Hyaluronic acid chemistry and biomedical material design.

Silk fibroin has applications in different medical fields such as tissue engineering, regenerative medicine, drug delivery and medical devices. Advances in silk chemistry and biomaterial designs have yielded exciting tools for generating new silk-based materials and technologies. Selective chemistries can enhance or tune the features of silk, such as mechanics, biodegradability, processability and biological interactions, to address challenges in medically relevant materials (hydrogels, films, sponges and fibres). This Review details the design and utility of silk biomaterials for different applications, with particular focus on chemistry. This Review consists of three segments: silk protein fundamentals, silk chemistries and functionalization mechanisms. This is followed by a description of different crosslinking chemistries facilitating network formation, including the formation of composite biomaterials. Utility in the fields of tissue engineering, drug delivery, 3D printing, cell coatings, microfluidics and biosensors are highlighted. Looking to the future, we discuss silk biomaterial design strategies to continue to improve medical outcomes.

The microenvironment of neuron cells plays a crucial role in regulating neural development and regeneration. Hyaluronic acid (HA) biomaterial has been applied in a wide range of medical and biological fields and plays important roles in neural regeneration. PC12 cells have been reported to be capable of endogenous NGF synthesis and secretion. The purpose of this research was to assess the effect of HA biomaterial combining with PC12 cells conditioned media (PC12 CM) in neural regeneration. Using SH-SY5Y cells as an experimental model, we found that supporting with PC12 CM enhanced HA function in SH-SY5Y cell proliferation and adhesion. Through RP-nano-UPLC-ESI-MS/MS analyses, we identified increased expression of HSP60 and RanBP2 in SH-SY5Y cells grown on HA-modified surface with cotreatment of PC12 CM. Moreover, we also identified factors that were secreted from PC12 cells and may promote SH-SY5Y cell proliferation and adhesion. Here, we proposed a biomaterial surface enriched with neurotrophic factors for nerve regeneration application.

Nanoconstructs, such as liposomes, polymeric micelles, gold nanoparticles, carbon nanomaterials and nanocrystalline quantum dots, have been widely investigated for diagnostic, bioimaging and therapeutic applications [1]. Nanoconstructs have been also extensively explored in the field of dermatology. For example, liposomes and polymeric nanoparticles loaded with drugs were applied as topical administration agents for the treatment of skin diseases such as psoriasis, dermatitis and skin cancer [2,3]. Titanium dioxide and zinc oxide (ZnO) nanoparticles have been used as sun-screen formulation for the protection of skin from UV by the scattering, absorption and reflection of UV [4]. Silver nanoparticles are commercially used as wound and burn dressing agents with antibacterial effects [5]. Quantum dots and ZnO nanoparticles were utilized as bioimaging agents for the diagnosis of skin diseases [6,7]. Moreover, gold nanoconstructs and carbon nanomaterials have been actively investigated as promising agents for the photothermal ablation therapy of skin cancers due to their high light-to-heat conversion capability [8,9]. For further applications of nanoconstructs in dermatology, we need efficient transdermal delivery carriers of the nanoconstructs. The transdermal delivery has several advantages over other administration routes, such as oral delivery and needle based injection. The advantages include noninvasive treatment, self-administration, improved patient compliance and avoidance of hepatic first-pass metabolism or digestion system [10]. Despite these benefits, the low skin permeability of nanoconstructs such as polymers, proteins, hydrophilic drugs and nanoparticles limited their wide applications to the transdermal delivery. To facilitate the transdermal delivery of nanoconstructs, additional treatments have been adopted using penetration enhancers, iontophoresis, ultrasound and microneedles [11]. However, these methods require physical perturbations to the skin tissue, causing skin damage in some cases [12]. A noninvasive molecular carrier for transdermal delivery would have compelling advantages. The understanding for the characteristics of skin layers can be a good starting point for the development of transdermal delivery carriers of nanoconstructs. Stratum corneum (SC), the outermost skin layer, is the main barrier composed of densely packed dead cells forming hydrophobic surfaces. For the penetration of hydrophobic SC, hydrophobic molecules have clear benefits over hydrophilic molecules. The hydrophobic molecules can infiltrate into densely packed lipid layers in SC. However, nanoconstructs are usually formulated to have hydrophilic surfaces enhancing the physiological stability in the biological conditions. This contradicting requirement of hydrophobicity and hydrophilicity for transdermal delivery should be reconciled to enhance the physiological stability and the skin permeability by lipid disruption in SC. Recently, hyaluronic acid (HA) has been investigated as a promising transdermal delivery carrier. HA is a naturally occurring linear polysaccharide composed of repeating units of d-glucuronic acid and N-acetylEnhancing the transdermal penetration of nanoconstructs: could hyaluronic acid be the key?

Patients have benefited greatly from the steady stream of new medical devices approved each year by the US Food and Drug Administration and by regulatory agencies in countries around the world. But the number of such devices for tissue engineering and regenerative medicine is just a small fraction of these, perhaps 7 of the 194 devices (<5%) approved by the FDA since 2008 [1]. Few new tissue engineering and regenerative medicine devices have reached the clinic despite the proliferation of biomaterial matrices/scaffolds, cells, and regulators of cell function (e.g., growth factors); i.e., the three types of tool currently available for implantation or injection into a specific type of defect to facilitate regeneration (the paradigm of regenerative medicine), or for the formation of tissues and organs in vitro for subsequent implantation (the tissue engineering plan). While the number of new tools added to the tissue engineer's toolbox each year is continuing to grow dramatically, there are few tools being implemented for the production of new medical devices undergoing human trial (figure 1). What are the bottlenecks in getting the tools out of the toolbox and into clinical use to advance patient care?

The physiological functions of hyaluronan (HA) in the extracellular matrix of vertebrate tissues involve a range of specific protein interactions. In this study, the interaction of HA with the Link module from TSG-6 (Link_TSG6) and G1 domain of aggrecan (G1), were investigated by a biophysical analysis of translational diffusion in dilute solution using confocal fluorescence recovery after photobleaching (confocal FRAP). Both Link_TSG6 and G1 were shown to bind to polymeric HA and these interactions could be competed with HA(8) and HA(10) oligosaccharides, respectively. Equilibrium experiments showed that the binding affinity of Link_TSG6 to HA was maximal at pH 6.0, and reduced dramatically above and below this pH. In contrast, G1 had maximum binding at pH 7.0-8.0 and moderate to strong binding affinity over a much broader pH range (5.5-8.0). The K(D) determined for Link_TSG6 binding to HA showed a 100-fold increase in binding affinity between pH 7.4 and 6.0, whereas G1 showed a 75-fold decrease in binding affinity over the same pH range. The sharp difference observed in their pH binding suggests that pH controls the physiological function of TSG-6, with a low affinity for HA at neutral pH, but with increased affinity as the pH falls below pH 7. TSG-6 and aggrecan interact with HA through structurally homologous domains and the difference in pH-dependent binding can be understood in terms of differences in the presence and topographical distribution of key regulatory amino acids in Link_TSG6 and in the related tandem Link domains in aggrecan G1.

Application of hyaluronic acid in tissue engineering, regenerative medicine, and nanomedicine: A review

Hyaluronic acid (HA) is a natural, linear, endogenous polysaccharide that plays important physiological and biological roles in the human body. Nowadays, among biopolymers, HA is emerging as an appealing starting material for hydrogels design due to its biocompatibility, native biofunctionality, biodegradability, non-immunogenicity, and versatility. Since HA is not able to form gels alone, chemical modifications, covalent crosslinking, and gelling agents are always needed in order to obtain HA-based hydrogels. Therefore, in the last decade, different strategies for the design of physical and chemical HA hydrogels have been developed, such as click chemistry reactions, enzymatic and disulfide crosslinking, supramolecular assembly via inclusion complexation, and so on. HA-based hydrogels turn out to be versatile platforms, ranging from static to smart and stimuli-responsive systems, and for these reasons, they are widely investigated for biomedical applications like drug delivery, tissue engineering, regenerative medicine, cell therapy, and diagnostics. Furthermore, the overexpression of HA receptors on various tumor cells makes these platforms promising drug delivery systems for targeted cancer therapy. The aim of the present review is to highlight and discuss recent advances made in the last years on the design of chemical and physical HA-based hydrogels and their application for biomedical purposes, in particular, drug delivery. Notable attention is given to HA hydrogel-based drug delivery systems for targeted therapy of cancer and osteoarthritis.

Property-tailoring chemical modifications of hyaluronic acid for regenerative medicine applications.

Hyaluronic acid (HA) is crucial in regulating stem cells and enhancing their therapeutic efficacy in chronic inflammatory diseases such as interstitial cystitis/bladder pain syndrome )IC/BPS(. This study aimed to explore the potential of HA as a biomaterial for optimizing stem cell-based therapies in the treatment of IC/BPS. Due to its biocompatibility and bioactivity, HA serves as a supportive matrix that improves stem cell retention, survival, and function. Additionally, HA modulates stem cell behavior, promoting regeneration and anti-inflammatory response, which are essential for repairing the damaged bladder lining in animals. Its intrinsic anti-inflammatory properties further contribute to reducing inflammation and creating a favorable microenvironment for mesenchymal stem cells (MSCs). Furthermore, HA facilitates the controlled release of MSCs and other therapeutic agents, extending their benefits for chronic conditions like IC/BPS. The wide-ranging applications of HA in both animal models and human research underscore its potential as a therapeutic agent for various medical conditions. Preclinical studies have shown that HA supports tissue regeneration, reduces inflammation, and enhances stem cell retention, making it a valuable biomaterial for treating bladder inflammation, liver fibrosis, and cardiovascular disorders. In clinical settings, HA has been effectively applied in regenerative medicine, osteoarthritis management, wound healing, and drug delivery, demonstrating its biocompatibility and therapeutic effectiveness. These insights highlight HA’s role in translating experimental findings into clinical applications, paving the way for improved treatment approaches for chronic and inflammatory diseases. Overall, HA holds significant potential in enhancing the efficacy and long-term therapeutic outcomes of MSC-based treatments for chronic bladder disorders.

On behalf of the JALA scientific advisors and JALA editorial board, I am happy to present this year’s honorees of the prestigious JALA Ten. Each year, JALA seeks to highlight and honor the very best work of the year that will have a deep impact on how technology is used across a wide range of disciplines, including automation, life sciences and biomedical research, diagnostics, drug delivery, and regenerative medicine. Implementing the latest advances in microfluidics, nanotechnology, materials science, and other fields of research, this year’s JALA Ten honorees have and will continue to change the way research is performed and the way diseases are diagnosed and treated. As such, the work highlighted here should have far-reaching impact in our everyday lives. It demonstrates the promise that science brings toward a better future. While a number of areas of research will feel the impact of this year’s JALA Ten, one highlight of the collection is the diverse ways in which the honorees have advanced biological molecule detection and established foundations for tomorrow’s biosensors in life sciences research and medical diagnostics.1Lillehoj P.B. Kaplan C.W. He J. et al.Rapid, Electrical Impedance Detection of Bacterial Pathogens Using Immobilized Antimicrobial Peptides.J. Lab. Autom. 2014; 19: 42-49Google Scholar, 2Lee S.E. Chen Q. Bhat R. et al.Reversible Aptamer-Au Plasmon Rulers for Secreted Single Molecules.Nano Lett. 2015; 15: 4564-4570Google Scholar, 3Yeo D. Wiraja C. Chuah Y.J. et al.A Nanoparticle-Based Sensor Platform for Cell Tracking and Status/Function Assessment.Sci. Rep. 2015; 5 (DOI: 10.1038/srep14768.): 14768Google Scholar For example, Peter Lillehoj at Michigan State University (USA) and his collaborators have developed a microfluidic biosensor using immobilized antimicrobial peptides for highly specific, multiplexed detection of bacterial pathogens.1Lillehoj P.B. Kaplan C.W. He J. et al.Rapid, Electrical Impedance Detection of Bacterial Pathogens Using Immobilized Antimicrobial Peptides.J. Lab. Autom. 2014; 19: 42-49Google Scholar Showing both high pathogen detection specificity and accurate pathogen quantification, this work is an example of how microfluidic technology is changing how medical professionals will track and diagnose infectious diseases. On the other end of the spectrum, Somin Eunice Lee at the University of Michigan (USA) has harnessed the power of nanotechnology to develop a gold nanoparticle–based plasmon ruler that is capable of single-molecule measurements.2Lee S.E. Chen Q. Bhat R. et al.Reversible Aptamer-Au Plasmon Rulers for Secreted Single Molecules.Nano Lett. 2015; 15: 4564-4570Google Scholar Even more impressive is the application of this plasmon ruler toward the accurate detection and measurement of secreted single molecules in the cellular microenvironment. Understanding how cells communicate with their cellular microenvironment remains a challenge to study. Tools such as Lee’s plasmon ruler vastly improve life sciences researchers’ abilities to learn more about the interplay between cells and their microenvironments. Another area of research highlighted this year is the advancement of technology toward customizable platforms for increased personalized clinical and research applications.4Griffin D.R. Weaver W.M. Scumpia P.O. et al.Accelerated Wound Healing by Injectable Microporous Gel Scaffolds Assembled from Annealed Building Blocks.Nat. Mater. 2015; 14 (DOI: 10.1038/nmat4294.): 737-744Google Scholar, 5Huang P.H. Nama N. Mao Z. et al.A Reliable, Programmable Acoustofluidic Pump Powered by Oscillating Sharp-Edge Structures.Lab Chip. 2014; 14: 4319-4323Google Scholar, 6Zhang Y.S. Ribas J. Nadhman A. et al.A Cost-Effective Fluorescence Mini-Microscope with Adjustable Magnifications for Biomedical Applications.Lab Chip. 2015; 15: 3661-3669Google Scholar, 7Bhargava K.C. Thompson B. Malmstadt N. et al.Discrete Elements for 3D Microfluidics.Proc. Natl. Acad. Sci. U.S.A. 2014; 111: 15013-15018Google Scholar The development of tunable injectable microporous gel scaffolds by researchers at the University of California, Los Angeles (USA) not only brings truly personalized medicine to regenerative medicine, but also allows researchers to quickly optimize culture conditions in research and drug development applications that require complex three-dimensional (3D) cell cultures.4Griffin D.R. Weaver W.M. Scumpia P.O. et al.Accelerated Wound Healing by Injectable Microporous Gel Scaffolds Assembled from Annealed Building Blocks.Nat. Mater. 2015; 14 (DOI: 10.1038/nmat4294.): 737-744Google Scholar The development of integrated platforms composed of modular technologies increases the diversity of clinical and research applications to which technology such as microfluidics can be applied and is highlighted here by work from Harvard University (USA)6Zhang Y.S. Ribas J. Nadhman A. et al.A Cost-Effective Fluorescence Mini-Microscope with Adjustable Magnifications for Biomedical Applications.Lab Chip. 2015; 15: 3661-3669Google Scholar and the University of Southern California (USA).7Bhargava K.C. Thompson B. Malmstadt N. et al.Discrete Elements for 3D Microfluidics.Proc. Natl. Acad. Sci. U.S.A. 2014; 111: 15013-15018Google Scholar Beyond building better biosensors and more effective and cost-efficient devices, work featured this year includes improvements in automation of large-cargo intracellular delivery, as well as innovating the use of inorganic and biomaterials in medical applications.8Wu Y.-C. Wu T.-H. Clemens D.L. et al.Massively Parallel Large Cargo Delivery into Mammalian Cells with Light Pulses.Nat. Methods. 2015; 12: 439-444Google Scholar, 9Dai X. Stogin B.B. Yang S. et al.Slippery Wenzel State.ACS Nano. 2015; 9: 9260-9267Google Scholar, 10Lu Y. Hu Q. Lin Y. et al.Transformable Liquid-Metal Nanomedicine.Nat. Commum. 2015; 6 (DOI: 10.1038/ncomms10066.): 10066Google Scholar This year’s honorees represent the best in research that advances translational science and technology, and we are proud to honor these researchers for their amazing work. JALA and SLAS would like to thank all the nominees as well as those who nominated them. We would also like to thank everyone who worked to discuss and select the 2016 JALA Ten. The last 6 years have allowed us to highlight innovative and exciting research breakthroughs that will greatly impact our lives. This year is no different and continues to raise the bar for innovation. We look forward to seeing what next year brings. Current sensing technologies for quantifying microbial pathogens rely on highly specific antibodies for molecular recognition that suffer from limited stability in high-temperature environments and can be difficult to obtain for similar pathogen species. Using synthetic antimicrobial peptides (AMPs) with species-specific targeting and binding capabilities, Peter Lillehoj of Michigan State University (USA) and his collaborators have developed a microfluidic biosensor for rapid, multiplexed detection of bacterial pathogens. AMP-coated sensors demonstrate strong preferential binding to their corresponding targeted cells with negligible cross-binding (Fig. 1) and impedance measurements correlated well to the cell concentration. With further development, this technology can provide a robust, portable platform for rapid pathogen detection. Tracking and studying single molecules in living cells is of great interest to life sciences researchers; however, the tools to do so are highly limited. Even more difficult is studying the function of single molecules following cell secretion in order to better understand how these molecules function in cellular microenvironments. Somin Eunice Lee at the University of Michigan (USA) and her colleagues have developed an aptamer-gold plasmon ruler that now achieves reliable detection and study of secreted single molecules (Fig. 2). Furthermore, this plasmon ruler is reversible, allowing for detection of multiple events. High specificity of this reversible plasmon ruler is demonstrated with the specific detection of the matrix metalloproteinase MMP3 over its family member MMP9. Implementation of this plasmon ruler into life sciences and biomedical research should allow for greater understanding of the functions of secreted molecules in the microenvironment. These plasmon rulers should also find use in phenotypic drug discovery and other translational applications. Even with the increasing popularity of phenotypic research methods such as high-content applications and the emergence of cell-based therapies, there is a dearth of cell labeling and tracking reagents that can reliably and quantifiably sense key cellular functions. Chenjie Xu and his colleagues at Nanyang Technological University (Singapore) have developed a nanoparticle-based platform for just this purpose. Encapsulating specific biosensor molecules in biodegradable polymeric nanoparticles, these nanosensors are able to serve as an intracellular source of sensor molecules for up to 30 days within cells (Fig. 3). This platform has a wide range of biomedical applications, as demonstrated by this team. Utilizing their nanosensors, they demonstrate real-time quantification of cellular processes such as nitric oxide production, as well as gene expression such as β-actin mRNA expression. The development of these nanosensors should serve as the foundation for a wide range of tools for molecular biology research, as well as phenotypic drug discovery. Scaffold materials for tissue engineering and 3D matrices for organ-on-a-chip technologies have been hampered by a fundamental limitation in the ability to control material porosity separately from mechanical properties. This limitation is especially exacerbated when considering flowable biomaterials, which are useful for in vivo delivery, filling of wounds, or encapsulation of cells. Dino Di Carlo and Tatiana Segura and their teams at the University of California, Los Angeles (USA) have overcome these limitations with the development of a tunable injectable microporous gel scaffold (Fig. 4). Microporosity is important in a biomaterial, as it improves transport within a scaffold and enables rapid cellular ingrowth. This work demonstrates the first approach to achieve an injectable microporous material for tissue engineering, and this material promoted cellular growth in vitro and accelerated healing in vivo. Because of the modularity and tenability of these microporous annealed particle (MAP) gels, they should impact a wide range of regenerative medical applications, as well as drug development applications that require 3D cell culture or organ-on-a-chip methods of study. Tony Jun Huang’s research group at the Pennsylvania State University (USA) demonstrate a microfluidic pump, so-called sharp-edge-based acoustofluidic pump, that utilizes the acoustic streaming effects induced by acoustically oscillating tilted sharp-edge structures. Their pump is simply composed of a quarter-sized piezoelectric transducer and a microfluidic channel (Fig. 5). Upon the oscillation, the tilted sharp-edge structures generate acoustic streaming effects, which in turn generate net forces in the direction that the sharp-edge structured oriented. Thus, fluid pumping takes place because the generated net forces push the bulk fluid to flow forward. By simply modulating the driving signals to the piezoelectric transducer, the pump can generate not only flow rates ranging from nanoliters to several microliters per minute, but also flow rates of various flow profiles. Thus, the acoustofluidic pump has the potential to be integrated into this kind of portable testing platform, because of its small size, easy fabrication, controllability, and tunability. Microscopes remain a key component of any life sciences research; however, they remain expensive and are often limited in specific functions when made more affordable. Y. S. Zhang from Harvard University (USA) has innovated a miniature microscope with built-in fluorescence capability for biomedical applications (Fig. 6). This mini-microscope has adjustable magnifications from 8× to 60× and achieves a high resolution of <2 μm. Using off-the-shelf components and a webcam, the mini-microscope system is inexpensive (<$10), and its modularity allows for convenient integration with a wide variety of preexisting platforms, such as cell culture plates and microfluidic devices. Therefore, this mini-microscope is likely to find widespread applications in cell biology, tissue engineering, biosensing, microfluidics, and organs-on-chips, which may potentially replace conventional benchtop microscopy where long-term in situ and high-throughput imaging and analysis are required. Traditionally, microfluidic systems have been designed as monolithic devices, each with a single dedicated application. Noah Malmstadt and his team at the University of Southern California (USA) have developed a modular microfluidic platform where each module can be designed with discrete fluid handling, routing, or analysis function (Fig. 7). These modules are assembled together to create an application-defined microfluidic circuit. A key benefit of this approach is design predictability. The behavior of the assembled system can be predicted by circuit design methodologies because each module has well-understood fluidic characteristics. This modular approach to microfluidic circuit design allows for a truly 3D microfluidic design rather than less efficient and more complex planar layer-by-layer arrangements of channels. Additionally, because individual models are manufactured by 3D printing, fabrication is simpler and cheaper. This work represents a shift in microfluidic design that should allow microfluidic devices to be incorporated in a wider range of commercial applications. While there are a number of well-established methods for delivery of small-scale cargo, such as kilobit-sized nucleic acids, reliably delivering large cargo into cells has remained elusive. Developing a method for large-cargo delivery in a high-throughput manner is an even more difficult hurdle to overcome. Eric Pei-Yu Chiou and his team at the University of California, Los Angeles (USA) have accomplished such a feat with their new intracellular delivery platform called biophotonic laser-assisted surgery tool (BLAST) (Fig. 8). BLAST enables the delivery of cargo up to several microns in size into 100,000 cells in 1 min, which is five orders of magnitude faster than prior methods. Cargo is delivered into the cytosol of cells directly without undesirable endosome trapping. High efficiency, high cell viability, and nearly simultaneous delivery of diverse types of cargo into numerous cells under constant physiological conditions allow reliable measurements in a variety of biological settings. Because of the reliability of the high-throughput platform and the increase in cargo size, BLAST can be applied to a broad range of applications, including delivery of large nanoparticle complexes, functional proteins, and large amounts of both DNA and RNA. As such, this platform should improve life sciences research, from basic molecular biological research to drug development and biomedical clinical research. The ability to repel liquids regardless of how they wet the surface has important technological implications for numerous industrial and biomedical processes, ranging from condensation heat transfer to water harvesting to antifouling of medical devices. However, maintaining liquid mobility on engineered surfaces under various conditions has been an engineering challenge for more than a decade. Now, researchers at the Pennsylvania State University (USA) led by Tak-Sing Wong have invented a new class of liquid-repellent surface, known as slippery rough surface, which can repel liquids in any state of wetness for the first time (Fig. 9). Their surfaces, modeled after lotus leaves and the pitcher plant, have been developed by engineering hierarchical nano- and microscale textures and infusing liquid lubricant into the nanotextures to create a highly slippery rough surface. The new surface may open up new opportunities for scientific studies and engineering applications related to wetting, adhesion, transport phenomena, and biofouling. While the use of inorganic nanomaterials in drug delivery remains an area of active research in nanomedicine, clinical translation of inorganic material-based drug delivery systems has been difficult due to their toxicity and clearance failures. A promising new approach that overcomes these hurdles was developed by Zhen Gu and his team at the University of North Carolina at Chapel Hill (USA) and North Carolina State University (USA). Utilizing a liquid-phase eutectic gallium-indium core and a thiolated polymeric shell, a transformable liquid-metal-based nanosphere drug delivery particle is formed (Fig. 10). When functionalized with hyaluronic acid, these drug delivery complexes are capable of efficiently delivering chemotherapeutics and more effectively inhibiting tumor growth than conventional chemotherapeutics. Degradable in mildly acidic environments, this new liquid-metal-based approach opens new avenues to exploring the use of inorganic materials in a variety of nanomedical applications.

The unique role of hyaluronan (HA) in mammal life can be easily understood considering that the first reaction in mammals involves this molecule. In fact, the spermatozoa, before fertilization, use a specific hyaluronidase to digest a large stratum of HA surrounding the oocyte in order to reach it. HA is an extracellular matrix polymer with extraordinary structure and functions: it is a simple, linear, and unbranched polymer chain without sulfate or phosphate groups. Nevertheless, it has a key role in several physiological and pathophysiological processes in mammals. HA is ubiquitous in mammalian tissues with several specific functions, such as influencing cell proliferation and migration, angiogenesis, and inflammation. Considering the simple structure of HA, to exert several important functions in tissues, this polymer can only be modified in its concentration and size. Hence, HA content in tissues is carefully controlled by different mechanisms, including covalent modification of the synthetic enzymes and epigenetic control of their gene expression. HA function is also critical in several diseases including cancer, diabetes, and chronic inflammation. Among these biological roles, the biophysical properties of HA allow its use as a hydrogel in regenerative medicine, cosmetics, and drug delivery. HA takes advantage from its capacity to form gels even at the concentration of 1% producing non-immunogenic scaffolds with very intriguing mechanical properties. The HA hydrogels are useful tools in regenerative medicine as biocompatible material for advanced therapeutic uses, also as drug delivery system.

Hyaluronic acid-based multifunctional carriers for applications in regenerative medicine: A review

This article reports tunable crosslinking, reversible phase transition, and three-dimensional printing (3DP) of hyaluronic acid (HyA) hydrogels via dynamic coordination of Fe3+ ions with their innate carboxyl groups for the first time. The concentrations of Fe3+ and H+ ions and the reaction time determine the tunable ratios of mono-, bi-, and tridentate coordination, leading to the low-to-high crosslinking densities and reversible solid-liquid phase transition of HyA hydrogels. At the monodentate-dominant coordination, the liquid hydrogels have low crosslinking densities (HyA_L). At the mixed coordination of mono-, bi-, and tridentate bonding, the solid hydrogels have medium crosslinking densities (HyA_M). At the tridentate-dominant coordination, the solid hydrogels have high crosslinking densities (HyA_H). The reversible solid-liquid phase transitions among HyA_L, HyA_M, and HyA_H were achieved via controlling the concentrations of Fe3+ and H+ ions and reaction time. When the crosslinking densities are between HyA_L and HyA_M, the hydrogels become 3D printable (HyA_P). HyA_P hydrogels were 3D-printed successfully using cold-stage or direct writing methods, and the 3D constructs achieved better structural stability using the latter method. In the direct exposure culture with bone marrow-derived mesenchymal stem cells, the 3D-printed HyA_H (HyA_H_3D) and HyA_H hydrogels showed higher average cell adhesion densities than the HyA_M, HyA_P, and HyA_L hydrogel groups under both direct and indirect contact conditions. For all hydrogel groups, cell adhesion densities under direct contact conditions were statistically lower than the same groups under indirect contact conditions. In this article, we elucidated the mechanisms of dynamic coordination and the relationships among the key parameters in controlling the tunable crosslinking, reversible phase transition, and 3DP of HyA hydrogels without blending with other polymers or adding functional groups. This approach can be potentially adapted to crosslink and 3D print other polymeric hydrogels with carboxyl groups, which is promising for a wide range of applications.

Development of hyaluronic acid-silica composites via in situ precipitation for improved penetration efficiency in fast-dissolving microneedle systems

Cancer cell migration in collagen-hyaluronan composite extracellular matrices