Supramolecular Hydrogel Based on pNIPAm Microgels Connected via Host⁻Guest Interactions

Leveraging novel innovative thermoresponsive polymers in microneedles for targeted intradermal deposition

Microneedles have garnered considerable attention over the years as a versatile pharmaceutical platform that could be leveraged to deliver drugs into and across the skin. In the current work, poly (N-isopropylacrylamide) (PNIPAm) is synthesized and characterized as a novel material for the development of a physiologically responsive microneedle-based drug delivery system. Typically, this polymer transitions reversibly between a swell state at lower temperatures and a more hydrophobic state at higher temperatures, enabling precise drug release. This study demonstrates that dissolving microneedles patches made from PNIPAm, incorporating BIS-PNIPAm, a crosslinked polymer variant, exhibit enhanced mechanical properties, evident from a smaller height reduction in microneedle (∼10 %). Although microneedles using PNIPAm alone were achievable, it displayed poor mechanical strength, requiring the inclusion of additional polymeric excipients like PVA to enhance mechanical properties. In addition, the incorporation of a thermoresponsive polymer did not have a significant (p > 0.05) impact on the insertion properties of the needles as all formulations inserted to a similar depth of 500 µm into ex vivo skin. Furthering this, the needles were loaded with a model payload, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate (DID) and the deposition of the cargo was monitored via multiphoton microscopy that showed that a deposit is formed at a depth of ≈200 µm. Also, it was revealed that crosslinked-PNIPAm (Bis-PNIPAm) formulations exhibited notable skin accumulationof the dye only after 4 h, independent of the excipient matrix used. This phenomenon was absent in non-crosslinked PNIPAm formulations, indicating a deposit formation in Bis-PNIPAm microneedle formulation. Collectively, this proof-of-concept study has advanced our understanding on the possibility to use PNIPAm for dissolving microneedle fabrication which could be harnessed for the deposition of nanoparticles into the dermis, for extended drug release within the skin.

Dynamically Cross-Linked Granular Hydrogels for 3D Printing and Therapeutic Delivery

Granular hydrogels have recently emerged as promising biomaterials for tissue engineering and 3D-printing applications, addressing the limitations of bulk hydrogels while exhibiting desirable properties such as injectability and high porosity. However, their structural stability can be improved with post-injection interparticle cross-linking. In this study, we developed granular hydrogels with interparticle cross-linking through reversible and dynamic covalent bonds. We fragmented photo-cross-linked bulk hydrogels to produce aldehyde or hydrazide-functionalized microgels using chondroitin sulfate. Mixing these microgels facilitated interparticle cross-linking through reversible hydrazone bonds, providing shear-thinning and self-healing properties for injectability and 3D printing. The resulting granular hydrogels displayed high mechanical stability without the need for secondary cross-linking. Furthermore, the porosity and sustained release of growth factors from these hydrogels synergistically enhanced cell recruitment. Our study highlights the potential of reversible interparticle cross-linking for designing injectable and 3D printable therapeutic delivery scaffolds using granular hydrogels. Overall, our study highlights the potential of reversible interparticle cross-linking to improve the structural stability of granular hydrogels, making them an effective biomaterial for use in tissue engineering and 3D-printing applications.

Self-healing hydrogels for bone defect repair

Severe bone defects can be caused by various factors, such as tumor resection, severe trauma, and infection. However, bone regeneration capacity is limited up to a critical-size defect, and further intervention is required. Currently, the most common clinical method to repair bone defects is bone grafting, where autografts are the "gold standard." However, the disadvantages of autografts, including inflammation, secondary trauma and chronic disease, limit their application. Bone tissue engineering (BTE) is an attractive strategy for repairing bone defects and has been widely researched. In particular, hydrogels with a three-dimensional network can be used as scaffolds for BTE owing to their hydrophilicity, biocompatibility, and large porosity. Self-healing hydrogels respond rapidly, autonomously, and repeatedly to induced damage and can maintain their original properties (i.e., mechanical properties, fluidity, and biocompatibility) following self-healing. This review focuses on self-healing hydrogels and their applications in bone defect repair. Moreover, we discussed the recent progress in this research field. Despite the significant existing research achievements, there are still challenges that need to be addressed to promote clinical research of self-healing hydrogels in bone defect repair and increase the market penetration.

Recent advances in host-guest supramolecular hydrogels for biomedical applications

The recognition-directed host-guest interaction is recognized as a valuable tool for creating supramolecular polymers. Functional hydrogels constructed through the dynamic and reversible host-guest complexation are endowed with a great many appealing features, such as superior self-healing, injectability, flexibility, stimuli-responsiveness and biocompatibility, which are crucial for biological and medicinal applications. With numerous topological structures and host-guest combinations established previously, recent breakthroughs in this area mostly focus on further improvement and fine-tuning of various properties for practical utilizations. The current contribution provides a comprehensive overview of the latest developments in host-guest supramolecular hydrogels, with a particular emphasis on the innovative molecular-level design strategies and hydrogel formation methodologies targeting at a wide range of active biomedical domains, including drug delivery, 3D printing, wound healing, tissue engineering, artificial actuators, biosensors, etc. Furthermore, a brief conclusion and discussion on the steps forward to bring these smart hydrogels to clinical practice is also presented.

Aggressive strategies for regenerating intervertebral discs: stimulus-responsive composite hydrogels from single to multiscale delivery systems

As our research on the physiopathology of intervertebral disc degeneration (IVD degeneration, IVDD) has advanced and tissue engineering has rapidly evolved, cell-, biomolecule- and nucleic acid-based hydrogel grafting strategies have been widely investigated for their ability to overcome the harsh microenvironment of IVDD. However, such single delivery systems suffer from excessive external dimensions, difficult performance control, the need for surgical implantation, and difficulty in eliminating degradation products. Stimulus-responsive composite hydrogels have good biocompatibility and controllable mechanical properties and can undergo solution-gel phase transition under certain conditions. Their combination with ready-to-use particles to form a multiscale delivery system may be a breakthrough for regenerative IVD strategies. In this paper, we focus on summarizing the progress of research on the stimulus response mechanisms of regenerative IVD-related biomaterials and their design as macro-, micro- and nanoparticles. Finally, we discuss multi-scale delivery systems as bioinks for bio-3D printing technology for customizing personalized artificial IVDs, which promises to take IVD regenerative strategies to new heights.

Additive Soft Matter Design by UV-Induced Polymer Hydrogel Inter-Crosslinking

In recent years, stimuli-responsive hydrogels have gained tremendous interest in designing complex smart 4D materials for applications ranging from biomedicine to soft electronics that can change their properties on demand over time. However, at present, a hydrogel’s response is often induced by merely a single stimulus, restricting its broader applicability. The controlled hierarchical assembly of various hydrogel building blocks, each with a tailored set of mechanical and physicochemical properties as well as programmed stimulus response, may potentially enable the design and fabrication of multi-responsive polymer parts that process complex operations, like signal routing dependent on different stimuli. Since inter-connection stability of such building blocks directly accompanies the transmission of information across building blocks and is as important as the building property itself to create complex 4D materials, we provide a study on the utility of an inter-crosslinking mechanism based on UV-induced 2,3-dimethylmaleimide (DMMI) dimerization to inter-connect acrylamide-based and N-isopropylacrylamide-based millimeter-sized cubic building blocks, respectively. The resulting dual-crosslinked assemblies are freestanding and stable against contraction–expansion cycles in solution. In addition, the approach is also applicable for connecting microfluidically fabricated, micrometer-sized hydrogel spheres, with the resulting assemblies being processable and mechanical stable, likewise resisting contraction–expansion in different solvents, for instance.

Relationship between Structure and Rheology of Hydrogels for Various Applications

Hydrogels have gained a lot of attention with their widespread use in different industrial applications. The versatility in the synthesis and the nature of the precursor reactants allow for a varying range of hydrogels with different mechanical and rheological properties. Understanding of the rheological behavior and the relationship between the chemical structure and the resulting properties is crucial, and is the focus of this review. Specifically, we include detailed discussion on the correlation between the rheological characteristics of hydrogels and their possible applications. Different rheological tests such as time, temperature and frequency sweep, among others, are described and the results of those tests are reported. The most prevalent applications of hydrogels are also discussed.

Fast Thermoresponsive Poly(oligoethylene glycol methacrylate) (POEGMA)-Based Nanostructured Hydrogels for Reversible Tuning of Cell Interactions

Reactive electrospinning is demonstrated as a viable method to create fast-responsive and degradable macroporous thermoresponsive hydrogels based on poly(oligoethylene glycol methacrylate) (POEGMA). Hydrazide- and aldehyde-functionalized POEGMA precursor polymers were coelectrospun to create hydrazone cross-linked nanostructured hydrogels in a single processing step that avoids the need for porogens, phase separation-driving additives, or scaffold postprocessing. The resulting nanostructured hydrogels can respond reversibly and repeatedly to changes in external temperature within seconds, in contrast to the minutes-to-hours response time observed with bulk hydrogels. Furthermore, nearly quantitative cell delamination can be achieved within 2 min of incubation at 4 °C, resulting in the recovery of as many or more (as well as more proliferatively active) cells from the substrate relative to the conventional trypsinization protocol. The combined macroporosity, nanoscale feature size, and interfacial switching potential of these nanostructured hydrogels thus offer promise for manipulating cell-hydrogel interactions as well as other applications in which rapid responses to external stimuli are desirable.

Temperature-Dependent Nanomechanical Properties of Adsorbed Poly-NIPAm Microgel Particles Immersed in Water

The temperature dependence of nanomechanical properties of adsorbed poly-NIPAm microgel particles prepared by a semibatch polymerization process was investigated in an aqueous environment via indentation-based atomic force microscopy (AFM) methods. Poly-NIPAm microgel particles prepared by the classical batch process were also characterized for comparison. The local mechanical properties were measured between 26 and 35 °C, i.e., in the temperature range of the volume transition. Two different AFM tips with different shapes and end radii were utilized. The nanomechanical properties measured by the two kinds of tips showed a similar temperature dependence of the nanomechanical properties, but the actual values were found to depend on the size of the tip. The results suggest that the semibatch synthesis process results in the formation of more homogeneous microgel particles than the classical batch method. The methodological approach reported in this work is generally applicable to soft surface characterization in situ.

Synthesis, characterization and in vitro cytotoxicity studies of poly-N-isopropyl acrylamide gel nanoparticles and films

In this work, we show synthesis that leads to thermoreponsive poly-N-isopropyl acrylamide (pNIPAM) nanogels with sizes below 100 nm, irrespectively of the surfactant to crosslinker ratio. We also show that in many environments the temperature induced pNIPAM collapse at Lower Critical Solution Temperature (LCST) of 32.5 °C is accompanied by gel nanoparticles' aggregation. Thus, the proper information on the nanoparticle (NP) structure and deswelling can be obtained only if the routinely measured hydrodynamic radius is supplemented by information on the molecular weight, which can be obtained from the intensity of scattered light. We measured the dynamics and reversibility of the deswelling and subsequent aggregation processes. Furthermore, we show that the highly concentrated pNIPAM gel NPs reversibly form bulk hydrogel networks of varied interconnected porous structure. We show, that in case of drying pNIPAM gel NPs above the LCST, it is possible to obtain films with 20-fold increase in storage modulus (G') compared to hydrogel networks measured at room temperature. They exhibit temperature hysteresis behavior around LCST of 32.5 °C similar to pNIPAM films. Finally, we show that these hydrogel films, lead to extended proliferation of cells across three different types: fibroblast, endothelial and cancer cells. Additionally, none of the films exhibited any cytotoxic effects. Our study brings new insights into physicochemical characterization of pNIPAM gel NPs and networks behavior in realistic conditions of in vitro measurements, especially by means of dynamic light scattering as well as final unique properties of both gel NPs and formed porous films for possible tissue engineering applications.

Isothermal kinase-triggered supramolecular assemblies as drug sensitizers

Protein kinases, the main regulators of a vast map of cellular processes, are the most attractive targets in drug discovery. Despite a few successful examples of protein kinase inhibitors, the drug discovery strategy of downregulating protein kinase activity has been quite limited and often fails even in animal models. Here, we utilize protein kinase A (PKA) activity to design PKA-triggered supramolecular assemblies with anticancer activities. Grafting a suitable peptide to PNIPAM raises the critical temperature of the LCST polymer above body temperature. Interestingly, the corresponding phosphorylated polymer has a critical temperature below body temperature, making this peptide-appended PNIPAM a suitable polymer for the PKA-triggered supramolecular assembly process. PKA-triggered assembly occurs selectively in PKA-upregulated MCF-7 cells, which disturbs the cytoskeleton and sensitizes cancer cells against doxorubicin. The chemosensitization is also observed in vivo to identify effective tumor inhibitors with satisfactory biocompatibility. Overall, this phosphorylation-induced (in principle, PKA-catalyzed) supramolecular assembly opens up a promising chemotherapy strategy for combating kinase-upregulated cancer.

A nonapeptide grafted LCST polymer undergoes enzymatic phosphorylation to assemble, which selectively disrupts PKA overexpressing cancer cells via kinetics targeting.

Tunable Fluorescence-Responsive Double Hydrophilic Block Polymers Induced by the Formation of Pseudopolyrotaxanes with Cucurbit[7]Uril

There is an urgent need for new strategies that allow the simultaneous detection and control of drug delivery. By making use of supramolecular host-guest interactions, a kind of pseudopolyrotaxanes, as a visualizable nanoscale drug carrier has been constructed by self-assembly of cucurbit[7]uril (CB[7]) with methoxy poly(ethylene glycol)-block-quaternized poly(4-vinyl pyridine) (mPEG-b-QP4VP) using 4-(chloromethyl)benzonitrile. Simple addition of CB[7] into an aqueous solution of mPEG-b-QP4VP resulted in noncovalent attachment of CB[7] to 4-cyanobenzyl-containing polymers, transforming the nonemissive mPEG-b-QP4VP micelles into highly fluorescent micelles. These pseudopolyrotaxanes micelles exhibited remarkable supramolecular assembly-induced emission enhancement and excellent biocompatibility, showing great potential for bioimaging applications. In addition, the efficient cellular uptake of the developed pseudopolyrotaxanes micelles loaded with the anticancer drug doxorubicin was a promising platform for simultaneous cell imaging and drug delivery, thereby widening their application in cancer theranostics.

Host⁻Guest Polymer Complexes

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