Correlation of Bulk Degradation and Molecular Release from Enzymatically Degradable Polymeric Hydrogels

pH-controlled breakup of fractal aggregates, microgels and gels formed by self-assembled amphiphilic triblock copolymers

The degradation of (micro)gels and fractal aggregates based on self-assembled amphiphilic triblock copolymers has been investigated in water by confocal microscopy and light scattering respectively. The triblock copolymer consisted of a central hydrophilic poly(acrylic acid) (pAA) block and two hydrophobic end blocks that contained an equal amount of randomly distributed n-butyl acrylate (nBA) and AA units. These latter units helped at tempering the hydrophobic end blocks resulting in the control and the fine tuning of the dynamics of the self-assembled triblock through the pH. Starting from a pH where the dynamics is frozen, the rate of breakup of the macroscopic gels, microgels and of fractal aggregates was measured after increasing the pH to different values. The mechanism of the breakup was found to be independent of the pH, but its rate increased exponentially with increasing pH. The degradation proceeded through the release of the polymers from the bulk into the surrounding aqueous phase.

Degradable and Multifunctional PEG-Based Hydrogels Formed by iEDDA Click Chemistry with Stable Click-Induced Supramolecular Interactions

The inverse electron demand Diels-Alder (iEDDA) reactions are highly efficient click chemistry increasingly utilized in bioconjugation, live cell labeling, and the synthesis and modification of biomaterials. iEDDA click reactions have also been used to cross-link tetrazine (Tz) and norbornene (NB) modified macromers [e.g., multiarm poly(ethylene glycol) or PEG]. In these hydrogels, Tz-NB adducts exhibit stable supramolecular interactions with a high hydrolytic stability. Toward engineering a new class of PEG-based click hydrogels with highly adaptable properties, we previously reported a new group of NB-derivatized PEG macromers via reacting hydroxyl-terminated PEG with carbic anhydride (CA). In this work, we show that hydrogels cross-linked by PEGNBCA or its derivatives exhibited fast and tunable hydrolytic degradation. Here, we show that PEGNBCA (either mono- or octafunctional) and its dopamine or tyramine conjugated derivatives (i.e., PEGNB-D and PEGNB-T) readily cross-link with 4-arm PEG-Tz to form a novel class of multifunctional iEDDA click hydrogels. Through modularly adjusting the macromers with unstable and stable iEDDA click-induced supramolecular interactions (iEDDA-CSI), we achieved highly tunable degradation, with full degradation in less than 2 weeks to over two months. We also show that secondary enzymatic reactions could dynamically stiffen these hydrogels. These hydrogels could also be spatiotemporally photopatterned through visible light-initiated photochemistry. Finally, the iEDDA-CSI hydrogels post ester hydrolysis displayed shear-thinning and self-healing properties, enabling injectable delivery.

Biomedical polymer scaffolds mimicking bone marrow niches to advance in vitro expansion of hematopoietic stem cells

Hematopoietic stem cell (HSC) transplantation provides an effective platform for the treatment of hematological disorders. However, the donor shortage of HSCs and immune responses severely restrict the clinical applications of HSCs. Compared to allogeneic transplantation, autogenous transplantation poses less risk to the immune system, but the problem associated with insufficient HSCs remains a substantial challenge. A significant strategy for obtaining sufficient HSCs is to promote the expansion of HSCs. In vivo, a bone marrow microenvironment supports the survival and hematopoiesis of HSCs. Therefore, it is crucial to establish a platform that mimics the features of a bone marrow microenvironment for the in vitro expansion of HSCs. Three-dimensional (3D) scaffolds have emerged as the most powerful tools to mimic cellular microenvironments for the growth and proliferation of stem cells. Biomedical polymers have been widely utilized as cell scaffolds due to their advantageous features including favorable biocompatibility, biodegradability, as well as adjustable physical and chemical properties. This review focuses on recent advances in the study of biomedical polymer scaffolds that mimic bone marrow microenvironments for the in vitro expansion of HSCs. Bone marrow transplantation and microenvironments are first introduced. Then, biomedical polymer scaffolds for the expansion of HSCs and future prospects are summarized and discussed.

Gelation Dynamics during Photo-Cross-Linking of Polymer Nanocomposite Hydrogels

Embedding nanomaterials into polymer hydrogels enables the design of functional materials with tailored chemical, mechanical, and optical properties. Nanocapsules that protect interior cargo and disperse readily through a polymeric matrix have drawn particular interest for their ability to integrate chemically incompatible systems and to further expand the parameter space for polymer nanocomposite hydrogels. The properties of polymer nanocomposite hydrogels depend on the material composition and processing route, which were explored systematically in this work. The gelation kinetics of network-forming polymer solutions with and without silica-coated nanocapsules bearing polyethylene glycol (PEG) surface ligands were investigated using in situ dynamic rheology measurements. Network-forming polymers comprised either 4-arm or 8-arm star PEG with terminal anthracene groups, which dimerize upon irradiation with ultraviolet (UV) light. The PEG-anthracene solutions exhibited rapid gel formation upon UV exposure (365 nm); gel formation was observed as a crossover from liquid-like to solid-like behavior during in situ small-amplitude oscillatory shear rheology. This crossover time was non-monotonic with polymer concentration. Far below the overlap concentration (c/c* ≪ 1), spatially separated PEG-anthracene molecules were subject to forming intramolecular loops over intermolecular cross-links, thereby slowing the gelation process. Near the polymer overlap concentration (c/c* ∼ 1), rapid gelation was attributed to the ideal proximity of anthracene end groups from neighboring polymer molecules. Above the overlap concentration (c/c* > 1), increased solution viscosities hindered molecular diffusion, thereby reducing the frequency of dimerization reactions. Adding nanocapsules to PEG-anthracene solutions resulted in faster gelation than nanocapsule-free PEG-anthracene solutions with equivalent effective polymer concentrations. The final elastic modulus of nanocomposite hydrogels increased with nanocapsule volume fraction, signifying synergistic mechanical reinforcement by nanocapsules despite not being cross-linked into the polymer network. Overall, these findings quantify the impact of nanocapsule addition on the gelation kinetics and mechanical properties of polymer nanocomposite hydrogels, which are promising materials for applications in optoelectronics, biotechnology, and additive manufacturing.

Dual-jet electrospun PDLGA/PCU nonwovens as promising mesh implant materials with controlled release of sirolimus and diclofenac

Dual-jet electrospinning was employed to produce two-component, partially degradable drug releasing nonwovens with interlacing of poly(D,L-lactide-co-glycolide) (PDLGA) and different poly(carbonate urethanes) (PCUs). Diclofenac sodium and sirolimus were released simultaneously from the copolyester carrier. The research focused on determining of release profiles of drugs, depending on the hydrophilicity of introduced PCU nanofibers. The influence of drugs incorporation on the hydrolytic degradation of the PDLGA and mechanical properties of nonwovens was also studied. Evaluation for interaction with cells in vitro was investigated on a fibroblast cell line in cytotoxicity and surface adhesion tests. Significant changes in drugs release rate, depending on the applied PCU were observed. It was also noticed, that hydrophilicity of drugs significantly influenced the hydrolytic degradation mechanism and surface erosion of the PDLGA, as well as the tensile strength of nonwovens. Tests carried out on cells in an in vitro experiment showed that introduction of sirolimus caused a slight reduction in the viability of fibroblasts as well as a strong limitation in their capability to colonize the surface of fibers. Due to improvement of mechanical strength and the ability to controlled drugs release, the obtained material may be considered as prospect surgical mesh implant in the treatment of hernia.

Catalyst-Free Click Chemistry for Engineering Chondroitin Sulfate-Multiarmed PEG Hydrogels for Skin Tissue Engineering

The quest for an ideal biomaterial perfectly matching the microenvironment of the surrounding tissues and cells is an endless challenge within biomedical research, in addition to integrating this with a facile and sustainable technology for its preparation. Engineering hydrogels through click chemistry would promote the sustainable invention of tailor-made hydrogels. Herein, we disclose a versatile and facile catalyst-free click chemistry for the generation of an innovative hydrogel by combining chondroitin sulfate (CS) and polyethylene glycol (PEG). Various multi-armed PEG-Norbornene (A-PEG-N) with different molecular sizes were investigated to generate crosslinked copolymers with tunable rheological and mechanical properties. The crosslinked and mechanically stable porous hydrogels could be generated by simply mixing the two clickable Tetrazine-CS (TCS) and A-PEG-N components, generating a self-standing hydrogel within minutes. The leading candidate (TCS-8A-PEG-N (40 kD)), based on the mechanical and biocompatibility results, was further employed as a scaffold to improve wound closure and blood flow in vivo. The hydrogel demonstrated not only enhanced blood perfusion and an increased number of blood vessels, but also desirable fibrous matrix orientation and normal collagen deposition. Taken together, these results demonstrate the potential of the hydrogel to improve wound repair and hold promise for in situ skin tissue engineering applications.

The Impact of the Matricaria chamomilla L. Extract, Starch Solution and the Photoinitiator on Physiochemical Properties of Acrylic Hydrogels

Matricaria chamomilla L. extract is well-known for its therapeutic properties; thus, it shows potential to be used to modify materials designed for biomedical purposes. In this paper, acrylic hydrogels modified with this extract were prepared. The other modifier was starch introduced into the hydrogel matrix in two forms: room-temperature solution and elevated-temperature solution. Such hydrogels were synthesized via UV radiation, while two types of photoinitiator were used: 2-hydroxy-2-methylpropiophenone or phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide. The main task of performed research was to verify the impact of particular modifiers and photoinitiator on physicochemical properties of hydrogels. Studies involved determining their swelling ability, elasticity, chemical structure via FTIR spectroscopy and surface morphology via the SEM technique. Incubation of hydrogels in simulated physiological liquids, studies on the release of chamomile extract from their matrix and their biological analysis via MTT assay were also performed. It was demonstrated that all investigated variables affected the physicochemical properties of hydrogels. The modification of hydrogels with chamomile extract reduced their absorbency, decreased their thermal stability and increased the cell viability incubated with this material by 15%. Next, hydrogels obtained by using phenylbis(2,4,6-trimethylbenzoyl) phosphine oxide as a photoinitiator showed lower absorbency, more compact structure, better stability in SBF and a more effective release of chamomile extract compared to the materials prepared by using 2-hydroxy-2-methylpropiophenone. It was proved that, by applying adequate reagents, including both photoinitiator and modifiers, it is possible to obtain hydrogels with variable properties that will positively affect their application potential.