Niels M. B. Smeets

Designing Injectable, Covalently Cross‐Linked Hydrogels for Biomedical Applications

Hydrogels that can form spontaneously via covalent bond formation upon injection in vivo have recently attracted significant attention for their potential to address a variety of biomedical challenges. This review discusses the design rules for the effective engineering of such materials, and the major chemistries used to form injectable, in situ gelling hydrogels in the context of these design guidelines are outlined (with examples). Directions for future research in the area are addressed, noting the outstanding challenges associated with the use of this class of hydrogels in vivo.

Injectable and tunable poly(ethylene glycol) analogue hydrogels based on poly(oligoethylene glycol methacrylate)

Injectable PEG-analogue hydrogels based on poly(oligoethylene glycol methacrylate) have been developed based on complementary hydrazide and aldehyde reactive linear polymer precursors. These hydrogels display the desired biological properties of PEG, form covalent networks in situ following injection, and are easily modulated for improved control over their functionality and physiochemical properties.

Injectable hydrogels based on poly(ethylene glycol) and derivatives as functional biomaterials

Hydrogels based on poly(ethylene glycol) (PEG) and derivatives have attracted significant interest in recent years given their capacity to be well-tolerated in vivo in the context of drug delivery and tissue engineering applications. Injectable, in situ-gelling analogues of such hydrogels offer the additional advantages of being easy and non-invasive to administer via the injection of low-viscosity precursor polymer solutions, expanding their scope of potential applications. In this highlight, we first review the design criteria associated with the rational design of in situ-gelling hydrogels for in vivo applications. We then discuss recent progress in the design of injectable PEG hydrogels, specifically highlighting our ongoing work on PEG-analogue hydrogels based on poly(oligoethylene glycol methacrylate) for targeted biomedical applications.

Catalytic chain transfer and its derived macromonomers

An overview is given of cobalt-catalyzed chain transfer in free-radical polymerization and the chemistry and applications of its derived macromonomers. Catalytic chain transfer polymerization is a very efficient and versatile technique for the synthesis of functional macromonomers. Firstly the mechanism and kinetic aspects of the process are briefly discussed in solution/bulk and in emulsion polymerization, followed by a description of its application to produce functional macromonomers. The second part of this review briefly describes the behavior of the macromonomers as chain transfer agents and/or comonomers in second-stage radical polymerizations yielding polymers of more complex architectures. The review ends with a brief overview of post-polymerization modifications of the vinyl endfunctionality of the macromonomers yielding functional polymers with applications ranging from initiators in anionic polymerization to end-functional lectin-binding glycopolymers.

Injectable poly(oligoethylene glycol methacrylate)-based hydrogels with tunable phase transition behaviours: Physicochemical and biological responses

The potential of poly(oligoethylene glycol methacrylate) (POEGMA) hydrogels with varying thermosensitivities as soft materials for biomedical applications is demonstrated. Hydrogels are prepared from hydrazide and aldehyde functionalized POEGMA precursors, yielding POEGMA hydrogels with a volume phase transition temperature (VPTT) below (PO0), close to (PO10) and well above (PO100) physiological temperature. Hydrogels with VPTTs close to and above physiological temperature exhibit biological properties similar to those typically observed for poly(ethylene glycol) hydrogels (i.e. low protein adsorption, low cell adhesion and minimal inflammatory responses in vivo) while hydrogels with VPTTs lower than physiological temperature exhibit biological properties more analogous to poly(N-isopropylacrylamide) above its phase transition temperature (temperature-switchable cell adhesion, higher protein adsorption and somewhat more acute inflammation in vivo). As such, the use of POEGMA precursors with varying chain lengths of ethylene oxide grafts offers a versatile platform for the design of hydrogels with tunable physiological properties via simple copolymerization.

Poly(oligoethylene glycol methacrylate) Dip-Coating: Turning Cellulose Paper into a Protein-Repellent Platform for Biosensors

The passivation of nonspecific protein adsorption to paper is a major barrier to the use of paper as a platform for microfluidic bioassays. Herein we describe a simple, scalable protocol based on adsorption and cross-linking of poly(oligoethylene glycol methacrylate) (POEGMA) derivatives that reduces nonspecific adsorption of a range of proteins to filter paper by at least 1 order of magnitude without significantly changing the fiber morphology or paper macroporosity. A lateral-flow test strip coated with POEGMA facilitates effective protein transport while also confining the colorimetric reporting signal for easier detection, giving improved performance relative to bovine serum albumin (BSA)-blocked paper. Enzyme-linked immunosorbent assays based on POEGMA-coated paper also achieve lower blank values, higher sensitivities, and lower detection limits relative to ones based on paper blocked with BSA or skim milk. We anticipate that POEGMA-coated paper can function as a platform for the design of portable, disposable, and low-cost paper-based biosensors.

Injectable hydrogels with in situ-forming hydrophobic domains: oligo(D,L-lactide) modified poly(oligoethylene glycol methacrylate) hydrogels

Injectable, in situ-gelling nanostructured hydrogels have been prepared from hydrazide and aldehyde-functionalized polymer precursors based on a copolymer of oligo(ethylene glycol) methacrylate (OEGMA) and an oligo(lactic acid) macromonomer (OLA) with varying lactic acid chain lengths. The resulting hydrogels contain a mix of chemical (hydrazone bond formation between hydrazide and aldehyde groups) and physical (hydrophobic interactions between OLA chains) cross-links which form competitively as a function of the OLA chain length and density. An increase in the OLA chain length and density results in the formation of more physical cross-links and fewer chemical cross-links. Tuning the relative prevalence of physical and chemical cross-link formation facilitated largely independent tuning of gel mechanics relative to gel swelling and degradation. Small-angle neutron scattering of these OLA-containing hydrogels reveals a microstructure consisting of associative hydrophobic domains, based on an increased scattering intensity and decreased blob size relative to that observed for POEGMA hydrogels prepared without the OLA co-monomer. The presence of hydrophobic OLA domains increases the uptake and slows the release of bovine serum albumin, a protein well-known to associate with hydrophobic domains. Coupled with the observed cytocompatibility of the reactive precursor polymers used to prepare the hydrogels, we anticipate significant potential applications of these hydrogels for the prolonged release of hydrophobic cargoes.

A New Method for the Preparation of Concentrated Translucent Polymer Nanolatexes from Emulsion Polymerization

A novel method for the preparation of concentrated, colloidally stable, translucent polymer nanolatexes is presented. Herein nanolatexes are obtained from emulsion polymerization, utilizing the potential of catalytic chain transfer to enhance the particle nucleation efficiency. Low amounts of emulsifier are required (<8% w/w based on monomer) while the nanolatexes concentration can be increased to 40% w/w. The nanolatexes are translucent in appearance, which was correlated to the average particle size and width of the particle size distribution using Mie theory. Increasing the nanolatex concentration was found to have no deteriorating effect on either the optical or colloidal properties. Preparing translucent nanolatexes via this method is advantageous, as the amount of emulsifier is significantly reduced without sacrificing the optical transparency or the high interfacial surface area of the polymer colloids.

A Highly Sensitive Immunosorbent Assay Based on Biotinylated Graphene Oxide and the Quartz Crystal Microbalance

A high-sensitivity flow-based immunoassay is reported based on a gold-coated quartz crystal microbalance (QCM) chip functionalized directly in the QCM without requiring covalent conjugation steps. Specifically, the irreversible adsorption of a biotinylated graphene oxide-avidin complex followed by loading of a biotinylated capture antibody is applied to avoid more complex conventional surface modification chemistries and enable chip functionalization and sensing all within the QCM instrument. The resulting immunosensors exhibit significantly lower nonspecific protein adsorption and stronger signal for antigen sensing relative to simple avidin-coated sensors. Reproducible quantification of rabbit IgG concentrations ranging from 0.1 ng/mL to 10 μg/mL (6 orders of magnitude) can be achieved depending on the approach used to quantify the binding with simple mass changes used to detect higher concentrations and a horseradish peroxidase-linked detection antibody that converts its substrate to a measurable precipitate used to detect very low analyte concentrations. Sensor fabrication and assay performance take ∼5 h in total, which is on par with or faster than other techniques. Quantitative sensing is possible in the presence of complex protein mixtures, such as human plasma. Given the broad availability of biotinylated capture antibodies, this method offers both an easy and flexible platform for the quantitative sensing of a variety of biomolecule targets.

“Off-the-shelf” thermoresponsive hydrogel design: tuning hydrogel properties by mixing precursor polymers with different lower-critical solution temperatures

“Off the shelf design” principles are applied to fabricate injectable poly(oligoethylene glycol methacrylate) (POEGMA) hydrogels with well-defined thermal phase transitions by mixing two sets of aldehyde and hydrazide-functionalized hydrogel precursors, one set with a low lower critical solution temperature (LCST) and one set with a high LCST, at different ratios. Macroscopically, gelation rates, swelling kinetics, degradation kinetics, and mechanical properties of hydrogels produced by mixing different precursor polymers can be predicted and designed based on the simple rule of mixtures. Microscopically, phase separated domains result in localized phase transitions within the hydrogels, inducing significant changes in protein affinity, drug release kinetics, transparency, and cell adhesion compared to hydrogels with only single-LCST precursors that may be leveraged in biomedical applications. Such an “off the shelf” design approach involving the simple mixing of a limited number (four) of precursor polymers with different LCSTs can thus achieve a full range of hydrogel properties while avoiding laborious polymerization steps each time a gel with different properties is required.