Injectable Microgel-Hydrogel Composites for Prolonged Small-Molecule Drug Delivery

Physicochemical, structural, and in-vitro release properties of carboxymethyl cellulose-based cryogel beads incorporating resveratrol-loaded microparticles for colon-targeted delivery system

The objective of this study was to investigate the physicochemical, structural, and in vitro release properties of carboxymethyl cellulose (CMC)-based cryogel beads incorporating resveratrol-loaded microparticles (MP) for colon-targeted delivery system. CMC-based cryogel beads were produced by ionic cross-linking with different concentrations (2%, 3%, and 4%) of AlCl3. Based on FE-SEM images, CMC-based cryogel beads showed a smoother surface and more compact internal structure with increasing AlCl3 concentrations, which was proven to be due to the new cross-linking between the -COO- group of CMC and Al3+ by FT-IR analysis. The encapsulation efficiency of the cryogel beads was significantly increased from 79.48% to 85.74% by elevating the concentrations of AlCl3 from 2% to 4%, respectively. In vitro release study showed that all CMC-based cryogel beads had higher stability for resveratrol than MP in simulated gastric conditions and can efficiently deliver resveratrol to colon without the premature release.

Microgels for bioprinting: recent advancements and challenges

Microgels have become a popular and powerful structural unit in the bioprinting field due to their advanced properties, ranging from the tiny size and well-connected hydrogel (nutrient) network to special rheological properties. Different microgels can be fabricated by a variety of fabrication methods including bulk crushing, auxiliary dripping, multiphase emulsion, and lithography technology. Traditionally, microgels can encapsulate specific cells and are used for in vitro disease models and in vivo organ regeneration. Furthermore, microgels can serve as a drug carrier to realize controlled release of drug molecules. Apart from being used as an independent application unit, recently, these microgels are widely applied as a specific bioink component in 3D bioprinting for in situ tissue repair or building special 3D structures. In this review, we introduce different methods used to generate microgels and the microgel-based bioink for bioprinting. Besides, the further tendency of microgel development in future is introduced and predicted to provide guidance for related researchers in exploring more effective ways to fabricate microgels and more potential bioprinting application cases as multifunctional bioink components.

Evolution of Hybrid Hydrogels: Next-Generation Biomaterials for Drug Delivery and Tissue Engineering

Hydrogels, being hydrophilic polymer networks capable of absorbing and retaining aqueous fluids, hold significant promise in biomedical applications owing to their high water content, permeability, and structural similarity to the extracellular matrix. Recent chemical advancements have bolstered their versatility, facilitating the integration of the molecules guiding cellular activities and enabling their controlled activation under time constraints. However, conventional synthetic hydrogels suffer from inherent weaknesses such as heterogeneity and network imperfections, which adversely affect their mechanical properties, diffusion rates, and biological activity. In response to these challenges, hybrid hydrogels have emerged, aiming to enhance their strength, drug release efficiency, and therapeutic effectiveness. These hybrid hydrogels, featuring improved formulations, are tailored for controlled drug release and tissue regeneration across both soft and hard tissues. The scientific community has increasingly recognized the versatile characteristics of hybrid hydrogels, particularly in the biomedical sector. This comprehensive review delves into recent advancements in hybrid hydrogel systems, covering the diverse types, modification strategies, and the integration of nano/microstructures. The discussion includes innovative fabrication techniques such as click reactions, 3D printing, and photopatterning alongside the elucidation of the release mechanisms of bioactive molecules. By addressing challenges, the review underscores diverse biomedical applications and envisages a promising future for hybrid hydrogels across various domains in the biomedical field.

Polymerized stimuli-responsive microgel hybrids of silver nanoparticles as efficient reusable catalyst for reduction reaction

We have showcased the potential of polymerized hydrogels (PGMs) with uniform-sized stimuli-responsive microgel particles as promising alternatives to prevent aggregation in solution based nanoparticle systems. In the current work, we implemented the PGM concept by embedding anionic stimuli-responsive microgels (PNIPAM-co-AAc)-silver (Ag) hybrids within a hydrogel matrix. These PGM@AgNP hybrid materials are used as catalysts for the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) in the presence of sodium borohydride. UV-VIS spectroscopy is used for studying catalytic activity. In the solution based system, the complete reduction of 4-NP to 4-AP took 30 minutes with pure Ag nanoparticles, 24 minutes with PNIPAM-Ag hybrid (Neutral) microgels and 15 minutes with PNIPAM-co-AAc-Ag (Anionic) hybrid microgels. In contrast PGM containing PNIPAM-co-AAc-Ag hybrids achieved full reduction in just 15 minutes, along with a 3-minute induction period. For pure Ag nanoparticles, the first-order rate constant is found to be 0.25 min-1, for PNIPAM-Ag hybrid (Neutral), it is 0.21 min-1 and for PNIPAM-co-AAc-Ag (Anionic), it is 0.5 min-1 where as for PGM containing anionic microgel hybrids it is found to be 0.8 min-1. Furthermore, the reusability of the PGM-Ag (anionic) materials for catalytic activity remains unaltered even after several washings. In summary, our study highlights the effectiveness of PGM@AgNP materials as efficient catalysts for the reduction of 4-nitrophenol to 4-aminophenol, indicating their versatile potential in various catalytic applications.

Non-Invasive Delivery of Insulin for Breaching Hindrances against Diabetes

Insulin is recognized as a crucial weapon in managing diabetes. Subcutaneous (s.c.) injections are the traditional approach for insulin administration, which usually have many limitations. Numerous alternative (non-invasive) slants through different routes have been explored by the researchers for making needle-free delivery of insulin for attaining its augmented absorption as well as bioavailability. The current review delineating numerous pros and cons of several novel approaches of non-invasive insulin delivery by overcoming many of their hurdles. Primary information on the topic was gathered by searching scholarly articles from PubMed added with extraction of data from auxiliary manuscripts. Many approaches (discussed in the article) are meant for the delivery of a safe, effective, stable, and patient friendly administration of insulin via buccal, oral, inhalational, transdermal, intranasal, ocular, vaginal and rectal routes. Few of them have proven their clinical efficacy for maintaining the glycemic levels, whereas others are under the investigational pipe line. The developed products are comprising of many advanced micro/nano composite technologies and few of them might be entering into the market in near future, thereby garnishing the hopes of millions of diabetics who are under the network of s.c. insulin injections.

Injectable On-Demand Pulsatile Drug Delivery Hydrogels Using Alternating Magnetic Field-Triggered Polymer Glass Transitions

Remote-controlled pulsatile or staged release has significant potential in a wide range of therapeutic treatments. However, most current approaches are hindered by the low resolution between the on- and off-states of drug release and the need for surgical implantation of larger controlled-release devices. Herein, we describe a method that addresses these limitations by combining injectable hydrogels, superparamagnetic iron oxide nanoparticles (SPIONs) that heat when exposed to an alternating magnetic field (AMF), and polymeric nanoparticles with a glass transition temperature (Tg) just above physiological temperature. Miniemulsion polymerization was used to fabricate poly(methyl methacrylate-co-butyl methacrylate) (p(MMA-co-BMA)) nanoparticles loaded with a model hydrophobic drug and tuned to have a Tg value just above physiological temperature (∼43 °C). Co-encapsulation of these drug-loaded nanoparticles with SPIONs inside a carbohydrate-based injectable hydrogel matrix (formed by rapid hydrazone cross-linking chemistry) enables injection and immobilization of the nanoparticles at the target site. Temperature cycling facilitated a 2.5:1 to 6:1 on/off rhodamine release ratio when the nanocomposites were switched between 37 and 45 °C; release was similarly enhanced by exposing the nanocomposite hydrogel to an AMF to drive heating, with enhanced release upon pulsing observed even 1 week after injection. Coupled with the apparent cytocompatibility of all of the nanocomposite components, these injectable nanocomposite hydrogels are promising as minimally invasive but remotely actuated release delivery vehicles capable of complex release kinetics with high on-off resolution.

Microgel-Assisted Delivery of Adenosine to Accelerate Fracture Healing

Extracellular adenosine plays a key role in promoting bone tissue formation. Local delivery of adenosine could be an effective therapeutic strategy to harness the beneficial effect of extracellular adenosine on bone tissue formation following injury. Herein, we describe the development of an injectable in situ curing scaffold containing microgel-based adenosine delivery units. The two-component scaffold includes adenosine-loaded microgels and functionalized hyaluronic acid (HA) molecules. The microgels were generated upon copolymerization of 3-acrylamidophenylboronic acid (3-APBA)- and 2-aminoethylmethacrylamide (2-AEMA)-conjugated HA (HA-AEMA) in an emulsion suspension. The PBA functional groups were used to load the adenosine molecules. Mixing of the microgels with the HA polymers containing clickable groups, dibenzocyclooctyne (DBCO) and azide (HA-DBCO and HA-Azide), resulted in a 3D scaffold embedded with adenosine delivery units. Application of the in situ curing scaffolds containing adenosine-loaded microgels following tibial fracture injury showed improved bone tissue healing in a mouse model as demonstrated by the reduced callus size, higher bone volume, and increased tissue mineral density compared to those treated with the scaffold without adenosine. Overall, our results suggest that local delivery of adenosine could potentially be an effective strategy to promote bone tissue repair.

Magnetic thermo-responsive branched polymer for fast extraction and enrichment of phenolic acids in olive oil with tunable and enhanced performance

Magnetic thermo-responsive branched polymer (Fe₃O₄@poly(glycidyl methacrylate)@poly(N-isopropylacrylamide)) was fabricated for the first time and applied for microwave-assisted magnetic solid phase extraction of phenolic acids in olive oil samples followed by ultra-high performance liquid chromatography-tandem mass spectrometry analysis in multiple reaction monitoring mode. Owing to the controllable molecular weight of poly(glycidyl methacrylate) synthesized by atom transfer radical polymerization and the thermo-responsive characteristic of poly(N-isopropylacrylamide), extraction performance could be efficiently tuned and enhanced. The whole sample pretreatment process was accomplished within 1 min with the help of the microwave. The nanocomposites were characterized by transmission electron microscope, scanning electron microscope, Fourier transform infrared spectroscopy, thermogravimetric analysis, vibrating sample magnetometer, water contact angles and dynamic light scattering. The adsorption experimental data fitted well with the Freundlich isotherm model and followed the pseudo-second-order kinetic model. The factors affecting the extraction process including adsorbent amount, adsorption time, sample volume, desorption conditions and interferents were investigated and optimized. Under the most favorable conditions, the developed method showed good linearity (R² ≥ 97.98%) in the range of 0.2-30 μg L^-1, low limits of detection (0.005-0.030 μg L^-1) and limits of quantification (0.016-0.098 μg L^-1) as well as satisfactory precision (RSDs≤4.85%). Our proposed method was successfully used for determination of phenolic acids in olive oil samples and satisfactory recoveries at three spiked concentration levels were in the range of 84.6-108.1% with RSDs less than 9.20%. Coupled with principal component analysis, our developed method proved promising for fast and convenient differentiation between extra virgin olive oils and refined olive oils.

Chitosan Hydrogels with Embedded Thermo- and pH-Responsive Microgels as a Potential Carrier for Controlled Release of Drugs

We report a promising strategy based on chitosan (CS) hydrogels and dual temperature- and pH-responsive poly(N-isopropylacrylamide-co-methacrylic acid) (PNIPAM-co-MAA) microgels to facilitate release of a model drug, moxifloxacin (MFX). In this protocol, first, the microgels were prepared using a free radical copolymerization method, and subsequently, these carboxyl-group-rich soft particles were incorporated inside the hydrogel matrix using an EDC-NHS amidation method. Interestingly, the resulting microgel-embedded hydrogel composites (MG-HG) acting as a double barrier system largely reduced the drug release rate and prolonged the delivery time for up to 68 h, which was significantly longer than that obtained using microgels or hydrogels alone (20 h). On account of the dual-responsive features of the embedded microgels and the variation of water-solubility of drug molecules as a function of pH, MFX could be released in a controllable manner by regulating the temperature and pH of the delivery medium. The release kinetics followed a Korsmeyer-Peppas model, and the drug delivery mechanism was described by Fickian diffusion. Both the gel precursors and the hydrogel composites exhibited low cytotoxicity against mammalian cell lines (HeLa and HEK-293) and no deleterious hemolytic activity up to a certain higher concentration, indicating excellent biocompatibility of the materials. Thus, the unprecedented combination of modularity of physical properties caused by soft particle entrapment, unique macromolecular architecture, biocompatibility, and the general utility of the stimuli-responsive polymers offers a great promise to use these composite materials in drug delivery applications.

Design of hydrogel-microgel composites with tailored small molecule release profiles

Stimuli-responsive hydrogel-microgel composites (HMC) were prepared by embedding poly(N-isopropylacrylamide)-based microgels in a poly(N-isopropylacrylamide)-based hydrogel. When the microgels were pre-loaded with the small molecule model drug crystal violet (CV) via electrostatics, the HMC was able to release the CV in a pH-triggered fashion. We found that the CV release rate was dependent on the solution temperature and the dimension of the material. Also, by changing the chemical composition and/or pore size of the hydrogel matrix, the CV release kinetics can be tuned. Moreover, when multiple microgels loaded with different model drugs were embedded in a single HMC, the HMC can be used to control the release rate of each drug analog individually in a pH-dependent fashion. By understanding how properties of a hydrogel can alter the release of small molecules from embedded microgels, new materials capable of controlled and triggered release of multiple small molecule drugs can be designed with myriad uses in the biomedical field.

Alginate-Based Hydrogels and Tubes, as Biological Macromolecule-Based Platforms for Peripheral Nerve Tissue Engineering: A Review

Unlike the central nervous system, the peripheral nervous system (PNS) has an inherent capacity to regenerate following injury. However, in the case of large nerve defects where end-to-end cooptation of two nerve stumps is not tension-free, autologous nerve grafting is often utilized to bridge the nerve gaps. To address the challenges associated with autologous nerve grafting, neural guidance channels (NGCs) have been successfully translated into clinic. Furthermore, hydrogel-based drug delivery systems have been extensively studied for the repair of PNS injuries. There are numerous biomaterial options for the production of NGCs and hydrogels. Among different candidates, alginate has shown promising results in PNS tissue engineering. Alginate is a naturally occurring polysaccharide which is biocompatible, non-toxic, non-immunogenic, and possesses modifiable properties. In the current review, applications, challenges, and future perspectives of alginate-based NGCs and hydrogels in the repair of PNS injuries will be discussed.

Recent progress in polymeric non-invasive insulin delivery

The design of carriers for insulin delivery has recently attracted major research attentions in the biomedical field. In general, the release of drug from polymers is driven via a variety of polymers. Several mechanisms such as matrix release, leaching of drug, swelling, and diffusion are usually adopted for the release of drug through polymers. Insulin is one of the most predominant therapeutic drugs for the treatment of both diabetes mellitus; type-I (insulin-dependent) and type II (insulin-independent). Currently, insulin is administered subcutaneously, which makes the patient feel discomfort, pain, hyperinsulinemia, allergic responses, lipodystrophy surrounding the injection area, and occurrence of miscarried glycemic control. Therefore, significant research interest has been focused on designing and developing new insulin delivery technologies to control blood glucose levels and time, which can enhance the patient compliance simultaneously through alternative routes as non-invasive insulin delivery. The aim of this review is to emphasize various non-invasive insulin delivery mechanisms including oral, transdermal, rectal, vaginal, ocular, and nasal. In addition, this review highlights different smart stimuli-responsive insulin delivery systems including glucose, pH, enzymes, near-infrared, ultrasound, magnetic and electric fields, and the application of various polymers as insulin carriers. Finally, the advantages, limitations, and the effect of each non-invasive route on insulin delivery are discussed in detail.

Multicompartment Hydrogels

Hydrogels belong to the most promising materials in polymer and materials science at the moment. As they feature soft and tissue-like character as well as high water-content, a broad range of applications are addressed with hydrogels, e.g. tissue engineering and wound dressings but also soft robotics, drug delivery, actuators and catalysis. Ways to tailor hydrogel properties are crosslinking mechanism, hydrogel shape and reinforcement, but new features can be introduced by variation of hydrogel composition as well, e.g. via monomer choice, functionalization or compartmentalization. Especially, multicompartment hydrogels drive progress towards complex and highly functional soft materials. In the present review the latest developments in multicompartment hydrogels are highlighted with a focus on three types of compartments, i.e. micellar/vesicular, droplets or multi-layers including various sub-categories. Furthermore, several morphologies of compartmentalized hydrogels and applications of multicompartment hydrogels will be discussed as well. Finally, an outlook towards future developments of the field will be given. The further development of multicompartment hydrogels is highly relevant for a broad range of applications and will have a significant impact on biomedicine and organic devices. This article is protected by copyright. All rights reserved.

Microwave-Assisted Synthesis of Modified Glycidyl Methacrylate-Ethyl Methacrylate Oligomers, Their Physico-Chemical and Biological Characteristics

In this study, well-known oligomers containing ethyl methacrylate (EMA) and glycidyl methacrylate (GMA) components for the synthesis of the oligomeric network [P(EMA)-co-(GMA)] were used. In order to change the hydrophobic character of the [P(EMA)-co-(GMA)] to a more hydrophilic one, the oligomeric chain was functionalized with ethanolamine, xylitol (Xyl), and L-ornithine. The oligomeric materials were characterized by nuclear magnetic resonance and Fourier transform infrared spectroscopy, scanning electron microscopy, and differential thermogravimetric analysis. In the final stage, thanks to the large amount of -OH groups, it was possible to obtain a three-dimensional hydrogel (HG) network. The HGs were used as a matrix for the immobilization of methylene blue, which was chosen as a model compound of active substances, the release of which from the matrix was examined using spectrophotometric detection. The cytotoxic test was performed using fluid extracts of the HGs and human skin fibroblasts. The cell culture experiment showed that only [P(EMA)-co-(GMA)] and [P(EMA)-co-(GMA)]-Xyl have the potential to be used in biomedical applications. The studies revealed that the obtained HGs were porous and non-cytotoxic, which gives them the opportunity to possess great potential for use as an oligomeric network for drug reservoirs in in vitro application.

Microfluidic Chip Device for In Situ Mixing and Fabrication of Hydrogel Microspheres via Michael-Type Addition

Hydrogel microspheres are sought for a variety of biomedical applications, including therapeutic and cellular delivery, sensors, and lubricants. Robust fabrication of hydrogel microspheres with uniform sizes and properties can be achieved using microfluidic systems that rely on droplet formation and subsequent gelation to form microspheres. Such systems work well when gelation is initiated after droplet formation but are not practical for timed gelation systems where gelation is initiated prior to droplet formation; premature gelation can lead to device blockage, variable microsphere diameter due to viscosity changes in the precursor solution, and limited numbers of microspheres produced in a single run. To enable microfluidic fabrication of microspheres from timed gelation hydrogel systems, an in situ mixing region is needed so that various hydrogel precursor components can be added separately. Here, we designed and evaluated three mixing devices for their effectiveness at mixing hydrogel precursor solutions prior to droplet formation and subsequent gelation. The serpentine geometry was found to be the most effective and was further improved with the inclusion of a pillar array to increase agitation. The optimized device was shown to fully mix precursor solutions and enable the fabrication of monodisperse polyethylene glycol microspheres, offering great potential for use with timed gelation hydrogel systems.

Translational Applications of Hydrogels

Advances in hydrogel technology have unlocked unique and valuable capabilities that are being applied to a diverse set of translational applications. Hydrogels perform functions relevant to a range of biomedical purposes-they can deliver drugs or cells, regenerate hard and soft tissues, adhere to wet tissues, prevent bleeding, provide contrast during imaging, protect tissues or organs during radiotherapy, and improve the biocompatibility of medical implants. These capabilities make hydrogels useful for many distinct and pressing diseases and medical conditions and even for less conventional areas such as environmental engineering. In this review, we cover the major capabilities of hydrogels, with a focus on the novel benefits of injectable hydrogels, and how they relate to translational applications in medicine and the environment. We pay close attention to how the development of contemporary hydrogels requires extensive interdisciplinary collaboration to accomplish highly specific and complex biological tasks that range from cancer immunotherapy to tissue engineering to vaccination. We complement our discussion of preclinical and clinical development of hydrogels with mechanical design considerations needed for scaling injectable hydrogel technologies for clinical application. We anticipate that readers will gain a more complete picture of the expansive possibilities for hydrogels to make practical and impactful differences across numerous fields and biomedical applications.

3D-Printable Hierarchical Nanogel-GelMA Composite Hydrogel System

The strength of the extracellular matrix (ECM) is that it is hierarchical in terms of matrix built-up, matrix density and fiber structure, which allows for hormones, cytokines, and other small biomolecules to be stored within its network. The ECM-like hydrogels that are currently used do not possess this ability, and long-term storage, along with the need for free diffusion of small molecules, are generally incompatible requirements. Nanogels are able to fulfill the additional requirements upon successful integration. Herein, a stable hierarchical nanogel–gelatin methacryloyl (GelMA) composite hydrogel system is provided by covalently embedding nanogels inside the micropore network of GelMA hydrogel to allow a controlled local functionality that is not found in a homogenous GelMA hydrogel. Nanogels have emerged as a powerful tool in nanomedicine and are highly versatile, due to their simplicity of chemical control and biological compatibility. In this study, an N-isopropylacrylamide-based nanogel with primary amine groups on the surface was modified with methacryloyl groups to obtain a photo-cross-linking ability similar to GelMA. The nanogel-GelMA composite hydrogel was formed by mixing the GelMA and the photo-initiator within the nanogel solution through UV irradiation. The morphology of the composite hydrogel was observed by scanning electron microscopy, which clearly showed the nanogel wrapped within the GelMA network and covering the surface of the pore wall. A release experiment was conducted to prove covalent bonding and the stability of the nanogel inside the GelMA hydrogel. In addition, 3D printability studies showed that the nanogel-GelMA composite ink is printable. Therefore, the suggested stable hierarchical nanogel-GelMA composite hydrogel system has great potential to achieve the in situ delivery and controllable release of bioactive molecules in 3D cell culture systems.

Development of Nanosilicate-Hydrogel Composites for Sustained Delivery of Charged Biopharmaceutics

Nanocomposite hydrogels containing two-dimensional nanosilicates (NS) have emerged as a new technology for the prolonged delivery of biopharmaceuticals. However, little is known about the physical-chemical properties governing the interaction between NS and proteins and the release profiles of NS-protein complexes in comparison to traditional poly(ethylene glycol) (PEG) hydrogel technologies. To fill this gap in knowledge, we fabricated a nanocomposite hydrogel composed of PEG and laponite and identified simple but effective experimental conditions to obtain sustained protein release, up to 23 times slower as compared to traditional PEG hydrogels, as determined by bulk release experiments and fluorescence correlation spectroscopy. Slowed protein release was attributed to the formation of NS-protein complexes, as NS-protein complex size was inversely correlated with protein diffusivity and release rates. While protein electrostatics, protein concentration, and incubation time were important variables to control protein-NS complex formation, we found that one of the most significant and less appreciated variable to obtain a sustained release of bioactive proteins was the buffer chosen for preparing the initial suspension of NS particles. The buffer was found to control the size of nanoparticles, the absorption potential, morphology, and stiffness of hydrogels. From these studies, we conclude that the PEG-laponite composite fabricated is a promising new platform for sustained delivery of positively charged protein therapeutics.

Design and Development of Hybrid Hydrogels for Biomedical Applications: Recent Trends in Anticancer Drug Delivery and Tissue Engineering

The applications of hydrogels in biomedical field has been since multiple decades. Discoveries in biology and chemistry render this platform endowed with much engineering potentials and growing continuously. Novel approaches in constructing these materials have led to the production of complex hybrid hydrogels systems that can incorporate both natural and synthetic polymers and other functional moieties for mediated cell response, tunable release kinetic profiles, thus they are used and research for diverse biomedical applications. Recent advancement in this field has established promising techniques for the development of biorelevant materials for construction of hybrid hydrogels with potential applications in the delivery of cancer therapeutics, drug discovery, and re-generative medicines. In this review, recent trends in advanced hybrid hydrogels systems incorporating nano/microstructures, their synthesis, and their potential applications in tissue engineering and anticancer drug delivery has been discussed. Examples of some new approaches including click reactions implementation, 3D printing, and photopatterning for the development of these materials has been briefly discussed. In addition, the application of biomolecules and motifs for desired outcomes, and tailoring of their transport and kinetic behavior for achieving desired outcomes in hybrid nanogels has also been reviewed.

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.

Growth differentiation factor-5-gelatin methacryloyl injectable microspheres laden with adipose-derived stem cells for repair of disc degeneration

Nucleus pulposus (NP) degeneration is the major cause of degenerative disc disease (DDD). This condition cannot be treated or attenuated by traditional open or minimally invasive surgical options. However, a combination of stem cells, growth factors (GFs) and biomaterials present a viable option for regeneration. Injectable biomaterials act as carriers for controlled release of GFs and deliver stem cells to target tissues through a minimally invasive approach. In this study, injectable gelatin methacryloyl microspheres (GMs) with controllable, uniform particle sizes were rapidly biosynthesized through a low-cost electrospraying method. The GMs were used as delivery vehicles for cells and GFs, and they exhibited good mechanical properties and biocompatibility and enhanced the in vitro differentiation of laden cells into NP-like phenotypes. Furthermore, this integrated system attenuated the in vivo degeneration of rat intervertebral discs, maintained NP tissue integrity and accelerated the synthesis of extracellular matrix. Therefore, this novel therapeutic system is a promising option for the treatment of DDD.

Microencapsulation improves chondrogenesis in vitro and cartilaginous matrix stability in vivo compared to bulk encapsulation

The encapsulation of cells into microgels is attractive for applications in tissue regeneration. While cells are protected against shear stress during injection, the assembly of microgels after injection into a tissue defect also forms a macroporous scaffold that allows effective nutrient transport throughout the construct. However, in most of current strategies that form microgel-based macroporous scaffold or higher-order structures, cells are seeded during or post the assembly process and not microencapsulated in situ. The objective of this study is to investigate the chondrogenic phenotype of microencapsulated fetal chondrocytes in a biocompatible, assembled microgel system vs. bulk gels and to test the stability of the constructs in vivo. Here, we demonstrate that cell microencapsulation leads to increased expression of cartilage-specific genes in a TGF-β1-dependent manner. This correlates, as shown by histological staining, with the ability of microencapsulated cells to deposit cartilaginous matrix after migrating to the surface of the microgels, while keeping a macroscopic granular morphology. Implantation of precultured scaffolds in a subcutaneous mouse model results in vessel infiltration in bulk gels but not in assembled microgels, suggesting a higher stability of the matrix produced by the cells in the assembled microgel constructs. The cells are able to remodel the microgels as demonstrated by the gradual disappearance of the granular structure in vivo. The biocompatible microencapsulation and microgel assembly system presented in this article therefore hold great promise as an injectable system for cartilage repair.

Designing Microgels for Cell Culture and Controlled Assembly of Tissue Microenvironments

Micron-sized hydrogels, termed microgels, are emerging as multifunctional platforms that can recapitulate tissue heterogeneity in engineered cell microenvironments. The microgels can function as either individual cell culture units or can be assembled into larger scaffolds. In this manner, individual microgels can be customized for single or multi-cell co-culture applications, or heterogeneous populations can be used as building blocks to create microporous assembled scaffolds that more closely mimic tissue heterogeneities. The inherent versatility of these materials allows user-defined control of the microenvironments, from the order of singly encapsulated cells to entire three-dimensional cell scaffolds. These hydrogel scaffolds are promising for moving towards personalized medicine approaches and recapitulating the multifaceted microenvironments that exist in vivo.

Smart microgel-metal hybrid particles of PNIPAM-co-PAA@AgAu: synthesis, characterizations and modulated catalytic activity

Smart pH and thermoresponsive, poly(N-isopropyl acrylamide co acrylic acid) (PNIPAM-co-PAA) microgel particles are used as microreactors to prepare hybrids of gold (Au) and silver (Ag) nanoparticles (PNIPAM-co-PAA@AgAu) using a facile two steps in situ approach. These hybrid particles are characterized using the transmission electron microscope (TEM), UV-VIS spectrometer, and dynamic light scattering (DLS). TEM directly confirms the successful loading of metal nanoparticles onto microgels and the hybrid particles have a narrow size distribution. UV-VIS spectroscopy at different concentration ratios of silver/gold chloride strongly reveals the presence of plasmon peaks of both silver and gold between 10% to 25% of gold chloride concentration. DLS studies demonstrate that these hybrid microgels exhibit both pH and thermoresponsive properties comparatively with a lesser swelling than the pure microgels without loaded nanoparticles. Further, the catalytic activities of PNIPAM-co-PAA@AgAu hybrids are studied through a reduction of 4-nitrophenol (4-NP)-to-4-aminophenol (4-AP) in the presence of sodium borohydride at different pH. Interestingly, these hybrid particles exhibit modulating catalytic activity with variation in pH. The reduction kinetics decreases with increasing pH and the corresponding apparent rate constant exhibits two linear regimes with one at pH below pK_a and another at pH above pK_a of acrylic acid. This pH-modulated catalytic behavior of PNIPAM-co-PAA@AgAu hybrids is discussed based on pH-induced swelling/deswelling transition, the core-shell nature of microgel particles, and its intrinsic interplay with the diffusion of nitrophenols within the microgel network. Finally, our results are compared and discussed in the context of previously studied catalytic activities in different polymer-metal hybrids.

Photopolymerized Starchstarch Nanoparticle (SNP) network hydrogels

Starch is an attractive biomaterial given its low cost and high protein repellency, but its use in forming functional hydrogels is limited by its high viscosity and crystallinity. Herein, we demonstrate the use of fully amorphous starch nanoparticles (SNPs) as functional hydrogel building blocks that overcome these challenges. Methacrylation of SNPs enables hydrogel formation via photopolymerization, with the low viscosity of SNPs enabling facile preparation of pre-gel suspensions of up to 35 wt% SNPs relative to <10 wt% with linear starch. Small angle neutron scattering indicates a significantly different microstructure in SNP-based hydrogels compared to linear starch-based hydrogels due to the balance between inter- and intra-particle crosslinks, consistent with SNPs forming denser and stiffer hydrogels. Functionalized SNPs are highly cytocompatible at degree of substitution values <0.25 and, once gelled, can effectively repel cell adhesion. The physicochemical versatility and biological functionality of SNP-based hydrogels offer potential in various applications.

Advances of Nano-Structured Extended-Release Local Anesthetics

Extended-release local anesthetics (LAs) have drawn increasing attention with their promising role in improving analgesia and reducing adverse events of LAs. Nano-structured carriers such as liposomes and polymersomes optimally meet the demands of/for extended-release, and have been utilized in drug delivery over decades and showed satisfactory results with extended-release. Based on mature technology of liposomes, EXPAREL, the first approved liposomal LA loaded with bupivacaine, has seen its success in an extended-release form. At the same time, polymersomes has advances over liposomes with complementary profiles, which inspires the emergence of hybrid carriers. This article summarized the recent research successes on nano-structured extended-release LAs, of which liposomal and polymeric are mainstream systems. Furthermore, with continual optimization, drug delivery systems carry properties beyond simple transportation, such as specificity and responsiveness. In the near future, we may achieve targeted delivery and controlled-release properties to satisfy various analgesic requirements.

Hydrogels as Potential Nano-, Micro- and Macro-Scale Systems for Controlled Drug Delivery

This review is an extensive evaluation and essential analysis of the design and formation of hydrogels (HGs) for drug delivery. We review the fundamental principles of HGs (their chemical structures, physicochemical properties, synthesis routes, different types, etc.) that influence their biological properties and medical and pharmaceutical applications. Strategies for fabricating HGs with different diameters (macro, micro, and nano) are also presented. The size of biocompatible HG materials determines their potential uses in medicine as drug carriers. Additionally, novel drug delivery methods for enhancing treatment are discussed. A critical review is performed based on the latest literature reports.

Hydrogel microparticles for biomedical applications

Hydrogel microparticles (HMPs) are promising for biomedical applications, ranging from the therapeutic delivery of cells and drugs to the production of scaffolds for tissue repair and bioinks for 3D printing. Biologics (cells and drugs) can be encapsulated into HMPs of predefined shapes and sizes using a variety of fabrication techniques (batch emulsion, microfluidics, lithography, electrohydrodynamic (EHD) spraying and mechanical fragmentation). HMPs can be formulated in suspensions to deliver therapeutics, as aggregates of particles (granular hydrogels) to form microporous scaffolds that promote cell infiltration or embedded within a bulk hydrogel to obtain multiscale behaviours. HMP suspensions and granular hydrogels can be injected for minimally invasive delivery of biologics, and they exhibit modular properties when comprised of mixtures of distinct HMP populations. In this Review, we discuss the fabrication techniques that are available for fabricating HMPs, as well as the multiscale behaviours of HMP systems and their functional properties, highlighting their advantages over traditional bulk hydrogels. Furthermore, we discuss applications of HMPs in the fields of cell delivery, drug delivery, scaffold design and biofabrication.

Developing hyaluronic acid microgels for sustained delivery of platelet lysate for tissue engineering applications

Platelet lysate (PL), a blood product that contains high concentrations of growth factors (GFs), can be considered as a cost-effective source of multiple GFs. In this study, hyaluronic acid (HA) based microgels were developed for delivery of PL proteins. Spherical microgel were prepared using a water in oil emulsion method. First, hyaluronic acid was grafted with tyramine groups, after which prepared microdroplets were crosslinked via an enzymatic reaction in the presence of hydrogen peroxide and horseradish peroxidase. Because of electrostatic interactions, these microgels are promising carriers for positively charged proteins entrapment like most of the GFs. When microgels are incubated in PL solution, protein loading takes place which is mainly governed by nonspecific adsorption of plasma proteins. Although this hampered loading efficiency, loading could be increased by repeated washing and incubation steps. The loaded microgels presented a sustained release of PL growth factors for a period of two weeks. When PL enriched microgels were embedded in a HA bulk hydrogel, cell proliferation was higher compared to constructs without microgels. These findings suggest that the developed microgels are a potential candidate for sustained delivery of PL growth factors and present a solution to the issue of their short half-lives in vivo.

Challenges to Achieving Strong but Fully Degradable Adhesive Joints between Wet Cellulose Surfaces

The dramatic loss of strength upon exposure to water is one of the challenges preventing the widespread substitution of plastic packaging with paper and paperboard. Although treatment with conventional wet strength resins and other adhesive polymers can strengthen wet paper, it is at the expense of green credentials, including easy recycling or rapid composting. The goal of this work was to demonstrate the adhesive requirements for strong, wet cellulose-cellulose joints that can be recycled easily because the joint strength can be destroyed by the presence of a weak reducing agent. Cellulose membrane surfaces were first treated with a bound layer of carboxymethyl cellulose, modified to have covalently tethered hydrazide groups. Joints were fabricated by laminating two hydrazide-modified membranes with a polymeric adhesive bearing aldehyde functionality. Aldehydes spontaneously condense with hydrazide to give hydrazone bonds. When the adhesive was oxidized dextran, the wet laminates had an intermediate strength. Upon exposure to a reducing agent, the joint strength was reduced to nearly zero because every hydrazone moiety connecting two cellulose surfaces included a cleavable disulfide linkage. By contrast, glyoxalated cationic polyacrylamide gave very strong wet joints. However, the wet adhesion was dominated by polyelectrolyte complexation, and the presence of hydrazone linkages had little influence on the wet adhesion. We conclude that robust joint degradability will require cleavable linkages in the adhesive backbone while minimizing polyelectrolyte complexation.

Effect of Divalent Cation on Swelling Behavior of Anionic Microgels: Quantification and Dynamics of Ion Uptake and Release

Poly(N-vinylcaprolactam-co-itaconate) (P(VCL-co-IADME) microgels were synthesized varying the molar ratio between VCL and IADME via free radical precipitation polymerization in the presence of quaternary ammonium surfactant. In order to determine the effect of the divalent metal ions on the structure and the swelling behavior of the microgel systems, both neutral and charged forms of the hydrogels after hydrolysis were investigated. The triggered gel collapse caused by the divalent metal ion together with the quantification of the metal ion uptake was studied in detail by titration and ion chromatography methods and revealed the minimum concentration around 0.1 mM to trigger gel collapse on the treated gels. Uptake and release dynamics of the gels were followed by turbidity measurements and were in the time-range of 2 and 17 s, depending on the composition and the concentrations.