fabricating-degradable-thermoresponsive-hydrogels-on-multiple-length

Fabricating Degradable Thermoresponsive Hydrogels on Multiple Length Scales via Reactive Extrusion, Microfluidics, Self-assembly, and Electrospinning

While various smart materials have been explored for a variety of biomedical applications (e.g., drug delivery, tissue engineering, bioimaging, etc.), ...

McMaster University

The design and application of soft nanocomposite injectable hydrogels containing entrapped microgels for small-molecule drug delivery is demonstrated. Copolymer microgels based on N-isopropylacrylamide and acrylic acid were synthesized that exhibited both ionic and hydrophobic affinity for binding to bupivacaine, a cationic local anesthetic used as a model drug. Microgels were subsequently immobilized within an in situ-gelling hydrogel network cross-linked via hydrazide-aldehyde chemistry to generate hydrogel-microgel soft nanocomposites. Drug release could be sustained for up to 60 days from these nanocomposite hydrogels, significantly longer than that achievable using the constituent hydrogel or microgels alone (<1 week). Drug release kinetics could be readily tuned by varying the affinity of the microgel and hydrogel phases for drug-polymer interactions and the network density of the hydrogel phase.

Injectable microgel-hydrogel composites for prolonged small-molecule drug delivery.

The use of functional nanogels based on poly(N-isopropylacrylamide) for effectively scavenging compounds (here, the model drug bupivacaine) is demonstrated using an in vitro cell-based assay. Nanogels containing higher loadings of acidic functional groups or more core-localized functional group distributions bound more bupivacaine, while nanogel size had no significant effect on drug binding. Increasing the dose of nanogel applied also facilitated more bupivacaine binding for all nanogel compositions tested. Binding was driven predominantly by acid-base interactions between the nanogels (anionic) and bupivacaine (cationic) at physiological pH, although both non-specific absorption and hydrophobic partitioning also contributed to drug scavenging. Nanogels exhibited minimal cytotoxicity to multiple cell types and were well tolerated in vivo via peritoneal injections, although larger nanogels caused limited splenic toxicity at higher concentrations. The cell-based assay described herein is found to facilitate more robust drug uptake measurements for nanogels than conventional centrifugation-based assays, in which nanogels can be compressed (and thus drug released) during the measurement.

Nanogel scavengers for drugs: local anesthetic uptake by thermoresponsive nanogels.

Soft nanocomposite hydrogels consisting of thermoresponsive microgels physically entrapped or covalently cross-linked to a non-thermoresponsive hydrogel are synthesized and tested for their capacity to facilitate long-term drug release of a small molecule drug. Copolymer microgels based on N-isopropylacrylamide and acrylic acid were synthesized that exhibited ionic affinity for binding to bupivacaine, a cationic local anesthetic. These microgels were subsequently physically entrapped within an in situ-gelling carbohydrate-based hydrogel network cross-linked via hydrazide-aldehyde chemistry; alternately, hydrazide-functionalized microgels were prepared that covalently cross-linked to the bulk hydrogel phase. Both the overall rate of drug release and the magnitude of the burst release were significantly decreased when microgels were restricted from undergoing a phase transition between the preparation temperature of the nanocomposite (25°C) and the test temperature (37°C), whether deswelling was inhibited by increasing the cross-link density within the microgel itself or by cross-linking the microgel to the bulk hydrogel network. This result facilitates facile tuning of soft nanocomposite drug delivery systems to achieve targeted drug release kinetics.

Tuning drug release from smart microgel-hydrogel composites via cross-linking.

A simple, rapid, solvent-free, and scalable thermally driven self-assembly approach is described to produce monodisperse, covalently cross-linked, and degradable poly(N-isopropylacrylamide) (PNIPAM) microgels based on mixing hydrazide (PNIPAM-Hzd) and aldehyde (PNIPAM-Ald) functionalized PNIPAM precursors. Preheating of a seed PNIPAM-Hzd solution above its phase transition temperature produces nanoaggregates that are subsequently stabilized and cross-linked by the addition of PNIPAM-Ald. The ratio of PNIPAM-Hzd:PNIPAM-Ald used to prepare the microgels, the time between PNIPAM-Ald addition and cooling, the temperature to which the PNIPAM-Hzd polymer solution is preheated, and the concentration of PNIPAM-Hzd in the initial seed solution can all be used to control the size of the resulting microgels. The microgels exhibit similar thermal phase transition behavior to conventional precipitation-based microgels but are fully degradable into oligomeric precursor polymers. The microgels can also be lyophilized and redispersed without any change in colloidal stability or particle size and exhibit no significant cytotoxicity in vitro. We anticipate that microgels fabricated using this approach may facilitate translation of the attractive properties of such microgels in vivo without the concerns regarding microgel clearance that exist with other PNIPAM-based microgels. 

Temperature-Induced Assembly of Monodisperse, Covalently Cross-Linked, and Degradable Poly(N-isopropylacrylamide) Microgels Based on Oligomeric Precursors.

The potential of hydrophobically-modified poly(vinyl pyrrolidone) as a shear-responsive, self-associative hydrogel for ophthalmic applications is demonstrated. Hydrophobic modification was achieved via random copolymerization of N-vinylpyrrolidone with N-vinylformamide, the latter of which can be hydrolyzed to expose a desired degree of reactive amine groups permitting grafting of alkyl chlorides of varying alkyl chain lengths. The resulting materials formed highly shear-responsive physical hydrogels, exhibiting tunable shear thinning over 4-5 decades of viscosity from infinite shear to zero shear conditions that facilitates lubrication upon blinking and/or facile injection or drop-based delivery to the anterior or posterior segments of the eye. Viscosity changes due to self-association over time can also be tuned by changing the length of the hydrophobe, with C18-grafted materials exhibiting prolonged thickening over several weeks to form extremely stiff hydrogels and shorter grafts equilibrating significantly faster but forming weaker gels. The hydrogels remained transparent even at very high polymer concentrations (20 wt%) and are demonstrated to facilitate controlled release of a model drug (doxorubicin). The polymers exhibit minimal cytotoxicity in vitro to human corneal epithelial cells and retinal pigment epithelial cells, particularly when lower molecular weight backbone polymers were used. In vivo assessments in rabbits indicated no significant conjunctival edema or redness, secretion, corneal opacity, or iris involvement upon anterior application. Following intravitreal injection in rat eyes, no opacification of the lens, cornea or vitreous, nor any morphological or functional change to the posterior segment was observed. Examination of wholemount tissues and histology demonstrated no adverse effect from the injection or deposition of material. As such, these shear-thinning materials offer potential for drug delivery in both the anterior and posterior segments or as a vitreal replacement that can be easily administered or removed.

Hydrophobically-modified poly(vinyl pyrrolidone) as a physically-associative, shear-responsive ophthalmic hydrogel.

Highly monodisperse and hydrolytically degradable thermoresponsive microgels on the tens-to-hundreds of micron size scale have been fabricated based on simultaneous on-chip mixing and emulsification of aldehyde and hydrazide-functionalized poly(N-isopropylacrylamide) precursor polymers. The microfluidic chip can run for extended periods without upstream gelation and can produce monodisperse (<10% particle size variability) microgels on the size range of ∼30-90 μm, with size tunable according to the flow rate of the oil continuous phase. Fluorescence analysis indicates a uniform distribution of each reactive pre-polymer inside the microgels while micromechanical testing suggests that smaller microfluidic-produced microgels exhibit significantly higher compressive moduli compared to bulk hydrogels of the same composition, an effect we attribute to improved mixing (and thus crosslinking) of the precursor polymer solutions within the microfluidic device. The microgels retain the reversible volume phase transition behavior of conventional microgels but can be hydrolytically degraded back into their oligomeric precursor polymer fragments, offering potential for microgel clearance following use in vivo.

Article	Year
Injectable microgel-hydrogel composites for prolonged small-molecule drug delivery. Biomacromolecules\| PubMed ID: 22007750	2011
Nanogel scavengers for drugs: local anesthetic uptake by thermoresponsive nanogels. Acta biomaterialia\| PubMed ID: 22244983	2012
Tuning drug release from smart microgel-hydrogel composites via cross-linking. Journal of colloid and interface science\| PubMed ID: 23137903	2013
Temperature-Induced Assembly of Monodisperse, Covalently Cross-Linked, and Degradable Poly(N-isopropylacrylamide) Microgels Based on Oligomeric Precursors. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 25977976	2015
Hydrophobically-modified poly(vinyl pyrrolidone) as a physically-associative, shear-responsive ophthalmic hydrogel. Experimental eye research\| PubMed ID: 26044117	2015
Microfluidic production of degradable thermoresponsive poly(N-isopropylacrylamide)-based microgels. Soft matter\| PubMed ID: 29177347	2017

Daryl Sivakumaran

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