Gregory N. Grover

Emerging synthetic approaches for protein–polymer conjugations

Protein-polymer conjugates are important in diverse fields including drug delivery, biotechnology, and nanotechnology. This feature article highlights recent advances in the synthesis and application of protein-polymer conjugates by controlled radical polymerization techniques. Special emphasis on new applications of the materials, particularly in biomedicine, is provided.

Biocompatible Hydrogels by Oxime Click Chemistry

Oxime Click chemistry was used to form hydrogels that support cell adhesion. Eight-armed aminooxy poly(ethylene glycol) (PEG) was mixed with glutaraldehyde to form oxime-linked hydrogels. The mechanical properties, gelation kinetics, and water swelling ratios were studied and found to be tunable. It was also shown that gels containing the integrin ligand arginine-glycine-aspartic acid (RGD) supported mesenchymal stem cell (MSC) incorporation. High cell viability and proliferation of the encapsulated cells demonstrated biocompatibility of the material.

Protein-polymer conjugates: synthetic approaches by controlled radical polymerizations and interesting applications.

Protein-polymer conjugates are of interest to researchers in diverse fields. Attachment of polymers to proteins results in improved pharmacokinetics, which is important in medicine. From an engineering standpoint, conjugates are exciting because they exhibit properties of both the biomolecules and synthetic polymers. This allows the activity of the protein to be altered or tuned, anchoring to surfaces, and supramolecular self-assembly. Thus, there is broad interest in straightforward synthetic methods to prepare protein-polymer conjugates. Controlled radical polymerization (CRP) techniques have emerged as excellent strategies to make conjugates because the resulting polymers have narrow molecular weight distributions, targeted molecular weights, and attach to specific sites on proteins. Herein, recent advances in the synthesis and application of protein-polymer conjugates by CRP are highlighted.

Synthetic approach to homodimeric protein–polymer conjugates

Synthesis of well-defined homodimeric protein-polymer conjugates using RAFT polymerization is described.

Oxime Cross‐Linked Injectable Hydrogels for Catheter Delivery

Catheter delivery of therapeutic materials is important for developing minimally invasive treatment approaches. However, the majority of injectable materials gel rapidly upon mixing and/or heating to body temperature. The application of an oxime cross-linked hydrogel system is demonstrated. The system has a broad range of tunable gelation rates, is capable of injection through a catheter, and exhibits rapid gelation upon injection into tissue.

Myocardial matrix?polyethylene glycol hybrid hydrogels for tissue engineering

Similar to other protein-based hydrogels, extracellular matrix (ECM) based hydrogels, derived from decellularized tissues, have a narrow range of mechanical properties and are rapidly degraded. These hydrogels contain natural cellular adhesion sites, form nanofibrous networks similar to native ECM, and are biodegradable. In this study, we expand the properties of these types of materials by incorporating poly(ethylene glycol) (PEG) into the ECM network. We use decellularized myocardial matrix as an example of a tissue specific ECM derived hydrogel. Myocardial matrix-PEG hybrids were synthesized by two different methods, cross-linking the proteins with an amine-reactive PEG-star and photo-induced radical polymerization of two different multi-armed PEG-acrylates. We show that both methods allow for conjugation of PEG to the myocardial matrix by gel electrophoresis and infrared spectroscopy. Scanning electron microscopy demonstrated that the hybrid materials still contain a nanofibrous network similar to unmodified myocardial matrix and that the fiber diameter is changed by the method of PEG incorporation and PEG molecular weight. PEG conjugation also decreased the rate of enzymatic degradation in vitro, and increased material stiffness. Hybrids synthesized with amine-reactive PEG had gelation rates of 30 min, similar to the unmodified myocardial matrix, and incorporation of PEG did not prevent cell adhesion and migration through the hydrogels, thus offering the possibility to have an injectable ECM hydrogel that degrades more slowly in vivo. The photo-polymerized radical systems gelled in 4 min upon irradiation, allowing 3D encapsulation and culture of cells, unlike the soft unmodified myocardial matrix. This work demonstrates that PEG incorporation into ECM-based hydrogels can expand material properties, thereby opening up new possibilities for in vitro and in vivo applications.

Heterotelechelic polymers for capture and release of protein–polymer conjugates

A heterotelechelic biotin-maleimide polymer containing a cleavable disulfide bond was synthesized by RAFT polymerization and used to reversibly modify surfaces with proteins.

Tunable protein release from acetalated dextran microparticles: a platform for delivery of protein therapeutics to the heart post-MI.

The leading cause of death in the United States is cardiovascular disease. The majority of these cases result from heart failure post-myocardial infarction (MI). We present data providing evidence for use of acetalated dextran (AcDex) microparticles as a delivery vehicle for therapeutics to the heart post-MI. We harnessed the tunable degradation and acid-sensitivity of AcDex in the design of microparticles for intramyocardial injection. The particles released a model protein, myoglobin, and a sensitive growth factor, basic fibroblast growth factor (bFGF), over a wide range of time frames (from days to weeks) based on the percentage of cyclic acetals in the AcDex, which was easily controlled with acetalation reaction time. The release was shown in low pH environments, similar to what is found in an infarcted heart. bFGF maintained activity after release from the microparticles. Finally, biocompatibility of the microparticles was assessed.

A co-culture device with a tunable stiffness to understand combinatorial cell–cell and cell–matrix interactions

Cell behavior on 2-D in vitro cultures is continually being improved to better mimic in vivo physiological conditions by combining niche cues including multiple cell types and substrate stiffness, which are well known to impact cell phenotype. However, no system exists in which a user can systematically examine cell behavior on a substrate with a specific stiffness (elastic modulus) in culture with a different cell type, while maintaining distinct cell populations. We demonstrate the modification of a silicon reconfigurable co-culture system with a covalently linked hydrogel of user-defined stiffness. This device allows the user to control whether two separate cell populations are in contact with each other or only experience paracrine interactions on substrates of controllable stiffness. To illustrate the utility of this device, we examined the role of substrate stiffness combined with myoblast co-culture on adipose derived stem cell (ASC) differentiation and found that the presence of myoblasts and a 10 kPa substrate stiffness increased ASC myogenesis versus co-culture on stiff substrates. As this example highlights, this technology better controls the in vitro microenvironment, allowing the user to develop a more thorough understanding of the combined effects of cell-cell and cell-matrix interactions.

Synthesis of Michael Acceptor Ionomers of Poly(4-Sulfonated Styrene-co-Poly(ethylene Glycol) Methyl Ether Acrylate)

Ionomers containing sodium 4-styrene sulfonate (4SS) and poly(ethylene glycol) methyl ether acrylate (PEGA) were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. The polymerization was mediated by 1-phenylethyl dithiobenzoate chain transfer agent in a dimethylformamide/water solvent system. Well-defined copolymers of pPEGA-co-4SS were produced with molecular weights ranging from 10 kDa to 40 kDa and polydispersity indices (PDIs) of 1.06-1.18 by gel permeation chromatography (GPC) against monodisperse poly(methyl methacrylate) (PMMA) standards. Post polymerization, the dithioester was reduced and trapped in situ with divinyl sulfone to produce a well-defined, semitelechelic pPEGA-co-4SS Michael acceptor polymer. UV-vis, infrared, and (1)H NMR spectroscopy confirmed that the integrity of the polymer backbone was maintained and that the vinyl sulfone was successfully incorporated at the chain end.