There has been considerable progress in recent years in addressing the clinical and pharmacological limitations of hydrogels for drug delivery applications but substantial challenges remain. Here we ...discuss recent progress in overcoming these challenges, particularly with regards to effectively delivering hydrogels inside the body without implantation, prolonging the release kinetics of drugs from hydrogels, and expanding the nature of drugs which can be delivered using hydrogel-based approaches.
Injectable, covalently in situ forming hydrogels based on poly(N-isopropylacrylamide) have been designed on the basis of mixing hydrazide-functionalized nucleophilic precursor polymers with ...electrophilic precursor polymers functionalized with a combination of ketone (slow reacting) and aldehyde (fast reacting) functional groups. By tuning the ratio of aldehyde:ketone functional groups as well as the total number of ketone groups in the electrophilic precursor polymer, largely independent control over hydrogel properties including gelation time (from seconds to hours), degradation kinetics (from hours to months), optical transmission (from 1 to 85%), and mechanics (over nearly 1 order of magnitude) can be achieved. In addition, ketone-functionalized precursor polymers exhibit improved cytocompatibility at even extremely high concentrations relative to polymers functionalized with aldehyde groups, even at 4-fold higher functional group densities. Overall, increasing the ketone content of the precursor copolymers can result in in situ-gellable hydrogels with improved transparency and biocompatibility and equivalent mechanics and stimuli-responsiveness while only modestly sacrificing the speed of gel formation.
Abstract With the goal of designing a clinically-relevant expansion strategy for human adipose-derived stem/stromal cells (ASCs), methods were developed to synthesize porous microcarriers derived ...purely from human decellularized adipose tissue (DAT). An electrospraying approach was applied to generate spherical DAT microcarriers with an average diameter of 428 ± 41 μm, which were soft, compliant, and stable in long-term culture without chemical crosslinking. Human ASCs demonstrated enhanced proliferation on the DAT microcarriers relative to commercially-sourced Cultispher-S microcarriers within a spinner culture system over 1 month. ASC immunophenotype was maintained post expansion, with a trend for reduced expression of the cell adhesion receptors CD73, CD105, and CD29 under dynamic conditions. Upregulation of the early lineage-specific genes PPARγ, LPL , and COMP was observed in the ASCs expanded on the DAT microcarriers, but the cells retained their multilineage differentiation capacity. Comparison of adipogenic and osteogenic differentiation in 2-D cultures prepared with ASCs pre-expanded on the DAT microcarriers or Cultispher-S microcarriers revealed similar adipogenic and enhanced osteogenic marker expression in the DAT microcarrier group, which had undergone a higher population fold change. Further, histological staining results suggested a more homogeneous differentiation response in the ASCs expanded on the DAT microcarriers as compared to either Cultispher-S microcarriers or tissue culture polystyrene. A pilot chondrogenesis study revealed higher levels of chondrogenic gene and protein expression in the ASCs expanded on the DAT microcarriers relative to all other groups, including the baseline controls. Overall, this study demonstrates the promise of applying dynamic culture with tissue-specific DAT microcarriers as a means of deriving regenerative cell populations.
The interactions of a range of water-soluble drugs of different charges and hydrophobicities with carboxylic acid-functionalized poly(N-isopropylacrylamide)-based microgels containing different ...functional group distributions are investigated to determine the impact of drug properties and microgel morphologies on drug uptake and release. The radial distribution of carboxylic acid functional groups in the microgel and the hydrophobicities of the cationic drugs both strongly affect drug partitioning between the solution and microgel phases. Microgels with surface-localized functional group distributions bind less cationic drug than bulk-functionalized microgels, likely due to the formation of a locally collapsed “skin layer” at the acid−base drug binding sites at the microgel surface. In this way, cationic drugs induce a local phase transition that can be used to regulate small molecule diffusion in and out of the gel. As the drug hydrophobicity is increased, the skin layer becomes more condensed and less drug uptake is achieved. In the case of anionic or neutral drugs, high drug uptakes are achieved independent of the functional group distribution within the microgel. High drug uptake is also observed when nonfunctionalized poly(N-isopropylacrylamide) microgels are used as the uptake matrix, suggesting the importance of hydrophobic partitioning in regulating drug−microgel interactions.
Cellulose nanocrystals (CNCs) are emerging nanomaterials that form chiral nematic liquid crystals above a critical concentration (C*) and additionally orient within electromagnetic fields. The ...control over CNC alignment is significant for materials processing and end use; to date, magnetic alignment has been demonstrated using only strong fields over extended or arbitrary time scales. This work investigates the effects of comparatively weak magnetic fields (0–1.2 T) and CNC concentration (1.65–8.25 wt %) on the kinetics and degree of CNC ordering using small-angle X-ray scattering. Interparticle spacing, correlation length, and orientation order parameters (η and S) increased with time and field strength following a sigmoidal profile. In a 1.2 T magnetic field for CNC suspensions above C*, partial alignment occurred in under 2 min followed by slower cooperative ordering to achieve nearly perfect alignment in under 200 min (S = −0.499 where S = −0.5 indicates perfect antialignment). At 0.56 T, nearly perfect alignment was also achieved, yet the ordering was 36% slower. Outside of a magnetic field, the order parameter plateaued at 52% alignment (S = −0.26) after 5 h, showcasing the drastic effects of relatively weak magnetic fields on CNC alignment. For suspensions below C*, no magnetic alignment was detected.
A direct, all-aqueous electrospinning method for fabricating degradable nanofibrous hydrogel networks is reported in which hydrazide and aldehyde-functionalized poly(oligoethylene glycol ...methacrylate) (POEGMA) polymers are simultaneously electrospun and cross-linked. The resulting networks are spatially well-defined, mechanically stable (both dry and wet), and offer extremely fast swelling responses, suggesting potential utility as smart hydrogels and tunable tissue engineering matrices.
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.
The design criteria for injectable,
in situ
-gelling hydrogels are reviewed in conjunction with highlights on recent progress in the preparation of injectable PEG and PEG-analogue poly(oligoethylene glycol methacrylate) (POEGMA) hydrogels.
Injectable, in situ-gelling magnetic composite materials have been fabricated by using aldehyde-functionalized dextran to cross-link superparamagnetic nanoparticles surface-functionalized with ...hydrazide-functionalized poly(N-isopropylacrylamide) (pNIPAM). The resulting composites exhibit high water contents (82–88 wt.%) while also displaying significantly higher elasticities (G′ >60 kPa) than other injectable hydrogels previously reported. The composites hydrolytically degrade via slow hydrolysis of the hydrazone cross-link at physiological temperature and pH into degradation products that show no significant cytotoxicity. Subcutaneous injections indicate only minor chronic inflammation associated with material degradation, with no fibrous capsule formation evident. Drug release experiments indicate the potential of these materials to facilitate pulsatile, “on-demand” changes in drug release upon the application of an external oscillating magnetic field. The injectable but high-strength and externally triggerable nature of these materials, coupled with their biological degradability and inertness, suggest potential biological applications in tissue engineering and drug delivery.
While injectable hydrogels have several advantages in the context of biomedical use, their generally weak mechanical properties often limit their applications. Herein, we describe in situ-gelling ...nanocomposite hydrogels based on poly(oligoethylene glycol methacrylate) (POEGMA) and rigid rod-like cellulose nanocrystals (CNCs) that can overcome this challenge. By physically incorporating CNCs into hydrazone cross-linked POEGMA hydrogels, macroscopic properties including gelation rate, swelling kinetics, mechanical properties, and hydrogel stability can be readily tailored. Strong adsorption of aldehyde- and hydrazide-modified POEGMA precursor polymers onto the surface of CNCs promotes uniform dispersion of CNCs within the hydrogel, imparts physical cross-links throughout the network, and significantly improves mechanical strength overall, as demonstrated by quartz crystal microbalance gravimetry and rheometry. When POEGMA hydrogels containing mixtures of long and short ethylene oxide side chain precursor polymers were prepared, transmission electron microscopy reveals that phase segregation occurs with CNCs hypothesized to preferentially locate within the stronger adsorbing short side chain polymer domains. Incorporating as little as 5 wt % CNCs results in dramatic enhancements in mechanical properties (up to 35-fold increases in storage modulus) coupled with faster gelation rates, decreased swelling ratios, and increased stability versus hydrolysis. Furthermore, cell viability can be maintained within 3D culture using these hydrogels independent of the CNC content. These properties collectively make POEGMA–CNC nanocomposite hydrogels of potential interest for various biomedical applications including tissue engineering scaffolds for stiffer tissues or platforms for cell growth.
While injectable in situ cross-linking hydrogels have attracted increasing attention as minimally invasive tissue scaffolds and controlled delivery systems, their inherently disorganized and ...isotropic network structure limits their utility in engineering oriented biological tissues. Traditional methods to prepare anisotropic hydrogels are not easily translatable to injectable systems given the need for external equipment to direct anisotropic gel fabrication and/or the required use of temperatures or solvents incompatible with biological systems. Herein, we report a new class of injectable nanocomposite hydrogels based on hydrazone cross-linked poly(oligoethylene glycol methacrylate) and magnetically aligned cellulose nanocrystals (CNCs) capable of encapsulating skeletal muscle myoblasts and promoting their differentiation into highly oriented myotubes in situ. CNC alignment occurs on the same time scale as network gelation and remains fixed after the removal of the magnetic field, enabling concurrent CNC orientation and hydrogel injection. The aligned hydrogels show mechanical and swelling profiles that can be rationally modulated by the degree of CNC alignment and can direct myotube alignment both in two- and three-dimensions following coinjection of the myoblasts with the gel precursor components. As such, these hydrogels represent a critical advancement in anisotropic biomimetic scaffolds that can be generated noninvasively in vivo following simple injection.
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