Evaluating the polymerization effectiveness and biocompatibility of bio-sourced, visible light-based photoinitiator systems

The use of photopolymerization is expanding across a multitude of biomedical applications, from drug delivery to bioprinting. Many of these current and emerging photopolymerization systems employ visible light, as motivated by safety and energy efficiency considerations. However, the "library" of visible light initiators is limited compared with the wealth of options available for UV polymerization. Furthermore, the synthesis of traditional photoinitiators relies on diminishing raw materials, and several traditional photoinitiators are considered emerging environmental contaminants. As such, there has been recent focus on identifying and characterizing biologically sourced, visible light-based photoinitiator systems that can be effectively used in photopolymerization applications. In this regard, several bio-sourced molecules have been shown to act as photoinitiators, primarily through Type II photoinitiation mechanisms. However, whether bio-sourced molecules can also act as effective synergists in these reactions remains unknown. In this study, we evaluated the effectiveness of bio-sourced synergist candidates, with a focus on amino acids, due to their amine functional groups, in combination with two bio-sourced photoinitiator molecules: riboflavin and curcumin. We tested the effectiveness of these photoinitiator systems under both violet (405 nm) and blue (460-475 nm) light using photo-rheology. We found that several synergist candidates, namely lysine, arginine, and histidine, increased the polymerization effectiveness of riboflavin when used with both violet and blue light. With curcumin, we found that almost all tested synergist candidates slightly decreased the polymerization effectiveness compared with curcumin alone under both light sources. These results show that bio-sourced molecules have the potential to be used as synergists with bio-sourced photoinitiators in visible light photopolymerization. However, more work must be done to fully characterize these reactions and to investigate more synergist candidates. Ultimately, this information is expected to expand the range of available visible light-based photoinitiator systems and increase their sustainability.

Photocrosslinked carboxymethylcellulose-based hydrogels: Synthesis, characterization for curcumin delivery and wound healing

Skin could protect our body and regenerate itself to against dysfunctional and disfiguring scars when faced with external injury. As wound dressings, hydrogels are biocompatible, hydrophilic and have a 3D structure similar to the extracellular matrix (ECM). In particular, hydrogels with drug-releasing capabilities are in acute wound healing. In this paper, photocrosslinked hydrogels served as wound dressing based on sodium carboxymethylcellulose (CMC) were prepared to promote wound healing. Photocrosslinked hydrogels were prepared by grafting lysine and allyl glycidyl ether (AGE) onto CMC and encapsulating curcumin (Cur). The synthesized hydrogels had the unique 3D porous structure with a swelling ratio up to 1300 % in aqueous solution. The drug release ratios of the hydrogels were 20.8 % in acid environment, and 14.4 % in alkaline environment. Notably, the hydrogels showed good biocompatibility and antibacterial properties and also exhibited the ability to accelerate the process of skin wound healing while prevent inflammation and scar formation when applied to a mouse skin wound model. As a result, the prepared hydrogels Gel-CLA@Cur showed great potential in wound healing.

Use of photosensitive molecules in the crosslinking of biopolymers: applications and considerations in biomaterials development

The development of diverse types of biomaterials has significantly contributed to bringing new biomedical strategies to treat clinical conditions. Applications of these biomaterials can range from mechanical support and protection of injured tissues to joint replacement, tissue implants, and drug delivery systems. Among the strategies commonly used to prepare biomaterials, the use of electromagnetic radiation to initiate crosslinking stands out. The predominance of photo-induced polymerization methods relies on a fast, efficient, and straightforward process that can be easily adjusted to clinical needs. This strategy consists of irradiating the components that form the material with photons in the near ultraviolet-visible wavelength range (i.e., ∼310 to 750 nm) in the presence of a photoactive molecule. Upon photon absorption, photosensitive molecules can generate excited species that initiate photopolymerization through different reaction mechanisms. However, this process could promote undesired side reactions depending on the target zone or treatment type (e.g., oxidative stress and modification of biomolecules such as proteins and lipids). This review explores the basic concepts behind the photopolymerization process of ex situ and in situ biomaterials. Particular emphasis was put on the photosensitization initiated by the most employed photosensitizers and the photoreactions that they mediate in aqueous media. Finally, the undesired oxidation reactions at the bio-interface and potential solutions are presented.

Electron-Beam-Initiated Crosslinking of Methacrylated Alginate and Diacrylated Poly(ethylene glycol) Hydrogels

An ideal wound dressing not only needs to absorb excess exudate but should also allow for a moist wound-healing environment as well as being mechanically strong. Such a dressing can be achieved by combining both a natural (alginate) and synthetic (poly(ethylene glycol) polymer. Interestingly, using an electron beam on (meth)acrylated polymers allows their covalent crosslinking without the use of toxic photo-initiators. The goal of this work was to crosslink alginate at different methacrylation degrees (26.1 and 53.5% of the repeating units) with diacrylated poly(ethylene glycol) (PEGDA) using electron-beam irradiation at different doses to create strong, transparent hydrogels. Infrared spectroscopy showed that both polymers were homogeneously distributed within the irradiated hydrogel. Rheology showed that the addition of PEGDA into alginate with a high degree of methacrylation and a polymer concentration of 6 wt/v% improved the storage modulus up to 15,867 ± 1102 Pa. Gel fractions > 90% and swelling ratios ranging from 10 to 250 times its own weight were obtained. It was observed that the higher the storage modulus, the more limited the swelling ratio due to a more crosslinked network. Finally, all species were highly transparent, with transmittance values > 80%. This may be beneficial for the visual inspection of healing progression. Furthermore, these polymers may eventually be used as carriers of photosensitizers, which is favorable in applications such as photodynamic therapy.

Flavin mononucleotide in visible light photoinitiating systems for multiple-photocrosslinking and photoencapsulation strategies

Visible light-induced photocrosslinking techniques have attracted significant attention for their flexibility, controllability, safety, and energy conservation, especially in tissue engineering and biofabrication, compared to UV photocrosslinking. Despite these advantages, current photoinitiators are constrained by various challenges, including inadequate photoinitiation efficiency, low biocompatibility, poor water solubility, and limited compatibility with diverse crosslinking systems. Here, a water-soluble derivative of riboflavin, flavin mononucleotide (FMN-), was used to assess its potential as an initiator in multiple-photocrosslinking systems, including radical photopolymerization, dityrosine, and ditryptophan coupling crosslinking, under blue light irradiation. Blue light irradiation facilitated an efficient electron transfer reaction between FMN- and persulfate, owing to their suitable spectral compatibility and photoactivity. The resulting oxidizing free radicals and excited triplet state of FMN- served as initiating active species for the multiple-photocrosslinking reactions. The combination of FMN- and potassium persulfate (KPS) exhibited exceptional photoinitiation efficiency for various biomaterials, including silk fibroin, gelatin, poly(ethylene glycol) diacrylate, and carboxymethyl cellulose modified with amino acids. Furthermore, the cytocompatibility of the FMN-/KPS photoinitiator was demonstrated by the survival rates of 3T3-LI fibroblasts encapsulated in it, which exceeded 95 % when compared to a commercial initiator. STATEMENT OF SIGNIFICANCE: By introducing persulfate, the photoinitiation efficiency of flavin mononucleotide was significantly improved. The application scenarios of flavin mononucleotide and persulfate combinations were also greatly extended, including radical photopolymerization, dityrosine, diphenylalanine, and ditryptophan coupling crosslinking. Among them, the coupling crosslinking of amino acids (di-phenylalanine, and di-tryptophan) modified carboxymethyl cellulose, to our knowledge, was first reported. The excellent cytocompatibility of cell encapsulation further proved that the combinations of flavin mononucleotide and persulfate have great potential in tissue engineering.

Green synthesis of quaternized chitosan nanogel using emulsion-photopolymerization as redox-responsive drug carrier

We report the green synthesis of trimethyl chitosan-functionalized poly(2-hydroxyethyl methacrylate) (PHEMA-TMC) nanogels via surfactant-free emulsion photopolymerization. TMC, a quaternized derivative of chitosan, was synthesized through methylation of chitosan, resulting in quaternary and tertiary amine groups as the main substitution products. TMC tertiary amine moiety and riboflavin (RF) acted as a redox photo-initiating system to generate free radicals for the polymerization under light irradiation. The effects of polymerization parameters such as irradiation time, concentrations of TMC and RF were investigated using MBA as crosslinker. Under the optimal condition of 1 % TMC, 4 % HEMA, 0.8 μM RF, 5 % MBA, and 4 h of polymerization time, the cationic PHEMA-TMC nanogel was synthesized with 76 % monomer conversion and an average diameter of about 106 nm. Moreover, the disulfide-crosslinked PHEMA-TMC nanogel was also synthesized using the disulfide dimethacrylate crosslinker, which exhibited a redox-induced degradation and release of encapsulated melatonin, potentially useful as a redox-responsive drug delivery carrier.

A "T.E.S.T." hydrogel bioadhesive assisted by corneal cross-linking for in situ sutureless corneal repair

Corneal transplantation is an effective clinical treatment for corneal diseases, which, however, is limited by donor corneas. It is of great clinical value to develop bioadhesive corneal patches with functions of "Transparency" and "Epithelium & Stroma generation", as well as "Suturelessness" and "Toughness". To simultaneously meet the "T.E.S.T." requirements, a light-curable hydrogel is designed based on methacryloylated gelatin (GelMA), Pluronic F127 diacrylate (F127DA) & Aldehyded Pluronic F127 (AF127) co-assembled bi-functional micelles and collagen type I (COL I), combined with clinically applied corneal cross-linking (CXL) technology for repairing damaged cornea. The patch formed after 5 min of ultraviolet irradiation possesses transparent, highly tough, and strongly bio-adhesive performance. Multiple cross-linking makes the patch withstand deformation near 600% and exhibit a burst pressure larger than 400 mmHg, significantly higher than normal intraocular pressure (10-21 mmHg). Besides, the slower degradation than GelMA-F127DA&AF127 hydrogel without COL I makes hydrogel patch stable on stromal beds in vivo, supporting the regrowth of corneal epithelium and stroma. The hydrogel patch can replace deep corneal stromal defects and well bio-integrate into the corneal tissue in rabbit models within 4 weeks, showing great potential in surgeries for keratoconus and other corneal diseases by combining with CXL.

Green Chemistry for Crosslinking Biopolymers: Recent Advances in Riboflavin-Mediated Photochemistry

Riboflavin (RF), which is also known as vitamin B2, is a water-soluble vitamin. RF is a nontoxic and biocompatible natural substance. It absorbs light (at wavelengths of 380 and 450 nm) in the presence of oxygen to form reactive singlet oxygen (¹O₂). The generated singlet oxygen acts as a photoinitiator to induce the oxidation of biomolecules, such as amino acids, proteins, and nucleotides, or to initiate chemical reactions, such as the thiol-ene reaction and crosslinking of tyramine and furfuryl groups. In this review, we focus on the chemical mechanism and utilization of the photochemistry of RF, such as protein crosslinking and hydrogel formation. Currently, the crosslinking method using RF as a photoinitiator is actively employed in ophthalmic clinics. However, a significant broadening is expected in its range of applications, such as in tissue engineering and drug delivery.

Visible light-curable methacrylated glycol chitosan hydrogel patches for prenatal closure of fetal myelomeningocele

In this study, we prepared visible light-curable methacrylated glycol chitosan (MGC) hydrogel patches for the prenatal treatment of fetal myelomeningocele (MMC) and investigated their feasibility using a retinoic acid-induced fetal MMC rat model. 4, 5, and 6 w/v% of MGC were selected as candidate precursor solutions, and photo-cured for 20 s, because the resulting hydrogels were found to possess concentration dependent tunable mechanical properties and structural morphologies. Moreover, these materials exhibited no foreign body reactions with good adhesive properties in animal studies. The inflammation scoring assessment in vivo exhibited the absence of foreign body reactions in MGC hydrogel treated lesion. The complete epithelial coverage of MMC was made with using 6 w/v% MGC hydrogel followed by well-organized granulation along with noticeable decrease of abortion rate and wound size that highlight the therapeutic potential for the prenatal treatment of fetal MMC.

Dynamic modulation and epoxy functionalization of protein-mediated enoate ester-based hybrid cryogels

The current work is focused on the preparation of protein-mediated poly(hydroxyethyl methacrylate-co-glycidyl methacrylate) copolymer as a self-template for in situ synthesis of hybrid gels. Gelatin, collagen, biotin, and l-arginine were used to create hybrid materials with adjustable swelling and elastic properties. Hybrid cryogels tended to swell more than hybrid hydrogels due to their porous nature. Collaged-doped cryogels had the highest swelling, whereas gelatin-doped hybrids showed enhanced elastic modulus. All hybrid gels exhibited pH-sensitive swelling to controlled release applications depending on the degree of protonation of NH₂ and COOH groups in the side chains. At low pH conditions, hybrid cryogels exhibited a higher swelling tendency compared to hydrogels. Ion-stimulus-response of hybrid gels was studied to evaluate the effect of salt concentration and features of ambient ions on swelling. Depending on the polyelectrolytic or polyampholytic nature, the extent of swelling in NaCl and KCl solutions varied according to the charge distribution in the network chains. Hybrid gels showed excellent adsorption performance for methyl orange by the presence of epoxy, hydroxyl groups, amino and carboxyl groups providing sufficient active sites. Adsorption capacity of hybrid cryogels is higher than that of hydrogels. The removal rate 97/%, reached an equilibrium state in a short period, suggested that collagen-doped hybrid cryogels have a potential application to remove dyestuff from wastewater. In relation to the decrease of methyl orange concentration in solution, adsorption process followed pseudo-second-order kinetic model. Avrami model has provided a better experimental-calculated fit and adsorption thermodynamics analysis indicated that the adsorption was a spontaneous process with a negative standard free energy. The characteristic findings from this research will provide insights into the design and application of enoate-ester and protein-based combinations in the food, biomedical and cosmetic fields.

Water-soluble/visible-light-sensitive naphthalimide derivative-based photoinitiating systems: 3D printing of antibacterial hydrogels

Adaptability of hydrogels allows these structures to be used in a variety of industries, including biomedicine, soft electronics, and sensors. In this study, 10 different naphthalimide derivatives were prepared (five...

Highly Absorbent Silk Fibroin Protein Xerogel

Highly absorbent polymers have a wide range of applications in biomaterials, agriculture, physiological products of daily uses, and others. Silk fibroin, as a natural biomaterial with excellent biocompatibility and tunable mechanical properties, shows good prospects in the field of biomedicine applications. However, the dried fibroin hydrogel has very low absorbency. In this work, silk fibroin protein is used as the carrier, riboflavin as the photosensitizer, and accordingly, the hydrogel is prepared by free radical cross-linking under ultraviolet light. The fibroin in the hydrogel contains mainly the random coil structure. The covalent bond cross-linking hinders the crystallization of the silk fibroin, thereby an amorphous silk fibroin hydrogel is obtained. After drying, this xerogel can absorb water 90 times more than its own mass and assimilates a good amount of water within a minute. In vitro and in vivo rabbit ear hemostasis experiments show that this fabricated xerogel has good hemostatic properties. Therefore, this xerogel exhibits good promise for rapid hemostasis of wounds and absorbing other body exudates.

Water-Soluble Photoinitiators in Biomedical Applications

Light-initiated polymerization processes are currently an important tool in various industrial fields. The advancement of technology has resulted in the use of photopolymerization in various biomedical applications, such as the production of 3D hydrogel structures, the encapsulation of cells, and in drug delivery systems. The use of photopolymerization processes requires an appropriate initiating system that, in biomedical applications, must meet additional criteria such as high water solubility, non-toxicity to cells, and compatibility with visible low-power light sources. This article is a literature review on those compounds that act as photoinitiators of photopolymerization processes in biomedical applications. The division of initiators according to the method of photoinitiation was described and the related mechanisms were discussed. Examples from each group of photoinitiators are presented, and their benefits, limitations, and applications are outlined.

UVA-activated riboflavin promotes collagen crosslinking to prevent root caries

Root caries is an increasingly problem in aging societies with severe implications for the general health and wellbeing of large numbers of people. Strengthening type-I collagen, a major organic component of human dentin, has proved effective in preventing root caries. This study sought to determine whether exposure to riboflavin followed by UVA irradiation (RF/UVA) could promote additional collagen crosslinking, and thus improve the acid and enzymatic resistance of human dentin under simulated oral environments. If so, it could offer potential for treatment of the intractable problem of root caries. The greatest flexural strengths were found in dentin exposed to a 0.1% riboflavin solution for 1 minute followed by 1,600 mW/cm² UVA irradiation for 10 minutes. Mineral loss and lesion depth were significantly lower in the RF/UVA group than in the control group. The microstructures of dentinal tubules and collagen networks after RF/UVA treatment retained their original forms after acidic and enzymatic degradation. In conclusion, RF/UVA treatment may be a new method for preventing root caries with promising prospects for clinical application.

Dynamic PEG-Peptide Hydrogels via Visible Light and FMN-Induced Tyrosine Dimerization

Photoresponsive hydrogels have become invaluable 3D culture matrices for mimicking aspects of the extracellular matrix. Recent efforts have focused on using ultraviolet (UV) light exposure and multifunctional macromers to induce secondary hydrogel crosslinking and dynamic matrix stiffening in the presence of cells. This contribution reports the design of a novel yet simple dynamic poly(ethylene glycol)-peptide hydrogel system through flavin mononucleotide (FMN) induced di-tyrosine crosslinking. These di-tyrosine linkages effectively increase hydrogel crosslinking density and elastic modulus. In addition, the degree of stiffening in hydrogels at a fixed PEG macromer content can be readily tuned by controlling FMN concentration or the number of tyrosine residues built-in to the peptide linker. Furthermore, tyrosine-bearing pendant biochemical motifs can be spatial-temporally patterned in the hydrogel network via controlling light exposure through a photomask. The visible light and FMN-induced tyrosine dimerization process produces a cytocompatible and physiologically relevant degree of stiffening, as shown by changes of cell morphology and gene expression in pancreatic cancer and stromal cells. This new dynamic hydrogel scheme should be highly desirable for researchers seeking a photoresponsive hydrogel system without complicated chemical synthesis and secondary UV light irradiation.

Comparison of visible and UVA phototoxicity in neural culture systems micropatterned with digital projection photolithography

Photopolymerization provides a favorable method for hydrogel formation due to its simplicity, convenience, and versatility. However, the light exposure required to initiate photopolymerization is known to have a cytotoxic effect on encapsulated cells. Here, a 3D in vitro model of the nervous system microenvironment, micropatterned through the use of digital projection photolithography using a single hydrogel formulation that cross-links similarly under ultraviolet A (UVA, 315-400 nm) and visible light (400-700 nm) exposure, is presented. This setup allowed for the investigation of neuronal responses to different light wavelengths and exposure times during photoencapsulation, while ruling out effects due to the hydrogel formulation or photoinitiators used. Cellular studies-including neurite viability, DNA fragmentation, and neurite outgrowth for both UVA and visible light irradiation, the most common spectra used in biological photomicropatterning applications-were performed to assess the effect of light source on neuronal cultures. These studies indicated that while cell death occurs after exposure to either spectrum, visible light was less phototoxic than UVA, when using comparable levels of irradiation, and interestingly, glial cells were more susceptible to phototoxicity than neuronal cells. Thus, while utilizing visible light for micropatterning and cell encapsulation for nervous system applications is beneficial, it is helpful to keep the light exposure low to ensure optimal neuronal survival and growth. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 134-144, 2019.

In situ photochemical crosslinking of hydrogel membrane for Guided Tissue Regeneration

OBJECTIVE

Periodontitis is an inflammatory disease that destroys the tooth-supporting attachment apparatus. Guided tissue regeneration (GTR) is a technique based on a barrier membrane designed to prevent wound space colonization by gingival cells. This study examined a new formulation composed of two polymers that could be photochemically cross-linked in situ into an interpenetrated polymer network (IPN) forming a hydrogel membrane.

METHODS

We synthetized and characterized silanized hydroxypropyl methylcellulose (Si-HPMC) for its cell barrier properties and methacrylated carboxymethyl chitosan (MA-CMCS) for its degradable backbone to use in IPN. Hydrogel membranes were cross-linked using riboflavin photoinitiator and a dentistry visible light lamp. The biomaterial's physicochemical and mechanical properties were determined. Hydrogel membrane degradation was evaluated in lysozyme. Cytocompatibility was estimated by neutral red uptake. The cell barrier property was studied culturing human primary gingival fibroblasts or human gingival explants on membrane and analyzed with confocal microscopy and histological staining.

RESULTS

The IPN hydrogel membrane was obtained after 120s of irradiation. The IPN showed a synergistic increase in Young moduli compared with the single networks. The CMCS addition in IPN allows a progressive weight loss compared to each polymer network. Cytocompatibility was confirmed by neutral red assay. Human cell invasion was prevented by hydrogel membranes and histological sections revealed that the biomaterial exhibited a barrier effect in contact with soft gingival tissue.

SIGNIFICANCE

We demonstrated the ability of an innovative polymer formulation to form in situ, using a dentist's lamp, an IPN hydrogel membrane, which could be an easy-to-use biomaterial for GTR therapy.

Biodegradable Microparticles for Simultaneous Detection of Counterfeit and Deteriorated Edible Products

In an era of globalized trade relations where food and pharmaceutical products cross borders effortlessly, consumers face counterfeit and deteriorated products at elevated rates. This paper presents multifunctional, biodegradable hydrogel microparticles that can provide information on the authenticity and the potential deterioration of the tagged food or pharmaceutical formulations. These microparticles integrate spatially patterned authenticity code with two sensors-the first one detects possible presence of pathogenic microbes through monitoring pH while the second one identifies products stored above optimal temperatures via optical monitoring of the microparticle degradation. Particles are synthesized from a biocompatible polymer and a photoinitiator, dextran modified with 2-hydroxyethylmethacrylate and riboflavin, respectively, using a continuous, high throughput method stop-flow lithography. The proposed synthesis approach also enables crosslinking with visible light bringing about additional flexibility to flow lithography. Model liquid and solid food and pharmaceutical products are successfully labeled with microparticles and the functionality of the sensors in aqueous solutions is demonstrated.

3D bioprinting for musculoskeletal applications

This review focuses on developments in the field of bioprinting for musculoskeletal tissue engineering, along with discussion on the various approaches for bone, cartilage and connective tissue fabrication. All approaches (cell-laden, cell-free and a combination of both) aim to obtain complex, living tissues able to develop and mature, using the same fundamental technology. To date, co-printing of cell-laden and cell-free materials has been revealed to be the most promising approach for musculoskeletal applications because materials with good bioactivity and good mechanical strength can be combined within the same constructs. Bioprinting for musculoskeletal applications is a developing field, and detailed discussion on the current challenges and future perspectives is also presented in this review.

Photocrosslinking of Silk Fibroin Using Riboflavin for Ocular Prostheses

A novel method to photocrosslink silk fibroin protein is reported, using riboflavin (vitamin B2) as a photoinitiator and the mechanism of crosslinking is determined. Exposure of riboflavin-doped liquid silk solution to light results in the formation of a transparent, elastic hydrogel. Several applications for this new material are investigated including corneal reshaping to restore visual acuity and photolithography.

25th anniversary article: Designer hydrogels for cell cultures: a materials selection guide

Cell culturing, whether for tissue engineering or cell biology studies, always involves placing cells in a non-natural environment and no material currently exist that can mimic the entire complexity of natural tissues and variety of cell-matrix interactions that is found in vivo. Here, we review the vast range of hydrogels, composed of natural or synthetic polymers that provide a route to tailored microenvironments.

Smart hydrogels exhibiting UCST-type volume changes under physiologically relevant conditions

Hydrogels composed of poly(allylurea) copolymers exhibited rapid temperature positive volume changes without hysteresis under physiologically relevant conditions.