A novel injectable chitosan/polyglutamate polyelectrolyte complex hydrogel with hydroxyapatite for soft-tissue augmentation

Self-Assembled Hydrogel Based on (Bio)polyelectrolyte Complex of Chitosan-Gelatin: Effect of Composition on Physicochemical Properties

Taking into account the trends in the field of green chemistry and the desire to use natural materials in biomedical applications, (bio)polyelectrolyte complexes ((bio)PECs) based on a mixture of chitosan and gelatin seem to be relevant systems. Using the approach of self-assembly from the dispersion of the coacervate phase of a (bio)PEC at different ratios of ionized functional groups of chitosan and gelatin (z), hydrogels with increased resistance to mechanical deformations and resorption in liquid media were obtained in this work in comparison to a hydrogel from gelatin. It was found that at z ≥ 1 a four-fold increase in the elastic modulus of the hydrogel occurred in comparison to a hydrogel based on gelatin. It was shown that hydrogels at z ≈ 1 had an increased sorption capacity and water sorption rate, as well as increased resistance to the in vitro model environment of phosphate-buffered saline (PBS) solution containing lysozyme at 37 °C. It was also shown that in PBS and simulated gastric fluid (SGF) solutions, the effect of the polyelectrolyte swelling of the hydrogels was significantly suppressed; however, at z ≥ 1, the (bio)PEC hydrogels had increased stability compared to the samples at z < 1 and based on gelatin.

Hydrogel Based on Riclin Cross-Linked with Polyethylene Glycol Diglycidyl Ether as a Soft Filler for Tissue Engineering

Hydrogels composed of natural polysaccharides have been widely used as filling materials, with a growing interest in medical cosmetology and skin care. However, conventional commercial dermal fillers still have limitations, particularly in terms of mechanical performance and durability in vivo. In this study, a novel injectable and implantable hydrogel with adjustable characteristics was prepared from succinoglycan riclin by introducing PEG diglycidyl ether as a cross-linker. FTIR spectra confirmed the cross-linking reaction. The riclin hydrogels exhibited shear-thinning behavior, excellent mechanical properties, and cytocompatibility through in vitro experiments. Furthermore, when compared with subcutaneous injection of a commercial hyaluronic acid hydrogel, the riclin hydrogels showed enhanced persistence and biocompatibility in Balb/c mice after 16 weeks. These results demonstrate the great potential of the riclin-based hydrogel as an alternative to conventional commercial soft tissue fillers.

pH-Sensitive Poly(acrylic acid)-g-poly(L-lysine) Charge-Driven Self-Assembling Hydrogels with 3D-Printability and Self-Healing Properties

We report the rheological behavior of aqueous solutions of a graft copolymer polyampholyte, constituted of polyacrylic acid (PAA) backbone grafted by Poly(L-lysine) (PAA-b-PLL). The graft copolymer self-assembles in aqueous media, forming a three-dimensional (3D) network through polyelectrolyte complexation of the oppositely charged PAA and PLL segments. Rheological investigations showed that the hydrogel exhibits interesting properties, namely, relatively low critical gel concentration, elastic response with slow dynamics, remarkable extended critical strain to flow, shear responsiveness, injectability, 3D printability and self-healing. Due to the weak nature of the involved polyelectrolyte segments, the hydrogel properties display pH-dependency, and they are affected by the presence of salt. Especially upon varying pH, the PLL secondary structure changes from random coil to α-helix, affecting the crosslinking structural mode and, in turn, the overall network structure as reflected in the rheological properties. Thanks to the biocompatibility of the copolymer constituents and the biodegradability of PLL, the designed gelator seems to exhibit potential for bioapplications.

Chemical stabilization of γ-polyglutamate by chitosan and the effect of co-solvents on the stability

In protein-based formulations, conformational distortions and attractive interactions may cause insoluble and undesired aggregates. In the case of ionic peptides, including cationic or anionic, commonly electrostatic interactions are the main factors that control structure assembling. In this study, it was proposed that grafting of chitosan (CS) to γ-polyglutamic acid (γ-PGA) might exhibit much strong inhibiting effect on the formation of protein aggregates due to multiple amino groups and hydrophilic properties. To guarantee stable and safe biopharmaceutical formulation, the potency of a variety of stabilizers including sugars (glucose, sucrose), polyols (sorbitol, glycerol), surfactant (Tween 20), salting-out salt (PBS), and also different pH values have been evaluated on stabilizing or destabilizing the native state of CS-g-PGA copolymer using FTIR, CD, DLS, and SDS-PAGE. The comparable analysis revealed that the stability of CS-g-PGA was strongly dependent on pH owing to the polyelectrolyte characteristics of the polymers. Altogether these results implied that CS at optimized conditions might be an important precursor for the pharmaceutical industry and function as a new polymer for aggregation suppression and protein stabilization.

Soft Materials by Design: Unconventional Polymer Networks Give Extreme Properties

Hydrogels are polymer networks infiltrated with water. Many biological hydrogels in animal bodies such as muscles, heart valves, cartilages, and tendons possess extreme mechanical properties including being extremely tough, strong, resilient, adhesive, and fatigue-resistant. These mechanical properties are also critical for hydrogels' diverse applications ranging from drug delivery, tissue engineering, medical implants, wound dressings, and contact lenses to sensors, actuators, electronic devices, optical devices, batteries, water harvesters, and soft robots. Whereas numerous hydrogels have been developed over the last few decades, a set of general principles that can rationally guide the design of hydrogels using different materials and fabrication methods for various applications remain a central need in the field of soft materials. This review is aimed at synergistically reporting: (i) general design principles for hydrogels to achieve extreme mechanical and physical properties, (ii) implementation strategies for the design principles using unconventional polymer networks, and (iii) future directions for the orthogonal design of hydrogels to achieve multiple combined mechanical, physical, chemical, and biological properties. Because these design principles and implementation strategies are based on generic polymer networks, they are also applicable to other soft materials including elastomers and organogels. Overall, the review will not only provide comprehensive and systematic guidelines on the rational design of soft materials, but also provoke interdisciplinary discussions on a fundamental question: why does nature select soft materials with unconventional polymer networks to constitute the major parts of animal bodies?

Supramolecular Structuring of Hyaluronan-Lactose-Modified Chitosan Matrix: Towards High-Performance Biopolymers with Excellent Biodegradation

Non-covalent interactions in supramolecular chemistry provide useful systems to understand biological processes, and self-assembly systems are suitable assets to build-up innovative products for biomedical applications. In this field, polyelectrolyte complexes are interesting, especially when polysaccharides are involved, due to their non-toxicity and bio-absorbability. In this work, we investigated a polyelectrolyte formed by hyaluronic acid (HA), a negatively charged linear polysaccharide, with Chitlac (Ch), a positively charged lactose-modified chitosan. The aim of the study was the investigation of a novel Ch–HA polyelectrolyte complex, to understand the interaction between the two polysaccharides and the stability towards enzymatic activity. By means of gel permeation chromatography–triple detector array (GPC–TDA), nuclear magnetic resonance (NMR), dynamic viscosity, Zeta Potential and scanning electron microscopy (SEM), the polyelectrolyte complex properties were identified and compared to individual polysaccharides. The complex showed monodisperse molecular weight distribution, high viscosity, negative charge, and could be degraded by specific enzymes, such as hyaluronidase and lysozyme. The results suggest a close interaction between the two polysaccharides in the complex, which could be considered a self-assembly system.

Laser Coatings via State-of-the-Art Additive Manufacturing: A Review

Ceramics and ceramic-reinforced metal matrix composites (CMMCs) demonstrate high wear resistance, excellent chemical inertness, and exceptional properties at elevated temperatures. These characteristics are suitable for their utilization in biomedical, aerospace, electronics, and other high-end engineering industries. The aforementioned performances make them difficult to fabricate via conventional manufacturing methods, requiring high costs and energy consumption. To overcome these issues, laser additive manufacturing (LAM) techniques, with high-power laser beams, were developed and extensively employed for processing ceramics and ceramic-reinforced CMMCs-based coatings. In respect to other LAM processes, laser melting deposition (LMD) excels in several aspects, such as high coating efficiency and lower labor cost. Nevertheless, difficulties such as poor bonding between coating and substrate, cracking, and reduced toughness are still encountered in some LMD coatings. In this article, we review recent developments in the LMD of ceramics and CMMCs-based coatings. Issues and solutions, along with development trends, are discussed and summarized in support of implementing this technology for current industrial use.

Combination Therapy of Killing Diseases by Injectable Hydrogels: From Concept to Medical Applications

The complexity of hard-to-treat diseases strongly undermines the therapeutic potential of available treatment options. Therefore, a paradigm shift from monotherapy toward combination therapy has been observed in clinical research to improve the efficiency of available treatment options. The advantages of combination therapy include the possibility of synchronous alteration of different biological pathways, reducing the required effective therapeutic dose, reducing drug resistance, and lowering the overall costs of treatment. The tunable physical properties, excellent biocompatibility, facile preparation, and ease of administration with minimal invasiveness of injectable hydrogels (IHs) have made them excellent candidates to solve the clinical and pharmacological limitations of present systems for multitherapy by direct delivery of therapeutic payloads and improving therapeutic responses through the formation of depots containing drugs, genes, cells, or a combination of them in the body after a single injection. In this review, currently available methods for the design and fabrication of IHs are systematically discussed in the first section. Next, as a step toward establishing IHs for future multimodal synergistic therapies, recent advances in cancer combination therapy, wound healing, and tissue engineering are addressed in detail in the following sections. Finally, opportunities and challenges associated with IHs for multitherapy are listed and further discussed.

Filling the Gap: A Correlation between Objective and Subjective Measures of Injectability

Various injectable biomaterials are developed for the minimally invasive delivery of therapeutics. Typically, a mechanical tester is used to ascertain the force required to inject these biomaterials through a given syringe-needle system. However, currently there is no method to correlate the force measured in the laboratory to the perceived effort required to perform that injection by the end user. In this article, the injection force (F) for a variety of biomaterials, displaying a range of rheological properties, is compared with the effort scores from a 50 person panel study. The maximum injection force measured at crosshead speed 1 mm s^-1 is a good proxy for injection effort, with an R² of 0.89. This correlation leads to the following conclusions: participants can easily inject 5 mL of substance for F < 12 N; considerable effort is required to inject 5 mL for 12 N < F < 38 N; great effort is required and <5 mL can be injected for 38 N < F < 64 N; and materials are entirely non-injectable for F > 64 N. These values may be used by developers of injectable biomaterials to make decisions about formulations and needle sizes early in the translational process.

Utilization of waste hemicelluloses lye for superabsorbent hydrogel synthesis

A high-performance superabsorbent hydrogel have been successfully fabricated by using waste hemicelluloses lye. Not any extra base was added into the synthesis system for achieving hydrophilic polymer composite. In addition, polyvinyl alcohol (PVA) was added the reaction system to entrap within the hemicelluloses-g-AA/bentonite matrix and form a semi-interpenetrating polymer networks (semi-IPN) for enhancing the swelling properties of the as-prepared polymer composite. SEM, FTIR, and TG were employed to characterize the morphologies, structure, and thermal stability of as-synthesized hydrogel composite. Moreover, liquid absorbency in distilled water and saline solutions, water absorption rate, water retainability, and water reusability of hemicelluloses-g-AA/bentonite (HAB) and hemicelluloses-g-AA/bentonite-PVA (HAB-PVA) hydrogels were also investigated systematically. The adsorption kinetics and isotherms of the composites were studied, and the synergy effect of PVA and bentonite were also proposed. This method provides a new avenue to design the new structure of superabsorbent hydrogel and treat the waste lye in green and sustainable chemical engineering processes.

pH-Sensitive Polyampholyte Microgels of Poly(Acrylic Acid-co-Vinylamine) as Injectable Hydrogel for Controlled Drug Release

pH-sensitive polyampholyte microgels of poly(acrylic acid-co-vinylamine) (P(AA-co-VAm)) were developed as an injectable hydrogel for controlled drug release. The microgels of P(AA-co-VAm) were prepared via inverse suspension polymerization of acrylic acid and N-vinylformamide followed by hydrolysis of poly(N-vinylformamide) (PNVF) chains of the resultant microgels under basic condition. The pH-sensitivity of the P(AA-co-VAm) microgels in zeta potential and swelling ratio were investigated using a zeta potential analyzer and optical microscope. The results showed that both the zeta potential and the swelling ratio of the microgels were highly affected by the solution pH. By changing the pH of P(AA-co-VAm) microgel dispersion, the interparticle interaction and the swelling ratio of the microgels could be well adjusted and a colloidal hydrogel could be fabricated at moderate pH, showing a pH-triggered reversible fluid-gel transition. Using the polyampholyte P(AA-co-VAm) microgels as an injectable hydrogel drug release system, a sustained drug release could be achieved, indicating the great potentials of the pH-sensitive P(AA-co-VAm) microgels for controlled drug delivery.

Influence of the Soluble⁻Insoluble Ratios of Cyclodextrins Polymers on the Viscoelastic Properties of Injectable Chitosan⁻Based Hydrogels for Biomedical Application

Injectable pre-formed physical hydrogels provide many advantages for biomedical applications. Polyelectrolyte complexes (PEC) formed between cationic chitosan (CHT) and anionic polymers of cyclodextrin (PCD) render a hydrogel of great interest. Given the difference between water-soluble (PCDs) and water-insoluble PCD (PCDi) in the extension of polymerization, the present study aims to explore their impact on the formation and properties of CHT/PCD hydrogel obtained from the variable ratios of PCDi and PCDs in the formulation. Hydrogels CHT/PCDi/PCDs at weight ratios of 3:0:3, 3:1.5:1.5, and 3:3:0 were elaborated in a double–syringe system. The chemical composition, microstructure, viscoelastic properties, injectability, and structural integrity of the hydrogels were investigated. The cytotoxicity of the hydrogel was also evaluated by indirect contact with pre-osteoblast cells. Despite having similar shear–thinning and self-healing behaviors, the three hydrogels showed a marked difference in their rheological characteristics, injectability, structural stability, etc., depending on their PCDi and PCDs contents. Among the three, all the best above-mentioned properties, in addition to a high cytocompatibility, were found in the hydrogel 3:1.5:1.5. For the first time, we gained a deeper understanding of the role of the PCDi/PCDs in the injectable pre-formed hydrogels (CHT/PCDi/PCDs), which could be further fine-tuned to enhance their performance in biomedical applications.

Oligoarginine mediated collagen/chitosan gel composite for cutaneous wound healing

In this study, the collagen/chitosan gel composite supplemented with a cell-penetrating peptide (CPP) (Oligoarginine, R8) was prepared. Then, the physicochemical properties of the new collagen/chitosan/CPPs gel obtained were analyzed and the related characteristics were evaluated by scanning electron microscopy (SEM), fourier transform infrared (FTIR), differential scanning calorimetry (DSC), differential thermal analyzer (DTA). Furthermore, we found that collagen/chitosan/CPPs gel composite was capable of inhibiting Staphylococcus aureus growth and had good ability to heal wounds. The mice test results showed that collagen/chitosan/CPPs gel had the highest healing rate, fastest healing speed in all the treatments. After 14 days, the group treated by collagen/chitosan/CPPs gel showed nearly complete wound surface healing rate of 98 ± 4.71%. In addition, histopathological examination suggested that collagen/chitosan/CPPs could promote cutaneous wound healing through enhancing granulation tissue formation, increasing collagen deposition and promoting angiogenesis in the wound tissue. Meanwhile, no significant cytotoxicity of the gel was observed. In conclusion, the collagen/chitosan/CPPs gel composite which has antibacterial activity renders a high therapeutic efficiency to heal wounds.

Study on an injectable biomedical paste using cross-linked sodium hyaluronate as a carrier of hydroxyapatite particles

Exploring the long-term filler for minimally invasive plastic surgery has been widely concerned. In the present study, a series of injectable paste composed of hydroxyapatite (HAp) spherical particles and cross-linked sodium hyaluronate (cHA) solution were prepared. The physicochemical properties of cHA as a carrier of high content HAp microspheres (>50%) and as-obtained injectable HAp/cHA paste were studied. The cross-linking degree (DC), viscosity and molecular weight (Mw and Mn) of cHA increased with the increasing of the cross-linker dosage from 7.5 to 17.5 wt% under the certain conditions. HAp/cHA pastes were fabricated by homogeneously blending different sizes of HAp microspheres with cHA solution. The stability, rheological performance and push-out force of the pastes were studied, and the influence factors were discussed. The results indicated that moderate crosslinked cHA with 60% middle size HAp (HAp-M60/cHA-15.0) had appropriate comprehensive property. Finally, the in vitro cell culture approved the paste had no cytotoxicity. Although the biological performance of the pastes still need to be investigated, this preliminary study demonstrates that it is possible to carry high content HAp in cHA, expecting the better volumetric maintenance after long term implantation.

Novel Biological Hydrogel: Swelling Behaviors Study in Salt Solutions with Different Ionic Valence Number

In this paper, poly γ-glutamic acid/ε-polylysine (γ-PGA/ε-PL) hydrogels were successful prepared. The γ-PGA/ε-PL hydrogels could be used to remove Na⁺, Ca²⁺, and Cr³⁺ from aqueous solution and were characterized by scanning electron microscopy. The performance of hydrogels were estimated under different ionic concentration, temperature, and pH. The results showed that the ionic concentration and the pH significantly influenced the swelling capacity of γ-PGA/ε-PL hydrogels. The swelling capacities of γ-PGA/ε-PL hydrogels were decreased with the increase of the ionic concentration. However, the swelling capacity of the γ-PGA/ε-PL hydrogel was increased with the increase of the pH. The swelling kinetics indicated that γ-PGA/ε-PL hydrogels presented a more limited swelling degree in metal ion solutions with higher ionic valence numbers than in ion solutions with lower ionic valence numbers. However, the swelling kinetics of γ-PGA/ε-PL hydrogels showed that they proposed a satisfactory description in NaCl and CaCl₂ solutions. The adsorption process was fitted with a pseudo-second-order rate equation model. Moreover, the desorption kinetics of γ-PGA/ε-PL hydrogels showed that they could release most of the adsorption ions. Considering the biocompatibility, biodegradability, and ionic-sensitive properties, we propose that these γ-PGA/ε-PL hydrogels have high potential to be used in environmental protection, medical treatment, and other related fields.

An injectable and physical levan-based hydrogel as a dermal filler for soft tissue augmentation

A novel levan-based injectable hydrogel was developed as a dermal filler having better in vivo stability and efficacy compared to HA-based hydrogel.

Cellular hydrogels based on pH-responsive chitosan-hydroxyapatite system

The development of bioactive injectable system as cell carrier with minimal impact on viability of encapsulated cells represents a great challenge. In the present work, we propose a new pH-responsive chitosan-hydroxyapatite-based hydrogel with sodium bicarbonate (NaHCO₃) as the gelling agent. The in situ synthesis of hydroxyapatite phase has resulted in stable composite suspension and final homogeneous hydrogel. The application of sodium bicarbonate has allowed non-cytotoxic fast gelation of chitosan-hydroxyapatite within 4min, and without excess of sodium ions concentration. Rheological properties of crosslinked hydrogel have demonstrated possible behaviour as 'strong physical hydrogel'. The live dead staining has confirmed good viability and dispersion, as well as proliferation of encapsulated cells by the culture time. Presented preliminary results show good potential of chitosan-hydroxyapatite/NaHCO₃ as a cell carrier, whose impact on the cell differentiation need to be confirmed by encapsulation of other cell phenotypes.

Preparation of γ-PGA hydrogels and swelling behaviors in salt solutions with different ionic valence numbers

In this paper, a novel poly γ-glutamic acid (γ-PGA) hydrogel was successfully synthesized by solution polymerization and ethylene glycol diglycidyl ether (EGDE) was used as crosslinker.

Preparation and properties of EDC/NHS mediated crosslinking poly (gamma-glutamic acid)/epsilon-polylysine hydrogels

In this paper, a novel pH-sensitive poly (amino acid) hydrogel based on poly γ-glutamic acid (γ-PGA) and ε-polylysine (ε-PL) was prepared by carbodiimide (EDC) and N-hydroxysuccinimide (NHS) mediated polymerization. The influence of PGA/PL molar ratio and EDC/NHS concentration on the structure and properties was studied. Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) proved that hydrogels were crosslinked through amide bond linkage, and the conversion rate of a carboxyl group could reach 96%. Scanning electron microscopy (SEM) results showed a regularly porous structure with 20 μm pore size in average. The gelation time in the crosslink process of PGA/PL hydrogels was within less than 5 min. PGA/PL hydrogels had excellent optical performance that was evaluated by a novel optotype method. Furthermore, PGA/PL hydrogels were found to be pH-sensitive, which could be adjusted to the pH of swelling media intelligently. The terminal pH of swelling medium could be controlled at 5 ± 1 after equilibrium when the initial pH was within 3-11. The swelling kinetics was found to follow a Voigt model in deionized water but a pseudo-second-order model in normal saline and phosphate buffer solution, respectively. The differential swelling degrees were attributed to the swelling theory based on the different ratio of -COOH/-NH2 and pore size in hydrogels. The results of mechanical property indicated that PGA/PL hydrogels were soft and elastic. Moreover, PGA/PL hydrogels exhibited excellent biocompatibility by cell proliferation experiment. PGA/PL hydrogels could be degraded in PBS solution and the degradation rate was decreased with the increase of the molar ratio of PL. Considering the simple preparation process and pH-sensitive property, these PGA/PL hydrogels might have high potential for use in medical and clinical fields.

Adaptable hydrogel networks with reversible linkages for tissue engineering

Adaptable hydrogels have recently emerged as a promising platform for three-dimensional (3D) cell encapsulation and culture. In conventional, covalently crosslinked hydrogels, degradation is typically required to allow complex cellular functions to occur, leading to bulk material degradation. In contrast, adaptable hydrogels are formed by reversible crosslinks. Through breaking and re-formation of the reversible linkages, adaptable hydrogels can be locally modified to permit complex cellular functions while maintaining their long-term integrity. In addition, these adaptable materials can have biomimetic viscoelastic properties that make them well suited for several biotechnology and medical applications. In this review, an overview of adaptable-hydrogel design considerations and linkage selections is presented, with a focus on various cell-compatible crosslinking mechanisms that can be exploited to form adaptable hydrogels for tissue engineering.

Injectable nanoparticle-loaded hydrogel system for local delivery of sodium alendronate

Systemic administration of bisphosphonates, e.g. sodium alendronate (Aln) is characterized by extremely low bioavailability and high toxicity. To omit aforementioned drawbacks an injectable system for the intra-bone delivery of Aln based on Aln-loaded nanoparticles (NPs-Aln) suspended in a hydrogel matrix (gellan gum, GG) was developed. Aln was encapsulated in poly(lactide-co-glycolide) (PLGA 85:15) by solid-oil-water emulsification. Drug release tests showed that within 25 days all the encapsulated drug was released from NPs-Aln and the release rate was highest at the beginning and decreased with time. In contrast, by suspending NPs-Aln in a GG matrix, the release rate was significantly lower and more constant in time. The GG-NPs-Aln system was engineered to be easily injectable and was able to reassemble its structure after extrusion as shown by rheological measurements. Invitro studies showed that the GG-NPs-Aln was cytocompatible with MG-63 osteoblast-like cells and it inhibited RANKL-mediated osteoclastic differentiation of RAW 264.7 cells. The injectability, the sustained local delivery of small doses of Aln and the biological activity render the GG-NPs-Aln system promising for the local treatment of osteoporosis and other bone tissue disorders.

Designing degradable hydrogels for orthogonal control of cell microenvironments

Degradable and cell-compatible hydrogels can be designed to mimic the physical and biochemical characteristics of native extracellular matrices and provide tunability of degradation rates and related properties under physiological conditions. Hence, such hydrogels are finding widespread application in many bioengineering fields, including controlled bioactive molecule delivery, cell encapsulation for controlled three-dimensional culture, and tissue engineering. Cellular processes, such as adhesion, proliferation, spreading, migration, and differentiation, can be controlled within degradable, cell-compatible hydrogels with temporal tuning of biochemical or biophysical cues, such as growth factor presentation or hydrogel stiffness. However, thoughtful selection of hydrogel base materials, formation chemistries, and degradable moieties is necessary to achieve the appropriate level of property control and desired cellular response. In this review, hydrogel design considerations and materials for hydrogel preparation, ranging from natural polymers to synthetic polymers, are overviewed. Recent advances in chemical and physical methods to crosslink hydrogels are highlighted, as well as recent developments in controlling hydrogel degradation rates and modes of degradation. Special attention is given to spatial or temporal presentation of various biochemical and biophysical cues to modulate cell response in static (i.e., non-degradable) or dynamic (i.e., degradable) microenvironments. This review provides insight into the design of new cell-compatible, degradable hydrogels to understand and modulate cellular processes for various biomedical applications.