The hydrogel shows no cytotoxicity when using 10T1/2 cells as a model cell line. The angiogenesis assay shows that FGF-2 released from the AlgS-Ph hydrogel promotes the formation of blood vessels. These results indicate that the AlgS-Ph hydrogel is a suitable candidate for the FGF-2 carrier.

Sulfonated, oxidized pectin-based double crosslinked bioprosthetic valve leaflets for synergistically enhancing hemocompatibility and cytocompatibility and reducing calcification

Clinically frequently-used glutaraldehyde (GA)-crosslinked bioprosthetic valve leaflets (BVLs) are still curbed by acute thrombosis, malignant immunoreaction, calcification, and poor durability. In this study, an anticoagulant heparin-like biomacromolecule, sulfonated, oxidized pectin (SAP) with a dialdehyde structure was first obtained by modifying citrus pectin with sulfonation of 3-amino-1-propane sulfonic acid and then oxidating with periodate. Notably, a novel crosslinking approach was established by doubly crosslinking BVLs with SAP and the nature-derived crosslinking agent quercetin (Que), which play a synergistic role in both crosslinking and bioactivity. The double crosslinked BVLs also presented enhanced mechanical properties and enzymatic degradation resistance owing to the double crosslinking networks formed via CN bonds and hydrogen bonds, respectively, and good HUVEC-cytocompatibility. The in vitro and ex vivo assay manifested that the double-crosslinked BVLs had excellent anticoagulant and antithrombotic properties, owing to the introduction of SAP. The subcutaneous implantation also demonstrated that the obtained BVLs showed a reduced inflammatory response and great resistance to calcification, which is attributed to quercetin with multiple physiological activities and depletion of aldehyde groups by hydroxyl aldehyde reaction. With excellent stability, hemocompatibility, anti-inflammatory, anti-calcification, and pro-endothelialization properties, the obtained double-crosslinked BVLs, SAP + Que-PP, would have great potential to substitute the current clinical GA-crosslinked BVLs.

Strain Construction and Process Development for Efficient Recombinant Production of Mannuronan C-5 Epimerases in Hansenula polymorpha

Alginates are linear polysaccharides produced by brown algae and some bacteria and are composed of β-D-mannuronic acid (M) and α-L-guluronic acid (G). Alginate has numerous present and potential future applications within industrial, medical and pharmaceutical areas and G rich alginates are traditionally most valuable and frequently used due to their gelling and viscosifying properties. Mannuronan C-5 epimerases are enzymes converting M to G at the polymer level during the biosynthesis of alginate. The Azotobacter vinelandii epimerases AlgE1-AlgE7 share a common structure, containing one or two catalytic A-modules (A), and one to seven regulatory R-modules (R). Despite the structural similarity of the epimerases, they create different M-G patterns in the alginate; AlgE4 (AR) creates strictly alternating MG structures whereas AlgE1 (ARRRAR) and AlgE6 (ARRR) create predominantly G-blocks. These enzymes are therefore promising tools for producing in vitro tailor-made alginates. Efficient in vitro epimerization of alginates requires availability of recombinantly produced alginate epimerases, and for this purpose the methylotrophic yeast Hansenula polymorpha is an attractive host organism. The present study investigates whether H. polymorpha is a suitable expression system for future large-scale production of AlgE1, AlgE4, and AlgE6. H. polymorpha expression strains were constructed using synthetic genes with reduced repetitive sequences as well as optimized codon usage. High cell density cultivations revealed that the largest epimerases AlgE1 (147 kDa) and AlgE6 (90 kDa) are subject to proteolytic degradation by proteases secreted by the yeast cells. However, degradation could be controlled to a large extent either by co-expression of chaperones or by adjusting cultivation conditions. The smaller AlgE4 (58 kDa) was stable under all tested conditions. The results obtained thus point toward a future potential for using H. polymorpha in industrial production of mannuronan C-5 epimerases for in vitro tailoring of alginates.

Alginate: Enhancement Strategies for Advanced Applications

Alginate is an excellent biodegradable and renewable material that is already used for a broad range of industrial applications, including advanced fields, such as biomedicine and bioengineering, due to its excellent biodegradable and biocompatible properties. This biopolymer can be produced from brown algae or a microorganism culture. This review presents the principles, chemical structures, gelation properties, chemical interactions, production, sterilization, purification, types, and alginate-based hydrogels developed so far. We present all of the advanced strategies used to remarkably enhance this biopolymer's physicochemical and biological characteristics in various forms, such as injectable gels, fibers, films, hydrogels, and scaffolds. Thus, we present here all of the material engineering enhancement approaches achieved so far in this biopolymer in terms of mechanical reinforcement, thermal and electrical performance, wettability, water sorption and diffusion, antimicrobial activity, in vivo and in vitro biological behavior, including toxicity, cell adhesion, proliferation, and differentiation, immunological response, biodegradation, porosity, and its use as scaffolds for tissue engineering applications. These improvements to overcome the drawbacks of the alginate biopolymer could exponentially increase the significant number of alginate applications that go from the paper industry to the bioprinting of organs.

Sulfated Alginate Reduces Pericapsular Fibrotic Overgrowth on Encapsulated cGMP-Compliant hPSC-Hepatocytes in Mice

Intra-peritoneal placement of alginate encapsulated human induced pluripotent stem cell-derived hepatocytes (hPSC-Heps) represents a potential new bridging therapy for acute liver failure. One of the rate-limiting steps that needs to be overcome to make such a procedure more efficacious and safer is to reduce the accumulation of fibrotic tissue around the encapsulated cells to allow the free passage of relevant molecules in and out for metabolism. Novel chemical compositions of alginate afford the possibility of achieving this aim. We accordingly used sulfated alginate and demonstrated that this material reduced fibrotic overgrowth whilst not impeding the process of encapsulation nor cell function. Cumulatively, this suggests sulfated alginate could be a more suitable material to encapsulate hPSC-hepatocyte prior to human use.

High resolution imaging of soft alginate hydrogels by atomic force microscopy

This work explores the largely unknown surface microstructure and elastic modulus of soft calcium-alginate hydrogels (E = 100-4500 Pa) in their hydrated state by atomic force microscopy (AFM) in quantitative imaging mode. Alginate concentration influenced the surface topography with surface roughness measured to be 101 ± 6 nm and 57 ± 1 nm for 0.5 and 2.0% (w/v) alginate, respectively. The calculated range of pore sizes increased with decreasing alginate concentration, with radii smaller than 360 nm, 570 nm and 1230 nm for 2.0%, 1.0% and 0.5% alginate, respectively. Small changes in calcium concentration (from 20 to 25 mM, 1.5% alginate) did not induce changes in surface microstructure, although it increased the elastic modulus mean values and distribution. Introducing oxidized or peptide-grafted alginate in the gels resulted in rougher surfaces, larger pore sizes and lower elasticity than the respective hydrogels with no modified alginate.

Pericapsular fibrotic overgrowth mitigated in immunocompetent mice through microbead formulations based on sulfated or intermediate G alginates

Cell encapsulation in alginate microbeads is a promising approach to provide immune isolation in cell therapy without immunosuppression. However, the efficacy is hampered by pericapsular fibrotic overgrowth (PFO), causing encapsulated cells to lose function. Stability of the microbeads is important to maintain immune isolation in the long-term. Here, we report alginate microbeads with minimal PFO in immunocompetent C57BL/6JRj mice. Microbead formulations included either alginate with an intermediate (47 %) guluronate (G) content (IntG) or sulfated alginate (SA), gelled in Ca²⁺/Ba²⁺ or Sr²⁺. A screening panel of eleven microbead formulations were evaluated for PFO, yielding multiple promising microbeads. Two candidate formulations were evaluated for 112 days in vivo, exhibiting maintained stability and minimal PFO. Microbeads investigated in a human whole blood assay revealed low cytokine and complement responses, while SA microbeads activated coagulation. Protein deposition on microbeads explanted from mice investigated by confocal laser scanning microscopy (CLSM) showed minimal deposition of complement C3. Fibrinogen was positively associated with PFO, with a high deposition on microbeads of high G (68 %) alginate compared to IntG and SA microbeads. Overall, stable microbeads containing IntG or SA may serve in long-term therapeutic applications of cell encapsulation. STATEMENT OF SIGNIFICANCE: Alginate-based hydrogels in the format of micrometer size beads is a promising approach for the immunoisolation of cells in cell therapy. Clinical trials in type 1 diabetes have so far had limited success due to fibrotic responses that hinder the diffusion of nutrients and oxygen to the encapsulated cells, resulting in graft failure. In this study, minimal fibrotic response towards micrometer size alginate beads was achieved by chemical modification of alginate with sulfate groups. Also, the use of alginate with intermediate guluronic acid content resulted in minimally fibrotic microbeads. Fibrinogen deposition was revealed to be a good indicator of fibrosis. This study points to both new microsphere developments and novel insight in the mechanisms behind the fibrotic responses.

Sulfated Alginate Hydrogels Prolong the Therapeutic Potential of MSC Spheroids by Sequestering the Secretome

Cell-based approaches to tissue repair suffer from rapid cell death upon implantation, limiting the window for therapeutic intervention. Despite robust lineage-specific differentiation potential in vitro, the function of transplanted mesenchymal stromal cells (MSCs) in vivo is largely attributed to their potent secretome comprising a variety of growth factors (GFs). Furthermore, GF secretion is markedly increased when MSCs are formed into spheroids. Native GFs are sequestered within the extracellular matrix (ECM) via sulfated glycosaminoglycans, increasing the potency of GF signaling compared to their unbound form. To address the critical need to prolong the efficacy of transplanted cells, alginate hydrogels are modified with sulfate groups to sequester endogenous heparin-binding GFs secreted by MSC spheroids. The influence of crosslinking method and alginate modification is assessed on mechanical properties, degradation rate, and degree of sulfate modification. Sulfated alginate hydrogels sequester a mixture of MSC-secreted endogenous biomolecules, thereby prolonging the therapeutic effect of MSC spheroids for tissue regeneration. GFs are sequestered for longer durations within sulfated hydrogels and retain their bioactivity to regulate endothelial cell tubulogenesis and myoblast infiltration. This platform has the potential to prolong the therapeutic benefit of the MSC secretome and serve as a valuable tool for investigating GF sequestration.

Sulfated alginate/polycaprolactone double-emulsion nanoparticles for enhanced delivery of heparin-binding growth factors in wound healing applications

Diabetic foot ulcers (DFUs) that are not effectively treated could lead to partial or complete lower limb amputations. The lack of connective tissue growth factor (CTGF) and insulin-like growth factor (IGF-I) in DFUs results in limited matrix deposition and poor tissue repair. To enhance growth factor (GF) availability in DFUs, heparin (HN)-mimetic alginate sulfate/polycaprolactone (AlgSulf/PCL) double emulsion nanoparticles (NPs) with high affinity and sustained release of CTGF and IGF-I were synthesized. The NPs size, encapsulation efficiency (EE), cytotoxicity, cellular uptake and wound healing capacity in immortalized primary human adult epidermal cells (HaCaT) were assessed. The sonication time and amplitude used for NPs synthesis enabled the production of particles with a minimum of 236 ± 25 nm diameter. Treatment of HaCaT cells with up to 50 μg mL^-1 of NPs showed no cytotoxic effects after 72 h. The highest bovine serum albumin EE (94.6 %, P = 0.028) and lowest burst release were attained with AlgSulf/PCL. Moreover, cells treated with AlgSulf/CTGF (250 ng mL^-1) exhibited the most rapid wound closure compared to controls while maintaining fibronectin synthesis. Double-emulsion NPs based on HN-mimetic AlgSulf represent a novel approach which can significantly enhance diabetic wound healing and can be expanded for applications requiring the delivery of other HN-binding GFs.

Heparin-modified alginate microspheres enhance neovessel formation in hiPSC-derived endothelial cells and heterocellular in vitro models by controlled release of vascular endothelial growth factor

A formidable challenge in regenerative medicine is the development of stable microvascular networks to restore adequate blood flow or to sustain graft viability and long-term function in implanted or ischemic tissues. In this work, we develop a biomimetic approach to increase the binding affinity of the extracellular matrix for the class of heparin-binding growth factors to localize and control the release of proangiogenic cues while maintaining their bioactivity. Sulfate and heparin moieties are covalently coupled to alginate, and alginate microspheres are produced and used as local delivery depots for vascular endothelial growth factor (VEGF). Release of VEGF from sulfate-alginate and heparin-alginate bulk hydrogels and microspheres was sustained over 14 days. In vitro evaluation with human induced pluripotent stem cell (hiPSC)-derived endothelial cells and aortic ring assay in a chemically defined hydrogel demonstrates development of primitive three-dimensional vessel-like networks in the presence of VEGF released from the chemically modified alginate microspheres. Furthermore, our results suggest that the sulfate groups available on the chemically modified alginate microspheres promote some new vessel formation even in VEGF-free samples. Based on this evidence, we conclude that sulfate- and heparin-alginate hydrogels are adaptive and bioactive delivery systems for revascularization therapy and translational vascular tissue engineering.

Antiviral Properties of Alginate-Based Biomaterials: Promising Antiviral Agents against SARS-CoV-2

The COVID-19 pandemic has made it essential to explore alternative antiviral materials. Alginate is a biodegradable, renewable, biocompatible, water-soluble and antiviral biopolymer with many potential biomedical applications. In this regard, this review shows 17 types of viruses that have been tested in contact with alginate and its related biomaterials. Most of these studies show that alginate-based materials possess little or no toxicity and are able to inhibit a wide variety of viruses affecting different organisms: in humans by the human immunodeficiency virus type 1, the hepatitis A, B, and C viruses, Sindbis virus, herpes simplex virus type 1 and 2, poliovirus type 1, rabies virus, rubella virus, and the influenza virus; in mice by the murine norovirus; in bacteria by the T4 coliphage, and in plants by the tobacco mosaic virus and the potato virus X. Many of these are enveloped positive-sense single-stranded RNA viruses, like SARS-CoV-2, which render alginate-based materials highly promising in the COVID-19 pandemic.

Thermally driven formation of polyphenolic carbonized nanogels with high anticoagulant activity from polysaccharides

We have demonstrated that alginate with negligible anticoagulant activity can be converted into carbonized nanogels with potent anticoagulant activity through a solid-state heating process. The conversion of alginate into graphene-like nanosheet (GNS)-embedded polyphenolic-alginate nanogels (GNS/Alg-NGs) has been carried out through condensation and carbonization processes. The GNS/Alg-NGs exhibit much stronger anticoagulant activity (>520-fold) compared to untreated alginate, mainly because their polyphenolic structures have a high binding affinity [dissociation constant (K_d) = 2.1 × 10^-10 M] toward thrombin. In addition, the thrombin clotting time delay caused by the GNS/Alg-NGs is 10-fold longer than that of natural polyphenolic compounds, such as quercetin, catechin, naringenin, caffeic acid, and ferulic acid. The thrombin- or kaolin-activated thromboelastography of whole-blood coagulation reveals that the GNS/Alg-NGs display a much stronger anticoagulant ability than that of untreated alginate and naturally sulfated polysaccharides (fucoidan). The GNS/Alg-NGs exhibit superior biocompatibility and anticoagulant activity, as observed with an in vivo rat model, revealing their potential as a blood thinner for the treatment of thrombotic disorders.

Alginate hydrogels functionalized with β-cyclodextrin as a local paclitaxel delivery system

Modification of drug delivery materials with beta-cyclodextrins (β-CyD) is known to increase solubility of poorly water-soluble drugs, protect drugs from degradation and sustain release. In this study, we developed a hydrogel drug delivery system for local paclitaxel delivery using the natural polysaccharide alginate functionalized with β-CyD-moieties. Paclitaxel was chosen due to its ability to form inclusion complexes with cyclodextrins. The rheological and mechanical properties of the prepared hydrogels were characterized, as well as in vitro release of the paclitaxel and in vitro activity on PC-3 prostate cancer cells. Introduction of β-CyD-moieties into the hydrogel reduces the mechanical properties of the gels compared to nonmodified gels. However, gelation kinetics were not markedly different. Furthermore, the β-CyD-modified alginate helped to reduce undesired crystallization of the paclitaxel in the gel and facilitated paclitaxel diffusion out of the gel network. Remarkably, the β-CyD grafted alginate showed increased capacity to complex paclitaxel compared to free HPβ-CyD. Release of both paclitaxel and degradation products were measured from the gels and were shown to have cytotoxic effects on the PC-3 cells. The results indicate that functionalized alginate with β-CyDs has potential as a material for drug delivery systems.

Alginate gels crosslinked with chitosan oligomers - a systematic investigation into alginate block structure and chitosan oligomer interaction

Three alginates with fundamentally different block structures, poly-M, poly-G, and poly-MG, have been investigated upon ionic crosslinking with chitosan oligosaccharides (CHOS), using circular dichroism (CD), rheology, and computer simulations, supporting the previously proposed gelling principle of poly-M forming zipper-like junction zones with chitosan (match in charge distance along the two polyelectrolytes) and revealing a unique high gel strength poly-MG chitosan gelling system. CD spectroscopy revealed an increased chiroptical activity exclusively for the poly-M chitosan gelling system, indicative of induced conformational changes and higher ordered structures. Rheological measurement revealed gel strengths (G' < 900 Pa) for poly-MG (1%) CHOS (0.3%) hydrogels, magnitudes of order greater than displayed by its poly-M analogue. Furthermore, the ionically crosslinked poly-MG chitosan hydrogel increased in gel strength upon the addition of salt (G' < 1600 at 50 mM NaCl), suggesting a stabilization of the junction zones through hydrophobic interactions and/or a phase separation. Molecular dynamics simulations have been used to further investigate these findings, comparing interaction energies, charge distances and chain alignments. These alginates are displaying high gel strengths, are known to be fully biocompatible and have revealed a broad range of tolerance to salt concentrations present in biological systems, proving high relevance for biomedical applications.

Highly tough and ultrafast self-healable dual physically crosslinked sulfated alginate-based polyurethane elastomers for vascular tissue engineering

Since vascular diseases are regarded as a major cause of death worldwide, developing engineered biomimetic elastomers with physicochemical and biological properties resembling those of the natural vascular tissues, is vital for vascular tissue engineering (VTE). This study reports synthesis of highly tough supramolecular biologically active alginate-based supramolecular polyurethane (BASPU) elastomers that benefit from the presence of two physical networks with different strength of soft tertiary ammonium-soft sulfate pairs, as strong ionic bonds, and soft tertiary ammonium-hard carboxylate groups, as the weak bonds. The presence of sulfate groups resulted in low Young's modulus, high toughness and stretchability, proper energy dissipation, ultrafast self-healing and complete healing efficiency of BASPU. In vitro studies showed higher endothelial cells attachment, higher anticoagulation ability and significantly less platelet adhesion for BASPUs compared to the commercial vascular prosthesis. The histological studies of subcutaneously implanted scaffolds confirmed their low fibrosis and gradual biodegradation during 2 months of following.

Applications of Biocompatible Scaffold Materials in Stem Cell-Based Cartilage Tissue Engineering

Cartilage, especially articular cartilage, is a unique connective tissue consisting of chondrocytes and cartilage matrix that covers the surface of joints. It plays a critical role in maintaining joint durability and mobility by providing nearly frictionless articulation for mechanical load transmission between joints. Damage to the articular cartilage frequently results from sport-related injuries, systemic diseases, degeneration, trauma, or tumors. Failure to treat impaired cartilage may lead to osteoarthritis, affecting more than 25% of the adult population globally. Articular cartilage has a very low intrinsic self-repair capacity due to the limited proliferative ability of adult chondrocytes, lack of vascularization and innervation, slow matrix turnover, and low supply of progenitor cells. Furthermore, articular chondrocytes are encapsulated in low-nutrient, low-oxygen environment. While cartilage restoration techniques such as osteochondral transplantation, autologous chondrocyte implantation (ACI), and microfracture have been used to repair certain cartilage defects, the clinical outcomes are often mixed and undesirable. Cartilage tissue engineering (CTE) may hold promise to facilitate cartilage repair. Ideally, the prerequisites for successful CTE should include the use of effective chondrogenic factors, an ample supply of chondrogenic progenitors, and the employment of cell-friendly, biocompatible scaffold materials. Significant progress has been made on the above three fronts in past decade, which has been further facilitated by the advent of 3D bio-printing. In this review, we briefly discuss potential sources of chondrogenic progenitors. We then primarily focus on currently available chondrocyte-friendly scaffold materials, along with 3D bioprinting techniques, for their potential roles in effective CTE. It is hoped that this review will serve as a primer to bring cartilage biologists, synthetic chemists, biomechanical engineers, and 3D-bioprinting technologists together to expedite CTE process for eventual clinical applications.

Biomimetic sulfated glycosaminoglycans maintain differentiation markers of breast epithelial cells and preferentially inhibit proliferation of cancer cells

Glycosaminoglycans (GAG) are key elements involved in various physiological and pathological processes including cancer. Several GAG-based drugs have been developed showing significant results and potential use as cancer therapeutics. We previously reported that alginate sulfate (AlgSulf), a GAG-mimetic, reduces the proliferation of lung adenocarcinoma cells. In this study, we evaluated the preferential effect of AlgSulf on tumorigenic and nontumorigenic mammary epithelial cells in 2D, 3D, and coculture conditions. AlgSulf were synthesized with different degrees of sulfation (DSs) varying from 0 to 2.7 and used at 100 µg/mL on HMT-3522 S1 (S1) nontumorigenic mammary epithelial cells and their tumorigenic counterparts HMT-3522 T4-2 (T4-2) cells. The anti-tumor properties of AlgSulf were assessed using trypan blue and bromodeoxyuridine proliferation (BrdU) assays, immunofluorescence staining and transwell invasion assay.  Binding of insulin and epidermal growth factor (EGF) to sulfated substrates was measured using QCM-D and ELISA. In 2D, the cell growth rate of cells treated with AlgSulf was consistently lower compared to untreated controls (p<0.001) and surpassed the effect of the native GAG heparin (positive control). In 3D, AlgSulf preferentially hindered the growth rate and the invasion potential of tumorigenic T4-2 nodules while maintaining the formation of differentiated polarized nontumorigenic S1 acini. The preferential growth inhibition of tumorigenic cells by AlgSulf was confirmed in a coculture system (p<0.001). In the ELISA assay, a trend of EGF binding was detected for sulfated polysaccharides while QCM-D analysis showed negligible binding of insulin and EGF to sulfated substrates. The preferential effect mediated by the mimetic sulfated GAGs on cancer cells may in part be growth factor dependent. Our findings suggest a potential anticancer therapeutic role of AlgSulf for the development of anticancer drugs.

Exploiting Mannuronan C-5 Epimerases in Commercial Alginate Production

Alginates are one of the major polysaccharide constituents of marine brown algae in commercial manufacturing. However, the content and composition of alginates differ according to the distinct parts of these macroalgae and have a direct impact on the concentration of guluronate and subsequent commercial value of the final product. The Azotobacter vinelandii mannuronan C-5 epimerases AlgE1 and AlgE4 were used to determine their potential value in tailoring the production of high guluronate low-molecular-weight alginates from two sources of high mannuronic acid alginates, the naturally occurring harvested brown algae (Ascophyllum nodosum, Durvillea potatorum, Laminaria hyperborea and Lessonia nigrescens) and a pure mannuronic acid alginate derived from fermented production of the mutant strain of Pseudomonas fluorescens NCIMB 10,525. The mannuronan C-5 epimerases used in this study increased the content of guluronate from 32% up to 81% in both the harvested seaweed and bacterial fermented alginate sources. The guluronate-rich alginate oligomers subsequently derived from these two different sources showed structural identity as determined by proton nuclear magnetic resonance (1H NMR), high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) and size-exclusion chromatography with online multi-angle static laser light scattering (SEC-MALS). Functional identity was determined by minimum inhibitory concentration (MIC) assays with selected bacteria and antibiotics using the previously documented low-molecular-weight guluronate enriched alginate OligoG CF-5/20 as a comparator. The alginates produced using either source showed similar antibiotic potentiation effects to the drug candidate OligoG CF-5/20 currently in development as a mucolytic and anti-biofilm agent. These findings clearly illustrate the value of using epimerases to provide an alternative production route for novel low-molecular-weight alginates.

The use of heparin and heparin-like molecules in cancer treatment: a review

BACKGROUND

Heparin and heparin-like molecules have shown some promise in the treatment of several cancers. These molecules have roles in angiogenesis, cell proliferation, immune system modulation, cell migration, and cellular invasion. The pathways and mechanisms used by these molecules to inhibit the proliferation of cancer cells aid in understanding the utilization of these molecules in potential treatments. Our aim is to review the use of heparin and heparin-like molecules in cancer treatment, explore the results, and discuss their potential downfalls.

METHODS

Publications on heparin and heparin-like molecules and compounds were collected from the PubMed and EMBASE databases. Boolean operators and MeSH terms related to heparin, heparin-like molecules, and cancer were used to conduct this search. The articles were reviewed by the authors.

RESULTS

Several heparin mimetics are showing promise in cancer treatment. Various studies using mimetics alone or in combination with chemotherapy have been conducted and have yielded mixed results. They work on multiple target molecules, mostly receptors such as fibroblast growth factor and endothelial growth factor. The main types of cancers targeted by these drugs are multiple myeloma, pancreatic cancer, hepatocellular carcinoma (HCC), and other solid tumors.

CONCLUSION

Although limited clinical evidence of efficacy and potential pitfalls are present, heparin and heparin-like molecules have shown potential in the management of cancer patients. Additional research is required to fully understand the biological mechanisms utilized by these molecules in cancer treatment.

Strategies to Functionalize the Anionic Biopolymer Na-Alginate without Restricting Its Polyelectrolyte Properties

The natural anionic polyelectrolyte alginate and its derivatives are of particular interest for pharmaceutical and biomedical applications. Most interesting for such applications are alginate hydrogels, which can be processed into various shapes, self-standing or at surfaces. Increasing efforts are underway to functionalize the alginate macromolecules prior to hydrogel formation in order to overcome the shortcomings of purely ionically cross-linked alginate hydrogels that are hindering the progress of several sophisticated biomedical applications. Particularly promising are derivatives of alginate, which allow simultaneous ionic and covalent cross-linking to improve the physical properties and add biological activity to the hydrogel. This review will report recent progress in alginate modification and functionalization with special focus on synthesis procedures, which completely conserve the ionic functionality of the carboxyl groups along the backbone. Recent advances in analytical techniques and instrumentation supported the goal-directed modification and functionalization.

Electrospun Alginate Nanofibers Toward Various Applications: A Review

Alginate has been a material of choice for a spectrum of applications, ranging from metal adsorption to wound dressing. Electrospinning has added a new dimension to polymeric materials, including alginate, which can be processed to their nanosize levels in order to afford unique nanostructured materials with fascinating properties. The resulting nanostructured materials often feature high porosity, stability, permeability, and a large surface-to-volume ratio. In the present review, recent trends on electrospun alginate nanofibers from over the past 10 years toward advanced applications are discussed. The application of electrospun alginate nanofibers in various fields such as bioremediation, scaffolds for skin tissue engineering, drug delivery, and sensors are also elucidated.

In situ synthesis of core-shell carbon nanowires as a potent targeted anticoagulant

We have developed a one-pot synthesis of bio-carbon nanowires from the natural product sodium alginate at low temperature, without using any catalyst, for anticoagulation applications. Sodium alginate is carbonized and sulfated/sulfonated in situ by solid state heating of a mixture of sodium alginate and ammonium sulfite. By regulating the heating temperature and the ratio of ammonium sulfite to sodium alginate, we modulated the degree of sulfation/sulfonation and carbonization, as well as the morphology of the carbon nanomaterials. The core-shell sulfated/sulfonated bio-carbon nanowires (CNWs_Alg@SOx) made by the reaction of a mixture of ammonium sulfite and sodium alginate with a mass ratio of 5 (ammonium sulfite to sodium alginate) at 165 °C for 3 h, exhibit strong inhibition of thrombin activity due to their ultrahigh binding affinity towards it (dissociation constant (K_d) = 8.7 × 10^-11 M). The possible formation mechanism of the carbon nanowires has been proposed. The thrombin-clotting time delay caused by CNWs_Alg@SOx is ∼ 170 times longer than that caused by sodium alginate. Hemolysis and cytotoxicity assays demonstrated the high biocompatibility of CNWs_Alg@SOx. Furthermore, the thromboelastography of whole-blood coagulation and rat-tail bleeding assays further reveal that CNWs_Alg@SOx have a much stronger anticoagulation activity than sodium alginate and naturally sulfated polysaccharides (e.g., fucoidan). Our results suggest that the low-temperature prepared, cost-effective, and highly biocompatible CNWs_Alg@SOx show great potential as an efficient anticoagulant for the prevention and treatment of diseases associated with thrombosis.

In vitro and in vivo anticoagulant activity of heparin-like biomacromolecules and the mechanism analysis for heparin-mimicking activity

Heparin-like biomacromolecules (HepLBm), exhibiting similar chemical structure and biological properties to heparin, can be obtained by modifying either synthetic biopolymers or natural biomacromolecules with physical or chemical methods. In this work, a low-cost and biocompatible sodium alginate was chosen as a model biomacromolecule to design anticoagulant HepLBm with a similar sulfation degree to heparin. FTIR, ¹H NMR, and element analysis data were used to confirm the chemical structure of HepLBm. Hemolysis tests, clotting time, complement activation, and contact activation tests were carried out to determine the in vitro anticoagulant activity of HepLBm. In addition, systematic studies of blood cell count, coagulation function, and histopathology were performed to demonstrate the in vivo anticoagulant activity and toxicity of HepLBm with SD rat experiments. Furthermore, a series of linear molecules containing carboxyl groups, sulfonic groups, and hydroxyl groups were selected and their clotting time was tested to provide a mechanism analysis for the excellent anticoagulant activity of HepLBm. With the excellent in vitro/in vivo anticoagulant activity, good biocompatibility, and low cost, the HepLBm synthesized in this work would have great potential for substitution of heparin in many application fields, such as the surface modification of biomedical devices, extracorporeal anticoagulants, and other clinical fields.

An Update on the Use of Alginate in Additive Biofabrication Techniques

BACKGROUND

Solid free forming (SFF) technique also called additive manufacturing process is immensely popular for biofabrication owing to its high accuracy, precision and reproducibility.

METHOD

SFF techniques like stereolithography, selective laser sintering, fused deposition modeling, extrusion printing, and inkjet printing create three dimension (3D) structures by layer by layer processing of the material. To achieve desirable results, selection of the appropriate technique is an important aspect and it is based on the nature of biomaterial or bioink to be processed.

RESULT & CONCLUSION

Alginate is a commonly employed bioink in biofabrication process, attributable to its nontoxic, biodegradable and biocompatible nature; low cost; and tendency to form hydrogel under mild conditions. Furthermore, control on its rheological properties like viscosity and shear thinning, makes this natural anionic polymer an appropriate candidate for many of the SFF techniques. It is endeavoured in the present review to highlight the status of alginate as bioink in various SFF techniques.

Graphene-based advanced nanoplatforms and biocomposites from environmentally friendly and biomimetic approaches

Environmentally friendly and biomimetic approaches to fabricate graphene-based advanced nanoplatforms and biocomposites for biomedical applications are summarized in this review.

Polysaccharide utilization loci of North Sea Flavobacteriia as basis for using SusC/D-protein expression for predicting major phytoplankton glycans

Marine algae convert a substantial fraction of fixed carbon dioxide into various polysaccharides. Flavobacteriia that are specialized on algal polysaccharide degradation feature genomic clusters termed polysaccharide utilization loci (PULs). As knowledge on extant PUL diversity is sparse, we sequenced the genomes of 53 North Sea Flavobacteriia and obtained 400 PULs. Bioinformatic PUL annotations suggest usage of a large array of polysaccharides, including laminarin, α-glucans, and alginate as well as mannose-, fucose-, and xylose-rich substrates. Many of the PULs exhibit new genetic architectures and suggest substrates rarely described for marine environments. The isolates’ PUL repertoires often differed considerably within genera, corroborating ecological niche-associated glycan partitioning. Polysaccharide uptake in Flavobacteriia is mediated by SusCD-like transporter complexes. Respective protein trees revealed clustering according to polysaccharide specificities predicted by PUL annotations. Using the trees, we analyzed expression of SusC/D homologs in multiyear phytoplankton bloom-associated metaproteomes and found indications for profound changes in microbial utilization of laminarin, α-glucans, β-mannan, and sulfated xylan. We hence suggest the suitability of SusC/D-like transporter protein expression within heterotrophic bacteria as a proxy for the temporal utilization of discrete polysaccharides.

High-Performance Filaments from Fractionated Alginate by Polyvalent Cross-Linking: A Theoretical and Practical Approach

A series of alginate fractions with significant differences in molecular weight and uronic acid compositions were produced by consecutive fractionation and converted to thin and strong cross-linked polymer filaments via extrusion into calcium, aluminum, or polyaluminum (PolyAl) polyvalent solutions followed by drawing and drying. Models were elaborated to relate the alginate uronic acid composition to the tensile performance in both the wet gel filament and the dry filament states. The wet gel model was compared to the theory of the unidirectional elongation of charged polyelectrolyte gels based on the classical rubber elasticity of dilated polymer networks, extended to include the contributions of non-Gaussian chain extensions and the effect of electrostatic interactions. The theory of equilibrium swelling pressure was applied to describe the observed shrinkage of the alginate gels following immersion in a polyvalent solution. Congruent with the theoretical model of charged gels, the tensile performance of the gel filaments prepared from CaCl2 depended on the compositional ratio of guluronic acid dyads in the alginate fraction multiplied by the alginate concentration, while the tensile behavior of wet gel filaments prepared by AlCl3 instead resembled that of elastic solid materials and depended only on the alginate concentration. The dry filament tensile properties were greatly dependent on the preparation conditions, particularly the ratio of stress to alginate concentration and the nature of the ions present during filament drawing. The PolyAl solution effectively caused shrinkage of alginate to a strong extent, and the resulting filaments behaved as highly stiff materials able to withstand stresses of approximately 500 MPa and having elastic moduli as high as 28 GPa.

Three dimensional cell printing with sulfated alginate for improved bone morphogenetic protein-2 delivery and osteogenesis in bone tissue engineering

Three-dimensional (3D) cell printing is a unique technique that enables free-form fabrication of cell-laden hydrogel scaffolds with controllable features and interconnected pores for tissue engineering applications. To this end, bioink materials able to offer good printability and favorable cellular interaction are highly required. Herein, we synthesized alginate sulfate, which is a structural mimic of heparin that can strongly bind with growth factors to prolong their activities, and studied its feasibility for cell printing applications. Several bio-inks composed of alginate and alginate-sulfate were studied to characterize their material properties and their utilities in 3D printing. The inclusion of alginate-sulfate in bio-inks (alginate/alginate-sulfate) did not significantly influence their rheological properties and allowed for a good 3D printing processibility with distinct pores and features. Moreover, alginate/alginate-sulfate bio-inks exhibited an improved retention of bone morphogenetic protein 2 in 3D-printed scaffolds. Osteoblastic proliferation and differentiation in vitro were promoted by alginate/alginate-sulfate 3D-printed constructs with an optimal composition of 3% alginate and 2% alginate-sulfate. We envision that bio-inks displaying prolonged interactions with growth factors will be useful for tissue engineering applications including bone regeneration.

Systems for localized release to mimic paracrine cell communication in vitro

Paracrine cell communication plays a pivotal role for signal exchange between proximal cells in vivo. However, this localized, gradient type release of mediators at very low concentrations (pg/ml), relevant during physiological and pathological processes, is rarely reflected within in vitro approaches. This review gives an overview on state-of-the-art approaches, which transfer the paracrine cell-to-cell communication into in vitro cell culture model setups. The traditional methods like trans-well assays and more advanced microfluidic approaches are included. The review focusses on systems for localized release, mostly based on microparticles, which tightly mimic the paracrine interaction between single cells in 3D microenvironments. Approaches based on single microparticles, with the main focus on affinity-controlled storage and release of cytokines, are reviewed and their importance for understanding paracrine communication is highlighted. Various methods to study the cytokine release and their advantages and disadvantages are discussed. Basic principles of the release characteristics, like diffusion mechanisms, are quantitatively described, including the formation of resulting gradients around the local sources. In vitro cell experiments using such localized microparticle release systems in approaches to increase understanding of stem cell behavior within their niches and regulation of wound healing are highlighted as examples of successful localized release systems for mimicking paracrine cell communication.

Mussel-Inspired Synthesis of NIR-Responsive and Biocompatible Ag-Graphene 2D Nanoagents for Versatile Bacterial Disinfections

Pathogenic bacterial infection has been becoming a global threat toward people's health, especially the massive usage of antibiotics due to the lack of antibacterial agents with less side effects. Developing new nanoagents to fight pathogenic bacteria has provided enormous new possibilities in the treatment of bacterial infections, such as graphene-based two-dimensional (2D) antibacterial nanoagents with different bacterial inhibition capabilities; however, mussel-inspired design of near-infrared (NIR)-responsive and biocompatible Ag-graphene nanoagents possessing efficient and versatile bacterial disinfection activities have rarely been reported. In this study, we developed a new kind of antibacterial nanoagent, dopamine-conjugated polysaccharide sulfate-anchored and -protected Ag-graphene (Ag@G-sodium alginate sulfate ((SAS)) nanocomposite, to combat bacterial infection and contamination in different application fields. Ag@G-SAS exhibited robust antibacterial activity toward both Escherichia coli and Staphylococcus aureus; notably, the nanoagent can significantly inhibit S. aureus infection on wounded pig skin without or with NIR laser. Besides wound disinfection, the 2D Ag@G-SAS can also serve as a good layer-by-layer (LbL) building block for the construction of self-sterilizing coatings on biomedical devices. All of the results verified that the LbL-assembled Ag@G-SAS coating exhibited favorable bactericidal activity, extraordinary blood compatibilities, and good promotion ability for cell proliferation. Owing to the shielding effects of heparin-like polysaccharide sulfates, the Ag@G-SAS nanoagent showed limited cytotoxicity toward mammalian cells. Combining all of the advantages mentioned above, it is believed that the proposed Ag@G-SAS nanoagent and its LbL-assembled coatings may have versatile application potentials to avoid bacterial contaminations in different fields, such as wounded skin, disinfection of biomedical implants and devices, and food packaging sterilization.