Hydrogel-based Materials Research Articles

Chitosan/chondroitin sulfate hydrogels prepared in [Hmim][HSO4] ionic liquid

[Hmim][HSO4] ionic liquid (IL) and bio-renewable sources as chitosan (CHT) and chondroitin sulfate (CS) were used to yield hydrogel-based materials (CHT/CS). The use of IL to solubilize both polysaccharides was considered an innovative way based on “green chemistry” principle, aiming the production of CHT/CS blended systems. CHT/CS hydrogels were carried out in homogeneous medium from short dissolution times. The hydrogels were characterized and achieved with excellent stabilities (in the 1.2–10pH range), larger swelling capacities, as well as devoid of cytotoxicity towards the normal VERO and diseased HT29 cells. The CHT/CS hydrogels carried out in [Hmim][HSO4] could be applied in many technological purposes, like medical, pharmaceutical, and environmental fields.

Hydrogel-Based Materials for Delivery of Herbal Medicines.

Herbal medicine, as an integral component of oriental medicine, has assimilated into the lives of Asian people for millennia. The therapeutic efficiency of herbal extracts and ingredients has, however, been limited by various factors, including the lack of targeting capacity and poor bioavailability. Hydrogels are hydrophilic polymer networks that can imbibe a substantial amount of fluids. They are biocompatible, and may enable sustained drug release. Hydrogels, therefore, have attracted widespread studies in pharmaceutical formulation. This article first reviews the latest progress in the development of hydrogel-based materials as carriers of herbal medicines, followed by a discussion of the relationships between hydrogel properties and carrier performance. Finally, the promising potential of using hydrogels to combine medicinal herbs with synthetic drugs in one single treatment will be highlighted as an avenue for future research.

Shape-Morphing Materials from Stimuli-Responsive Hydrogel Hybrids

The formation of well-defined and functional three-dimensional (3D) structures by buckling of thin sheets subjected to spatially nonuniform stresses is common in biological morphogenesis and has become a subject of great interest in synthetic systems, as such programmable shape-morphing materials hold promise in areas including drug delivery, biomedical devices, soft robotics, and biomimetic systems. Given their ability to undergo large changes in swelling in response to a wide variety of stimuli, hydrogels have naturally emerged as a key type of material in this field. Of particular interest are hybrid systems containing rigid inclusions that can define both the anisotropy and spatial nonuniformity of swelling as well as nanoparticulate additives that can enhance the responsiveness and functionality of the material. In this Account, we discuss recent progress in approaches to achieve well-defined shape morphing in hydrogel hybrids. First, we provide an overview of materials and methods that facilitate fabrication of such systems and outline the geometry and mechanics behind shape morphing of thin sheets. We then discuss how patterning of stiff inclusions within soft responsive hydrogels can be used to program both bending and swelling, thereby providing access to a wide array of complex 3D forms. The use of discretely patterned stiff regions to provide an effective composite response offers distinct advantages in terms of scalability and ease of fabrication compared with approaches based on smooth gradients within a single layer of responsive material. We discuss a number of recent advances wherein control of the mechanical properties and geometric characteristics of patterned stiff elements enables the formation of 3D shapes, including origami-inspired structures, concatenated helical frameworks, and surfaces with nonzero Gaussian curvature. Next, we outline how the inclusion of functional elements such as nanoparticles can enable unique pathways to programmable and even reprogrammable shape-morphing materials. We focus to a large extent on photothermally reprogrammable systems that include one of a variety of additives that serve to efficiently absorb light and convert it into heat, thereby driving the response of a temperature-sensitive hydrogel. Such systems are advantageous in that patterns of light can be defined with very high spatial and temporal resolution in addition to offering the potential for wavelength-selective addressability of multiple different inclusions. We highlight recent advances in the preparation of light-responsive hybrid systems capable of undergoing reprogrammable bending and buckling into well-defined 3D shapes. In addition, we describe several examples where shape tuning of hybrid systems enables control over the motion of responsive hydrogel-based materials. Finally, we offer our perspective on open challenges and future areas of interest for the field.

Additive manufacturing of hydrogel-based materials for next-generation implantable medical devices.

Implantable microdevices often have static components rather than moving parts, and exhibit limited biocompatibility. This paper demonstrates a fast manufacturing method which can produce features in biocompatible materials down to tens of microns in scale, with intricate and composite patterns in each layer. By exploiting unique mechanical properties of hydrogels, we developed a "locking mechanism" for precise actuation and movement of freely moving parts, which can provide functions such as valves, manifolds, rotors, pumps, and delivery of payloads. Hydrogel components could be tuned within a wide range of mechanical and diffusive properties, and can be controlled after implantation without a sustained power supply. In a mouse model of osteosarcoma, triggering of release of doxorubicin from the device over ten days showed high treatment efficacy and low toxicity, at one-tenth of a standard systemic chemotherapy dose. Overall, this platform, called "iMEMS", enables development of biocompatible implantable microdevices with a wide range of intricate moving components that can be wirelessly controlled on demand, in a manner that solves issues of device powering and biocompatibility.

3D Cell Culture in Interpenetrating Networks of Alginate and rBM Matrix.

Altered tissue mechanical properties have been implicated in many key physiological and pathological processes. Hydrogel-based materials systems, made with native extracellular matrix (ECM) proteins, nonnative biopolymers, or synthetic polymers are often used to study these processes in vitro in 3D cell culture experiments. However, each of these materials systems present major limitations when used in mechanobiological studies. While native ECM-based hydrogels may enable good recapitulation of physiological behavior, the mechanics of these hydrogels are often manipulated by increasing or decreasing the protein concentration. This manipulation changes cell adhesion ligand density, thereby altering cell signaling. Alternatively, synthetic polymer-based hydrogels and nonnative biopolymer-based hydrogels can be mechanically tuned and engineered to present cell adhesion peptide motifs, but still may not fully promote physiologically relevant behavior. Here, we combine the advantages of native ECM proteins and nonnative biopolymers in interpenetrating network (IPN) hydrogels consisting of rBM matrix, which contains ligands native to epithelial basement membrane, and alginate, an inert biopolymer derived from seaweed. The following protocol details the generation of IPNs for mechanical testing or for 3D cell culture. This biomaterial system offers the ability to tune the stiffness of the 3D microenvironment without altering cell adhesion ligand concentration or pore size.

Difference between Chitosan Hydrogels via Alkaline and Acidic Solvent Systems.

Chitosan (CS) has generated considerable interest for its desirable properties and wide applications. Hydrogel has been proven to be a major and vital form in the applications of CS materials. Among various types of CS hydrogels, physical cross-linked CS hydrogels are popular, because they avoided the potential toxicity and sacrifice of intrinsic properties caused by cross-linking or reinforcements. Alkaline solvent system and acidic solvent system are two important solvent systems for the preparation of physical cross-linked CS hydrogels, and also lay the foundations of CS hydrogel-based materials in many aspects. As members of physical cross-linked CS hydrogels, gel material via alkaline solvent system showed significant differences from that via acidic solvent system, but the reasons behind are still unexplored. In the present work, we studied the difference between CS hydrogel via alkaline system and acidic system, in terms of gelation process, hydrogel structure and mechanical property. In-situ/pseudo in-situ studies were carried out, including fluorescent imaging of gelation process, which provided dynamic visualization. Finally, the reasons behind the differences were explained, accompanied by the discussion about design strategy based on gelation behavior of the two systems.

A correlative spatiotemporal microscale study of calcium phosphate formation and transformation within an alginate hydrogel matrix.

The modification of soft hydrogels with hard inorganic components is a method used to form composite materials with application in non-load-bearing bone tissue engineering. The inclusion of an inorganic component may provide mechanical enhancement, introduce osteoconductive or osteoinductive properties, or change other aspects of interactions between native or implanted cells and the material. A thorough understanding of the interactions between such components is needed to improve the rational design of such biomaterials. To achieve this goal, model systems which could allow study of the formation and transformation of mineral phases within a hydrogel network with a range of experimental methods and high spatial and time resolution are needed. Here, we report a detailed investigation of the formation and transformation process of calcium phosphate mineral within an alginate hydrogel matrix. A combination of optical microscopy, confocal Raman microspectroscopy and electron microscopy was used to investigate the spatial distribution, morphology and crystal phase of the calcium phosphate mineral, as well as to study transformation of the mineral phases during the hydrogel mineralization process and upon incubation in a simulated body fluid. It was found, that under the conditions used in this work, mineral initially formed as a metastable amorphous calcium phosphate phase (ACP). The ACP particles had a distinctive spherical morphology and transformed within minutes into brushite in the presence of brushite seed crystals or into octacalcium phosphate, when no seeds were present in the hydrogel matrix. Incubation of brushite-alginate composites in simulated body fluid resulted in formation of hydroxyapatite. The characterization strategy presented here allows for non-destructive, in situ observation of mineralization processes in optically transparent hydrogels with little to no sample preparation. The precipitation and transformations of calcium phosphates (CaP) is a complex process, where both formation kinetics and the stability of different mineral phases control the outcome. This situation is even more complex if CaP is precipitated in a hydrogel matrix, where one can expect the organic matrix to modulate crystallization by introducing supersaturation gradients or changing the nucleation and growth kinetics of crystals. In this study we apply a range of characterization techniques to study the mineral formation and transformations of CaP within an alginate matrix with spatiotemporal resolution. It demonstrates how a detailed investigation of the mineral precipitation and transformations can aid in the future rational design of hydrogel-based materials for bone tissue engineering and studies of biomineralization processes.

Dropwise gelation-dehydration kinetics during drop-on-demand printing of hydrogel-based materials

The present study aims to characterize and understand the dropwise gelation-dehydration phenomena during drop-on-demand (DOD) printing of hydrogel-based soft materials. Functional soft materials have broader impacts on many medical and engineering applications, but constructing soft materials into three-dimensional (3D) configuration with spatially varying properties is still extremely challenging. In order to establish a mechanistic understanding, a hypothesis was postulated that the porosity of hydrogel printed is determined by dropwise gelation and dehydration phenomena during the printing process. The underlying rationale is that many functional properties of the printed hydrogels are closely associated with the structural characteristics at the sub-droplet and droplet scales, specifically porosity. The porosity of a hydrogel droplet is thought to be determined by intra-droplet fluid–structure interactions during gelation and dehydration. In this study, thus, we characterized the gelation-dehydration and consequent microstructure of thermally responsive poly(N-isopropylacrylamind-co-acrylamide) (PNIPAM) copolymer droplets as a model hydrogel material. The gelation kinetics was studied by differential scanning calorimetry. Both macroscopic and microscopic structures of DOD printed hydrogels were characterized by a 3D profiler and scanning electron microscopy. Furthermore, a theoretical model to explain this complex transport processes was also developed. The results showed that the gelation is a rapid process and its impact is mainly observed at the deposition of droplets. Significant structural shrinkage of the printed hydrogel droplets was induced by dehydration. This shrinkage resulted in spatially varying intra-droplet porosity. A computational model of intra-droplet fluid–structure interactions was developed to explain this spatial variation of intra-droplet porosity. In addition, a new dimensionless parameter is proposed to gauge the significance of evaporation and interstitial water transport in the fluid–structure interactions. Significance of gelation kinetics, dehydration and complex fluid–structure interaction within the droplets was discussed to design a DOD printing process for 3D additive manufacturing of hydrogel-based soft materials.

Functionalized Polymer Microgel Particles Enable Customizable Production of Label-Free Sensor Arrays.

Probe molecule immobilization onto surfaces is a critical step in the production of many analytical devices, including labeled and label-free microarrays. New methods to increase the density and uniformity of probe deposition have the potential to significantly enhance the ultimate limits of detection and reproducibility. Hydrogel-based materials have been employed in the past to provide a 3D protein-friendly surface for deposition of antibodies and nucleic acids. However, these methods are susceptible to variation during polymerization of the hydrogel scaffold and provide limited opportunities for tuning deposition parameters on an antibody-by-antibody basis. In this work, a versatile hydrogel nanoparticle deposition method was developed for the production of label-free microarrays and tested in the context of antibody-antigen binding. Poly(N-isopropylacrylamide) nanoparticles (PNIPAM) were conjugated to antibodies using an avidin/biotin system and deposited onto surfaces using a noncontact printing system. After drying, these gel spots formed uniform and thin layers <10 nm in height. The conjugates were characterized with dynamic light scattering, scanning electron microscopy, and atomic force microscopy. We tested this format in the context of tumor necrosis factor-alpha (TNF-α) detection via arrayed imaging reflectometry (AIR), a label-free protein microarray method. This method of probe molecule deposition should be generally useful in the production of microarrays for label-free detection.

Responsive Polymer Photonics.

Responsive Polymer Photonics.

Hydrogel‐Based Nanocomposites and Mesenchymal Stem Cells: A Promising Synergistic Strategy for Neurodegenerative Disorders Therapy

Hydrogel-based materials are widely employed in the biomedical field. With regard to central nervous system (CNS) neurodegenerative disorders, the design of injectable nanocomposite hydrogels for in situ drug or cell release represents an interesting and minimally invasive solution that might play a key role in the development of successful treatments. In particular, biocompatible and biodegradable hydrogels can be designed as specific injectable tools and loaded with nanoparticles (NPs), to improve and to tailor their viscoelastic properties upon injection and release profile. An intriguing application is hydrogel loading with mesenchymal stem cells (MSCs) that are a very promising therapeutic tool for neurodegenerative or traumatic disorders of the CNS. This multidisciplinary review will focus on the basic concepts to design acellular and cell-loaded materials with specific and tunable rheological and functional properties. The use of hydrogel-based nanocomposites and mesenchymal stem cells as a synergistic strategy for nervous tissue applications will be then discussed.

Systematic Analysis of Injectable Materials and 3D Rapid Prototyped Magnetic Scaffolds: From CNS Applications to Soft and Hard Tissue Repair/Regeneration

Over the past years, polymer-based materials have attracted research interest in the field of tissue repair and regeneration. As reported in literature, different injectable systems have been proposed, trying to reduce surgical invasiveness. In a first step of the current research, the rheological and functional features of injecatble hydrogel-based materials for central nervous system applications or soft tissue regeneration (collagen/PEG semi-IPNs) as well as for hard tissue engineering (alginate/iron-doped hydroxyapatite) were evaluated. Then, the study was also devoted to the development of 3D nanocomposite poly(ɛ- caprolactone)/iron-doped hydroxyapatite scaffolds for bone tissue engineering, providing a preliminary approach to assess magnetic attraction forces.

Facilitating the mineralization of oligo(poly(ethylene glycol) fumarate) hydrogel by incorporation of hydroxyapatite nanoparticles

Exploring strategies to induce the mineralization of hydrogels is an important step toward the development of hydrogel-based materials for bone regeneration. In the current study, the effect of incorporating hydroxyapatite (HA) nanoparticles on the mineralization capacity of an inert poly(ethylene glycol) (PEG)-based hydrogel was investigated. HA nanoparticles were either directly loaded into oligo(poly(ethylene glycol) fumarate) (OPF) hydrogel or loaded into commonly used gelatin microsphere porogens that were subsequently integrated in the OPF matrix. Mineralization of composites after immersion of the samples in simulated body fluid up to 28 days was assessed. In contrast to the blank OPF hydrogel, the HA-containing constructs strongly mineralized such that the average rate of calcium uptake by the material was enhanced by orders of magnitude. The mineral formed was observed to be apatitic and needle shaped. The presented method allows modification of inert PEG-based hydrogels into bioactive biomaterials for applications in bone regeneration.

Gellan gum-based hydrogels for intervertebral disc tissue-engineering applications

Intervertebral disc (IVD) degeneration is a challenging clinical problem that urgently demands viable nucleus pulposus (NP) implant materials. The best suited biomaterial for NP regeneration has yet to be identified, but it is believed that biodegradable hydrogel-based materials are promising candidates. In this work, we have developed ionic- and photo-crosslinked methacrylated gellan gum (GG-MA) hydrogels to be used in acellular and cellular tissue-engineering strategies for the regeneration of IVDs. The physicochemical properties of the developed hydrogels were investigated by Fourier-transform infrared spectroscopy, (1) H nuclear magnetic resonance and differential scanning calorimetry. The swelling ability and degradation rate of hydrogels were also analysed in phosphate-buffered saline solution at physiological pH for a period of 30 days. Additionally, the morphology and mechanical properties of the hydrogels were assessed under a scanning electron microscope and dynamic compression, respectively. An in vitro study was carried out to screen possible cytotoxicity of the gellan gum-based hydrogels by culturing rat lung fibroblasts (L929 cells) with hydrogel leachables up to 7 days. The results demonstrated that gellan gum was successfully methacrylated. We observed that the produced GG-MA hydrogels possess improved mechanical properties and lower water uptake ability and degradation rate as compared to gellan gum. This work also revealed that GG-MA hydrogels are non-cytotoxic in vitro, thus being promising biomaterials to be used in IVD tissue-engineering strategies.

Rheological Characterization of Hyaluronic Acid Derivatives as Injectable Materials Toward Nucleus Pulposus Regeneration

Nucleus pulposus (NP) is the soft center of the intervertebral disc (IVD), able to resist compressive loads, while the annulus fibrosus withstands tension and gives mechanical strength. NP function may be altered as consequence of several pathologies or injury and when a damaged IVD does not properly play its role. In the past years, a great effort has been devoted to the design of injectable systems as NP substitutes. The different synthetic- and natural hydrogel-based materials proposed, present many drawbacks and, in particular, they do not seem to mimic the required behavior. In the search for natural-based systems a dodecylamide of hyaluronic acid (HA), HYADD3®, has been proved as bioactive and suitable vehicle to carry cells for NP tissue engineering, while a crosslinked HA ester, HYAFF120® showed interesting results if used as injectable acellular material. Even though these derivatives showed appropriate biological behavior up to now, data on mechanical behavior of these derivatives are still missing. In this frame, the aim of this study was to provide a rheological characterization of these HA derivatives to asses their biomechanical compatibility with the NP tissue. To this, the rheological properties of these derivatives were studied through dynamic shear tests before and after injection through needles used in the current surgical procedure. Both HA derivatives showed a 'gel-like' rheological behavior similar to the native NP tissue and this behavior was not altered by injection.

PEG-benzofulvene copolymer hydrogels for antibody delivery

Poly[monomethylnona(ethylene glycol) 1-methylene-3-(4-methylphenyl)-1 H-indene-2-carboxylate] (poly- 1b) a new polymer based on a PEG-functionalized benzofulvene macromonomer have been investigated as hydrogel-based material for complexation and release of immunoglobulin (IgG) at physiological mimicking conditions. The polymer ability to complex human IgG has been studied by preparing copolymer/protein complexes obtained by spontaneous protein interactions onto polymer hydrogel aggregates, and the protein release rate has been evaluated at physiological conditions. SEM analysis was used to visualize the copolymer/IgG aggregates and its microstructured deposition. Moreover, rheological studies performed at 37 °C allowed determining hydrogel mechanical properties. On the basis of these information and NMR transverse relaxation measurements, the estimation of hydrogel mesh size distribution was possible. Finally, biological studies performed with poly- 1b aqueous dispersions showed no cytotoxic effect on MCF-7 cell line, suggesting potential biocompatibility features for this polymer and making this new polymer a good potential candidate for the production of drug delivery systems.

Independent Control of Rigidity and Toughness of Polymeric Hydrogels

Refined control over the mechanical properties of hydrogel-based materials has increased as these materials have found broader application. We investigated various aspects of gel cross-linking to independently regulate the elastic modulus (E) and toughness (W). Alginate hydrogels were chosen as a model system, since alginate can be gelled via ionic or covalent cross-linking, and its block structure dictates the structure of ionic cross-links. Increasing the density of covalent cross-links increased E but led to more brittle gels. In contrast, increasing the density of ionic cross-links and length of the blocks responsible for the cross-linking increased both E and W. Oscillatory shear measurements suggested that ionic cross-links and their length were important in dissipating the energy of deformation due to a partial and stepwise de-cross-linking. In contrast, covalently cross-linked gels underwent energy accumulation. This study demonstrates a novel approach to independently control different mechanical properties of gels.