Enzyme-mediated redox initiation for hydrogel generation and cellular encapsulation

A Minimalist Method for Fully Oxygen-Tolerant RAFT Polymerization through Sulfur-Centered Trithiocarbonate Radical Initiation

In recent years, the fully oxygen-tolerant reversible deactivation radical polymerization (RDRP) has become a highly researched area. In this contribution, a new and minimalist method is successfully employed to accomplish fully oxygen-tolerant reversible addition-fragmentation chain transfer (RAFT) polymerization using bis(trithiocarbonate) disulfides (BisTTC) as an iniferter agent, where the released sulfur-centered trithiocarbonate (TTC) radical can initiate monomer. Furthermore, polymerization kinetics revealed the typical "living" features of this polymerization system. More importantly, by high-throughput screening, it is found that dodecyl-substituted TTC is responsible for the fully oxygen-tolerant RAFT polymerization though trithiocarbonate radical initiation and R radical deoxygenation. It is believed that trithiocarbonate radical initiation strategy provides a powerful and minimalist tool for fully oxygen-tolerant RDRPs.

PEG-Based Hydrogel Coatings: Design Tools for Biomedical Applications

Device failure due to undesired biological responses remains a substantial roadblock in the development and translation of new devices into clinical care. Polyethylene glycol (PEG)-based hydrogel coatings can be used to confer antifouling properties to medical devices-enabling minimization of biological responses such as bacterial infection, thrombosis, and foreign body reactions. Application of hydrogel coatings to diverse substrates requires careful consideration of multiple material factors. Herein, we report a systematic investigation of two coating methods: (1) traditional photoinitiated hydrogel coatings; (2) diffusion-mediated, redox-initiated hydrogel coatings. The effects of method, substrate, and compositional variables on the resulting hydrogel coating thickness are presented. To expand the redox-based method to include high molecular weight macromers, a mechanistic investigation of the role of cure rate and macromer viscosity was necessary to balance solution infiltration and gelation. Overall, these structure-property relationships provide users with a toolbox for hydrogel coating design for a broad range of medical devices.

Enzyme-manipulated hydrogelation of small molecules for biomedical applications

Enzyme-manipulated hydrogelation based on self-assembly of small molecules is an attractive methodology for development of functional biomaterials. Upon the catalysis of enzymes, small-molecular precursors are converted into assemblable building blocks, which arrange into high-ordered nanofibers via non-covalent interactions at the molecular level, and further trap water to form hydrogels at the macroscopic level. Such approach has numerous advantages of region- and enantioselectivity, and mild reaction conditions for encapsulation of biomedications or cells that are fragile to environmental change. In addition to the common applications as drug reservoirs or cell scaffolds, the utilization of endogenous enzymes as stimuli to initiate self-assembly in the living cells and tissue is considered as an intelligent spatiotemporally controllable hydrogelation strategy for biomedical applications. The enzyme-instructed in situ self-assembly and hydrogelation can modulate the cell behavior, and even present therapeutic bioactivities, which provides a new perspective in the field of disease treatment. In this review, we categorize distinct enzymatic stimuli and elaborate substrate design, catalytic characteristics, and mechanisms of self-assembly and hydrogelation. The biomedical applications in drug delivery, tissue engineering, bioimaging, and in situ gelation-produced bioactivity are outlined. Advantages and limitations regarding the state-of-the-art enzyme-driven hydrogelation technologies and future perspectives are also discussed. STATEMENT OF SIGNIFICANCE: Hydrogel is a semi-solid soft material containing a large amount of water. Due to the features of adjustable flexibility, extremely porous architecture, and the high similarity of structure to natural extracellular matrices, the hydrogel has broad application prospects in biomedicine. In recent 20 years, enzyme-manipulated hydrogelation based on self-assembly of small molecules has developed rapidly as an attractive methodology for the construction of functional biomaterials. Upon the catalysis of enzymes, small-molecular precursors are converted into assemblable building blocks, which arrange into high-ordered nanofibers via non-covalent interactions at the molecular level, and further trap water to form hydrogels at the macroscopic level. This review summarized the characteristics of enzymatic hydrogel, as well as the traditional application and emerging prospect of enzyme-instructed self-assembly and hydrogelation.

Enzyme Catalysis for Reversible Deactivation Radical Polymerization

Enzyme catalysis has been increasingly utilized in reversible deactivation radical polymerization (Enz-RDRP) on account of its mildness, efficiency, and sustainability. In this Minireview we discuss the key roles enzymes play in RDRP, including their ATRPase, initiase, deoxygenation, and photoenzyme activities. We use selected examples to highlight applications of Enz-RDRP in surface brush fabrication, sensing, polymerization-induced self-assembly, and high-throughput synthesis. We also give our reflections on the challenges and future directions of this emerging area.

Designing Inherently Photodegradable Cell-Adhesive Hydrogels for 3D Cell Culture

Light-based microfabrication techniques constitute an indispensable approach to fabricate tissue assemblies, benefiting from noncontact spatially and temporarily controlled manipulation of soft matter. Light-triggered degradation of soft materials, such as hydrogels, is important in tissue engineering, bioprinting, and related fields. The photoresponsiveness of hydrogels is generally not intrinsic and requires complex synthetic procedures wherein photoresponsive crosslinking groups are incorporated into the hydrogel. This paper demonstrates a novel biocompatible and inherently photodegradable poly(ethylene glycol) methacrylate (PEGMA)-based gelatin-methacryloyl (GelMA)-containing hydrogel that can be used to culture cells in 3D for at least 14 d. These gels are conveniently and quickly degraded via UV irradiation for 10 min to produce structured hydrogels of various geometries, sizes, and free-standing cell-laden hydrogel particles. These structures can be flexibly produced on demand. In particular, photodegradation can be temporarily delayed from photopolymerization, offering an alternative to hydrogel array production via photopolymerization with a photomask. The paper investigates the influences of hydrogel composition and swelling liquid on both its photodegradability and biocompatibility.

Tissue Fluid Triggered Enzyme Polymerization for Ultrafast Gelation and Cartilage Repair

The in situ gelation of injectable precursors is desirable in the field of tissue regeneration, especially in the context of irregular defect filling. The current driving forces for fast gelation include the phase-transition of thermally sensitive copolymers, click chemical reactions with tissue components, and metal coordination effect. However, the rapid formation of tough hydrogels remains a challenge. Inspired by aerobic metabolism, we herein propose a tissue-fluid-triggered cascade enzymatic polymerization process catalyzed by glucose oxidase and ferrous glycinate for the ultrafast gelation of acryloylated chondroitin sulfates and acrylamides. The highly efficient production of carbon radicals and macromolecules contribute to rapid polymerization for soft tissue augmentation in bone defects. The copolymer hydrogel demonstrated the regeneration-promoting capacity of cartilage. As the first example of using artificial enzyme complexes for in situ polymerization, this work offers a biomimetic approach to the design of strength-adjustable hydrogels for bio-implanting and bio-printing applications.

Bioactive hydrogel coatings of complex substrates using diffusion-mediated redox initiation

Hydrogels have long been established as materials with tunable stiffness and chemistry that enable controlled cellular interactions. When applied as coatings, hydrogels can be used to introduce biofunctionality to medical devices with minimal effect on bulk properties. However, it remains challenging to uniformly apply hydrogel coatings to three dimensional geometries without substantially changing the manufacturing process and potentially affecting device function. Herein, we report a new redox-based crosslinking method for applying conformable hydrogel coatings with tunable thickness and chemistry. This new diffusion-mediated strategy of redox initiation and hydrogel crosslinking enabled coating of a variety of three dimensional substrates without changing the primary fabrication process. Following adsorption of the reducing agent to the construct, hydrogel coating thickness was readily controlled by immersion time with desorption and diffusion of the reducing agent initiating hydrogel crosslinking from the surface. The process was used to generate a range of hydrogel properties by varying the macromer molecular weight and concentration. In addition, we demonstrated that these coatings can be applied sequentially to generate multilayered constructs with distinct features. Finally, incorporation of proteins into the bulk of the hydrogel coating or as a final surface layer permitted the controlled introduction of bioactivity that supported cell attachment. This work provides a versatile method for assembling bioactive coatings with a simple post-fabrication process that is amenable to diverse geometric substrates and chemistries.

Improved in situ seeding of 3D printed scaffolds using cell-releasing hydrogels

The design of tissue engineered scaffolds based on polymerized high internal phase emulsions (polyHIPEs) has emerged as a promising bone grafting strategy. We previously reported the ability to 3D print emulsion inks to better mimic the structure and mechanical properties of native bone while precisely matching defect geometry. In the current study, redox-initiated hydrogel carriers were investigated for in situ delivery of human mesenchymal stem cells (hMSCs) utilizing the biodegradable macromer, poly(ethylene glycol)-dithiothreitol. Hydrogel carrier properties including network formation time, sol-gel fraction, and swelling ratio were modulated to achieve rapid cure without external stimuli and a target cell-release period of 5-7 days. These in situ carriers enabled improved distribution of hMSCs in 3D printed polyHIPE grafts over standard suspension seeding. Additionally, carrier-loaded polyHIPEs supported sustained cell viability and osteogenic differentiation of hMSCs post-release. In summary, these findings demonstrate the potential of this in situ curing hydrogel carrier to enhance the cell distribution and retention of hMSCs in bone grafts. Although initially focused on improving bone regeneration, the ability to encapsulate cells in a hydrogel carrier without relying on external stimuli that can be attenuated in large grafts or tissues is expected to have a wide range of applications in tissue engineering.

Core-shell patterning of synthetic hydrogels via interfacial bioorthogonal chemistry for spatial control of stem cell behavior

A new technique is described for the patterning of cell-guidance cues in synthetic extracellular matrices (ECM) for tissue engineering applications. Using s-tetrazine modified hyaluronic acid (HA), bis-trans-cyclooctene (TCO) crosslinkers and monofunctional TCO conjugates, interfacial bioorthogonal crosslinking was used to covalently functionalize hydrogels as they were synthesized at the liquid-gel interface. Through temporally controlled introduction of TCO conjugates during the crosslinking process, the enzymatic degradability, cell adhesivity, and mechanical properties of the synthetic microenvironment can be tuned with spatial precision. Using human mesenchymal stem cells (hMSCs) and hydrogels with a core-shell structure, we demonstrated the ability of the synthetic ECM with spatially defined guidance cues to modulate cell morphology in a biomimetic fashion. This new method for the spatially resolved introduction of cell-guidance cues for the establishment of functional tissue constructs complements existing methods that require UV-light or specialized equipment.

Up in the air: oxygen tolerance in controlled/living radical polymerisation

The requirement for deoxygenation in controlled/living radical polymerisation (CLRP) places significant limitations on its widespread implementation by necessitating the use of large reaction volumes, sealed reaction vessels as well as requiring access to specialised equipment such as a glove box and/or inert gas source. As a result, in recent years there has been intense interest in developing strategies for overcoming the effects of oxygen inhibition in CLRP and therefore remove the necessity for deoxygenation. In this review, we highlight several strategies for achieving oxygen tolerant CLRP including: "polymerising through" oxygen, enzyme mediated deoxygenation and the continuous regeneration of a redox-active catalyst. In order to provide further clarity to the field, we also establish some basic parameters for evaluating the degree of "oxygen tolerance" that can be achieved using a given oxygen scrubbing strategy. Finally, we propose some applications that could most benefit from the implementation of oxygen tolerant CLRP and provide a perspective on the future direction of this field.

Oxidoreductase-Initiated Radical Polymerizations to Design Hydrogels and Micro/Nanogels: Mechanism, Molding, and Applications

Due to their 3D cross-linked networks and tunable physicochemical properties, polymer hydrogels with different sizes are applied widely in tissue engineering, drug-delivery systems, pollution regulation, ionic conducting electrolytes, agricultural drought-resistance, cosmetics, and the food industry. Novel, environmentally friendly, and efficient oxidoreductase-initiated radical polymerizations to design hydrogels and micro/nanogels have gained increasing attention. Herein, the recent advances on the use of novel enzyme-initiated systems for hydrogel polymerization, including the mechanisms, and molding of polymeric and hybrid-polymeric networks are reviewed. Preliminary progress related to interfacial enzymatic polymerization for the generation of hybrid micro/nanogels is introduced as an emerging initiating approach. In addition, certain biological applications in tissue engineering, bioimaging, and therapy are demonstrated step by step. Finally, some perspectives on the safety profile of enzymatic formed hydrogels, new enzymatic systems, and potential theranostic applications are discussed.

Nanoinitiator for enzymatic anaerobic polymerization and graft enhancement of gelatin-PAAM hydrogel

As an emerging method for mildly molding polymer hydrogel bioscaffolds, the enzymatically polymerized system is mainly based on the screening of various oxidoreductases to produce radicals, but the design of multifunctional nanoinitiators to facilitate hydrogel performance remains challenging. Here, we utilize N-hydroxyimide-modified silica nanoparticles as nanoinitiators to simultaneously trigger glucose oxidase anaerobic polymerization and nanoparticle-grafting enhancement of the gelatin-polyacrylamide (PAAM) hydrogel. The enzyme-nanoinitiator system produced nitrogen radicals, which were further converted into carbon radicals via GOx-catalyzed glucose reduction, as confirmed by electron paramagnetic resonance (EPR) analysis. Our stretchable hydrogel has a 12-fold increased fracture energy relative to traditional hydrogel due to grafting enhancement by the nanoinitiator. The temperature-dependent physical crosslinking of gelatin endowed our hydrogel facile printing ability. Cytotoxicity assay and 3D cell culture demonstrated the low toxicity of our hydrogel. As the first example of the use of nanoinitiators for enzymatic polymerization, this work provides a biocompatible platform to prepare or print hydrogel bioscaffolds with the required mechanical strength.

One-Enzyme Triple Catalysis: Employing the Promiscuity of Horseradish Peroxidase for Synthesis and Functionalization of Well-Defined Polymers

We demonstrate a new concept in polymer chemistry that the promiscuity of enzymes, as represented by horseradish peroxidase, can be employed for RAFT polymerization and thiol-ene and Diels-Alder reactions to synthesize well-defined functional polymers, via three different catalytic reactions mediated by one single enzyme.

Oxygen-mediated Polymerization Initiated by Oltipraz-derived Thiones

A pyrrolopyrazine-thione derived from oltipraz, a compound that has been investigated as a chemopreventive agent, affords radicals in the presence of thiols and oxygen via a redox cycle, an attribute that suggests its suitability as an initiator for oxygen-mediated polymerization. Here, we explore the utilization of this pyrrolopyrazine-thione, generated in situ from a precursor, as an initiator for the radical-mediated thiol-ene polymerization. While the pyrrolopyrazine-thione was shown to be capable of generating radicals in the presence of atmospheric oxygen and thiol groups, the reaction extents achievable were lower than desired owing to the presence of unwanted side reactions that would quench radical production and, subsequently, suppress polymerization. Moreover, we found that complex interactions between the pyrrolopyrazine-thione, its precursor, oxygen, and thiol groups determine whether or not the quenching reaction dominates over those favorable to polymerization.

Radical-Mediated Enzymatic Polymerizations

Polymerization reactions are commonly effected by exposing monomer formulations to some initiation stimulus such as elevated temperature, light, or a chemical reactant. Increasingly, these polymerization reactions are mediated by enzymes--catalytic proteins--owing to their reaction efficiency under mild conditions as well as their environmental friendliness. The utilization of enzymes, particularly oxidases and peroxidases, for generating radicals via reduction-oxidation mechanisms is especially common for initiating radical-mediated polymerization reactions, including vinyl chain-growth polymerization, atom transfer radical polymerization, thiol-ene step-growth polymerization, and polymerization via oxidative coupling. While enzyme-mediated polymerization is useful for the production of materials intended for subsequent use, it is especially well-suited for in situ polymerizations, where the polymer is formed in the place where it will be utilized. Such polymerizations are especially useful for biomedical adhesives and for sensing applications.

Printable hybrid hydrogel by dual enzymatic polymerization with superactivity

Hybrid hydrogels were fabricated via a new approach employing a dual enzyme-mediated redox initiation reaction and their applications for 3D printing and biocatalysis.

A new approach has been developed to fabricate tough hybrid hydrogels by employing dual enzyme-mediated redox initiation to achieve post-self-assembly cross-linking polymerization. The resulting hydrogel combines the merits of supramolecular hydrogels with polymeric hydrogels to achieve higher mechanical strength and porous networks. Designed 3D constructs were fabricated via in situ 3D printing. The in situ immobilized GOx/HRP in Gel II exhibited superactivity compared to free enzymes, which might be attributed to the synergistic effect of co-localized GOx and HRP minimizing the distances for mass transport between the gel and the bulk solution. This mechanically strong hybrid hydrogel maintained high reusability and thermal stability as well. In addition, in situ 3D cell culture was demonstrated, thus indicating that this biodegradable hybrid hydrogel is biocompatible with cells. The subsequent 3D cell printing further indicates that the hybrid hydrogel is a promising scaffold for bio-related applications such as biocatalysis and tissue engineering.

Viscosity-controlled printing of supramolecular-polymeric hydrogels via dual-enzyme catalysis

Hybrid hydrogels based on a guanidinium-containing oligopeptide are prepared via dual-enzyme-triggered reactions. An extended time window is available for in situ viscosity-controlled 3D printing.

Microgel coating of magnetic nanoparticles via bienzyme-mediated free-radical polymerization for colorimetric detection of glucose

This study describes a new strategy for the fabrication of magnetic core-shell microgels by free-radical polymerization triggered by the cascade reaction of glucose oxidase (GOx) and horseradish peroxidase (HRP). The mild polymerization around the interface of the magnetic nanoparticles permits the mild coating of the microgel layer with excellent characteristics for various applications in biocatalysis and medical diagnostics, as well as in clinical fields. The immobilized bienzyme within the microgel has a largely retained activity relative to the non-immobilized one. The confining effect of the microgel and the well designed distance between the two enzymes can benefit the diffusion of intermediates to the HRP active site. The final microgels can be incontestably employed as sensitive biosensors for colorimetric glucose detection.

Polymerization amplified detection for nanoparticle-based biosensing

Efficient signal amplification processes are key to the design of sensitive assays for biomolecule detection. Here, we describe a new assay platform that takes advantage of both polymerization reactions and the aggregation of nanoparticles to amplify signal. In our design, a cascade is set up in which radicals generated by either enzymes or metal ions are polymerized to form polymers that can entangle multiple gold nanoparticles (AuNPs) into aggregates, resulting in a visible color change. Less than 0.05% monomer-to-polymer conversion is required to initiate aggregation, providing high sensitivity toward the radical generating species. Good sensitivity of this assay toward horseradish peroxidase, catalase, and parts per billion concentrations of iron and copper is shown. Incorporation of the oxygen-consuming enzyme glucose oxidase (GOx), enables this assay to be performed in open air conditions at ambient temperature. We anticipate that such a design will provide a useful platform for sensitive detection of a broad range of biomolecules through polymerization-based amplification.

Oxygen-Mediated Enzymatic Polymerization of Thiol-Ene Hydrogels

Materials that solidify in response to an initiation stimulus are currently utilized in several biomedical and surgical applications; however, their clinical adoption would be more widespread with improved physical properties and biocompatibility. One chemistry that is particularly promising is based on the thiol-ene addition reaction, a radical-mediated step-growth polymerization that is resistant to oxygen inhibition and thus is an excellent candidate for materials that polymerize upon exposure to aerobic conditions. Here, thiol-ene-based hydrogels are polymerized by exposing aqueous solutions of multi-functional thiol and allyl ether PEG monomers, in combination with enzymatic radical initiating systems, to air. An initiating system based on glucose oxidase, glucose, and Fe²⁺ is initially investigated where, in the presence of glucose, the glucose oxidase reduces oxygen to hydrogen peroxide which is then further reduced by Fe²⁺ to yield hydroxyl radicals capable of initiating thiol-ene polymerization. While this system is shown to effectively initiate polymerization after exposure to oxygen, the polymerization rate does not monotonically increase with raised Fe²⁺ concentration owing to inhibitory reactions that retard polymerization at higher Fe²⁺ concentrations. Conversely, replacing the Fe²⁺ with horseradish peroxidase affords an initiating system is that is not subject to the iron-mediated inhibitory reactions and enables increased polymerization rates to be attained.

Interfacial thiol-ene photoclick reactions for forming multilayer hydrogels

Interfacial visible light-mediated thiol-ene photoclick reactions were developed for preparing step-growth hydrogels with multilayer structures. The effect of a noncleavage type photoinitiator eosin-Y on visible-light-mediated thiol-ene photopolymerization was first characterized using in situ photorheometry, gel fraction, and equilibrium swelling ratio. Next, spectrophotometric properties of eosin-Y in the presence of various relevant macromer species were evaluated using ultraviolet-visible light (UV-vis) spectrometry. It was determined that eosin-Y was able to reinitiate the thiol-ene photoclick reaction, even after light exposure. Because of its small molecular weight, most eosin-Y molecules readily leached out from the hydrogels. The diffusion of residual eosin-Y from preformed hydrogels was exploited for fabricating multilayer step-growth hydrogels. Interfacial hydrogel coating was formed via the same visible-light-mediated gelation mechanism without adding fresh initiator. The thickness of the thiol-ene gel coating could be easily controlled by adjusting visible light exposure time, eosin-Y concentration initially loaded in the core gel, or macromer concentration in the coating solution. The major benefits of this interfacial thiol-ene coating system include its simplicity and cytocompatibility. The formation of thiol-ene hydrogels and coatings neither requires nor generates any cytotoxic components. This new gelation chemistry may have great utilities in controlled release of multiple sensitive growth factors and encapsulation of multiple cell types for tissue regeneration.

Formation of Core-Shell Particles by Interfacial Radical Polymerization Initiated by a Glucose Oxidase-Mediated Redox System

A unique design paradigm to form core-shell particles based on interfacial radical polymerization is described. The interfacial initiation system is comprised of an enzymatic reaction between glucose and glucose oxidase (GOx) to generate hydrogen peroxide, which, in the presence of iron (Fe2+), generates hydroxyl radicals that initiate polymerization. Shell formation on prefabricated polymeric cores is achieved by localizing the initiation reaction to the interface of the core and a surrounding aqueous monomer formulation into which it is immersed. The interfacially confined initiation reaction is accomplished by incorporating one or more of the initiating species in the particle core and the remainder of the complementary initiating components in the surrounding media such that interactions and the resulting initiation reaction occur at the interface. This work is focused on engineering the reaction behavior and mass transport processes to promote interfacially confined polymerization, controlling the rate of shell formation, and manipulating the structure of the core-shell particle. Specifically, incorporating GOx in the precursor solution used to fabricate cores ranging from 100 to 200 μm, and the remainder of the complementary initiating components and monomer in the bulk solution prior to interfacial polymerization yielded shells whose average thickness was 20 μm after 4 min of immersion and at a bulk iron concentration of 12.5 mM. When the locations of glucose and GOx are interchanged, the average thickness of the shell was 15 or 100 μm for bulk iron concentrations of 45 and 12.5 mM, respectively. The initial locations of glucose and GOx also determine the degree of interpenetration of the core and the shell. Specifically, for a bulk iron concentration of 45 mM, the thickness of the interpenetrating layer averaged 12 μm when GOx was initially within the core, whereas no interpenetrating layer was observed when glucose was incorporated in the core. The polymeric shell formed by this technique is also demonstrated to be self-supporting following core degradation. This behavior is accomplished by fabricating the particle core hydrogel from monomers possessing degradable groups that can be irreversibly cleaved by light exposure following shell formation. When the coated particle was exposed to light, the shell remained intact while the core degraded as evidenced by a dramatic change in diffusion coefficient of fluorescent beads immobilized within the core.

Thiol-ene photopolymerizations provide a facile method to encapsulate proteins and maintain their bioactivity

Photoinitiated polymerization remains a robust method for fabrication of hydrogels, as these reactions allow facile spatial and temporal control of gelation and high compatibility for encapsulation of cells and biologics. The chain-growth reaction of macromolecular monomers, such as acrylated PEG and hyaluronan, is commonly used to form hydrogels, but there is growing interest in step-growth photopolymerizations, such as the thiol-ene "click" reaction, as an alternative. Thiol-ene reactions are not susceptible to oxygen inhibition and rapidly form hydrogels using low initiator concentrations. In this work, we characterize the differences in recovery of bioactive proteins when exposed to similar photoinitiation conditions during thiol-ene versus acrylate polymerizations. Following exposure to chain polymerization of acrylates, lysozyme bioactivity was approximately 50%; after step-growth thiol-ene reaction, lysozyme retained nearly 100% of its prereaction activity. Bioactive protein recovery was enhanced 1000-fold in the presence of a thiol-ene reaction, relative to recovery from solutions containing identical primary radical concentrations, but without the thiol-ene components. When the cytokine TGFβ was encapsulated in PEG hydrogels formed via the thiol-ene reaction, full protein bioactivity was preserved.

Kinetics of interfacial radical polymerization initiated by a glucose-oxidase mediated redox system

The reaction and coating kinetics for the glucose oxidase initiated interfacial polymerization are elaborated. The interfacial film grows rapidly and linearly with time, producing time-dependent controllable conformal coating thicknesses of up to a millimeter in less than 4 min. Bulk polymerization was only observed when the immersing media was stirred to induce higher mass transport rates. The dramatically different film thicknesses observed between different concentrations of glucose in the hydrogel and iron in the bulk media are demonstrated to be a result of an initial rapid growth phase following which the film grows at the same rate nearly independent of either the glucose or iron concentration. The polymerization rate and hence the thickness growth rate in this initial phase saturate at glucose and iron concentrations above 0.8 M and 0.63 mM, respectively. At iron concentrations above 0.05 mM, the film thickness at the end of 3 h of reaction monotonically decreased with increasing iron concentration from 5.7 mm to 4.2 mm. The glucose oxidase is trapped by the growing polymerization front and can be used as the sole enzymatic precursor to coat a second polymeric layer. However, the rate of film growth of the second layer is 14-fold lower than the rate of film growth when bulk enzyme is present during the second stage coating process.

Antigen-responsive, microfluidic valves for single use diagnostics

The growing need for medical diagnostics in resource limited settings is driving the development of simple, standalone immunoassay devices. A capillary flow device using polymerization based amplification is capable of blocking a microfluidic channel in response to target biomaterials, enabling multiple modes of detection that require little or no supplemental instrumentation.

Functionalized PEG hydrogels through reactive dip-coating for the formation of immunoactive barriers

Influencing the host immune system via implantable cell-delivery devices has the potential to reduce inflammation at the transplant site and increase the likelihood of tissue acceptance. Towards this goal, an enzymatically-initiated, dip-coating technique is adapted to fabricate conformal hydrogel layers and to create immunoactive polymer coatings on cell-laden poly(ethylene glycol) (PEG) hydrogels. Glucose oxidase (GOx)-initiated dip coatings enable the rapid formation of uniform, PEG-based coatings on the surfaces of PEG hydrogels, with thicknesses up to 500 μm where the thickness is proportional to the reaction time. Biofunctional coatings were fabricated by thiolating biomolecules that were subsequently covalently incorporated into the coating layer via thiol-acrylate copolymerization. The presence of these proteins was verified via fluorescent confocal microscopy and a modified ELISA, which indicated IgG concentrations as high as 13 ± 1 ng/coated cm² were achievable. Anti-Fas antibody, known to induce T cell apoptosis, was incorporated into coatings, with or without the addition of ICAM-1 to promote T cell interaction with the functionalized coating. Jurkat T cells were seeded atop functionalized coatings and the induction of apoptosis was measured as an indicator of coating bioactivity. After 48 h of interaction with the functionalized coatings, 61 ± 9% of all cells were either apoptotic or dead, compared to only 18 ± 5% of T cells on non-functionalized coatings. Finally, the cytocompatibility of the surface-initiated GOx coating process was confirmed by modifying gels with either encapsulated β-cells or 3T3 fibroblasts within a gel that contained a PEG methacrylate coating.

Glucose oxidase-mediated polymerization as a platform for dual-mode signal amplification and biodetection

We report the first use of a polymerization-based ELISA substrate solution employing enzymatically mediated radical polymerization as a dual-mode amplification strategy. Enzymes are selectively coupled to surfaces to generate radicals that subsequently lead to polymerization-based amplification (PBA) and biodetection. Sensitivity and amplification of the polymerization-based detection system were optimized in a microwell strip format using a biotinylated microwell surface with a glucose oxidase (GOx)–avidin conjugate. The immobilized GOx is used to initiate polymerization, enabling the detection of the biorecognition event visually or through the use of a plate reader. Assay response is compared to that of an enzymatic substrate utilizing nitroblue tetrazolium in a simplified assay using biotinylated wells. The polymerization substrate exhibits equivalent sensitivity (2 µg/mL of GOx-avidin) and over three times greater signal amplification than this traditional enzymatic substrate since each radical that is enzymatically generated leads to a large number of polymerization events. Enzyme-mediated polymerization proceeds in an ambient atmosphere without the need for external energy sources, which is an improvement upon previous PBA platforms. Substrate formulations are highly sensitive to both glucose and iron concentrations at the lowest enzyme concentrations. Increases in amplification time correspond to higher assay sensitivities with no increase in non-specific signal. Finally, the polymerization substrate generated a signal to noise ratio of 14 at the detection limit (156 ng/mL) in an assay of transforming growth factor-beta. Biotechnol. Bioeng. 2011; 108:1521–1528. © 2011 Wiley Periodicals, Inc.

Formation of three-dimensional hydrogel multilayers using enzyme-mediated redox chain initiation

Enzyme-mediated redox chain initiation involving glucose oxidase (GOX) was employed in an iterative solution dip-coating technique to polymerize multiple, three-dimensional hydrogel layers using mild aqueous conditions at ambient temperature and oxygen levels. To the best of our knowledge, sequential enzyme-mediated dip-coating resulting in an interfacial radical chain polymerization and subsequent formation of three-dimensional hydrogel layers has not been previously explored. Conformal, micrometer-scale, uniform poly(ethylene glycol) (PEG)-based hydrogel layers were polymerized within seconds and remained securely associated after incubation in water for 16 weeks. Incorporation of either small molecules (i.e., rhodamine-B acrylate, fluorescein acrylate) or fluorescent nanoparticles into crosslinked hydrogel layers during the polymerization reaction was also achieved. The encapsulation of 0.2 microm-diameter nanoparticles into hydrogels during polymerization of a 2-hydroxyethyl acrylate (HEA)/PEG(575) diacrylate monomer formulation, using the GOX-mediated initiation, resulted in minimal effects on polymerization kinetics, with final acrylate conversions of 95% (+/- 1%) achieved within minutes. The temporal control and spatial localization afforded by this interfacial redox approach resulted in the polymerization of uniform secondary layers ranging between 150 (+/- 10) microm and 650 (+/- 10) microm for 15 and 120 s immersion times, respectively. Moreover, increasing the PEG(575)-fraction within the initial hydrogel substrate from 10 to 50% decreased the subsequent layer thicknesses from 690 (+/- 30) microm to 490 (+/- 10) microm because of lowered glucose concentration at the hydrogel interface. The ability to sequentially combine differing initiation mechanisms with this coating approach was achieved by using GOX-mediated interfacial polymerization on hydrogel substrates initially photopolymerized in the presence of glucose. The strict control of layer thicknesses combined with the rapid, water-soluble, and mild polymerization will readily benefit applications requiring formation of stratified, complex, and three-dimensional polymer structures.