Effect of particle concentration on the microstructural and macromechanical properties of biocompatible magnetic hydrogels

Highly deformable and strongly magnetic semi-interpenetrating hydrogels based on alginate or cellulose

The effective implementation of many of the applications of magnetic hydrogels requires the development of innovative systems capable of withstanding a substantial load of magnetic particles to ensure exceptional responsiveness, without compromising their reliability and stability. To address this challenge, double-network hydrogels have emerged as a promising foundation, thanks to their extraordinary mechanical deformability and toughness. Here, we report a semi-interpenetrating polymer networks (SIPNs) approach to create diverse magnetic SIPNs hydrogels based on alginate or cellulose, exhibiting remarkable deformability under certain stresses. Achieving strong responsiveness to magnetic fields is a key objective, and this characteristic is realized by the incorporation of highly magnetic iron microparticles at moderately large concentrations into the polymer network. Remarkably, the SIPNs hydrogels developed in this research accommodate high loadings of magnetic particles without significantly compromising their physical properties. This feature is essential for their use in applications that demand robust responsiveness to applied magnetic fields and overall stability, such as a hydrogel luminescent oxygen sensor controlled by magnetic fields that we designed and tested as proof-of-concept. These findings underscore the potential and versatility of magnetic SIPNs hydrogels based on carbohydrate biopolymers as fundamental components in driving the progress of advanced hydrogels for diverse practical implementations.

Heparinized Acellular Hydrogels for Magnetically Induced Wound Healing Applications

The control of angiogenesis has the potential to be used for regulation of several pathological and physiological processes, which can be instrumental on the development of anticancer and wound healing therapeutical approaches. In this study, mesenchymal stem/stromal cells (MSCs) were seeded on magnetic-responsive gelatin, with or without heparin functionalization, and exposed to a static 0.08 T magnetic field (MF), for controlling their anti-inflammatory and angiogenic activity, with the aim of accelerating tissue healing. For the first time, it was examined how the amount of heparin and magnetic nanoparticles (MNPs) distributed on gelatin scaffolds affected the mechanical properties of the hydrogels and the morphology, proliferation, and secretome profiling of MSCs. The findings demonstrated that the addition of MNPs and heparin affects the hydrogel swelling capacity and renders distinct MSC proliferation rates. Additionally, MF acts as a topographical cue to guide MSCs alignment and increases the level of expression of specific genes and proteins that promote angiogenesis. The results also suggested that the presence of higher amounts of heparin (10 μg/cm3) interferes with the secretion and limits the capacity of angiogenic factors to diffuse through the hydrogel and into the culture medium. Ultimately, this study shows that acellular heparinized hydrogels efficiently retain the angiogenic growth factors released by magnetically stimulated MSCs thus rendering superior wound contraction (55.8% ± 0.4%) and cell migration rate (49.4% ± 0.4%), in comparison to nonheparinized hydrogels (35.2% ± 0.7% and 37.8% ± 0.7%, respectively). Therefore, these heparinized magnetic hydrogels can be used to facilitate angiogenesis in various forms of tissue damage including bone defects, skin wounds, and cardiovascular diseases, leading to enhanced tissue regeneration.

Chitosan/Alginate Nanogels Containing Multicore Magnetic Nanoparticles for Delivery of Doxorubicin

In this study, multicore-like iron oxide (Fe3O4) and manganese ferrite (MnFe2O4) nanoparticles were synthesized and combined with nanogels based on chitosan and alginate to obtain a multimodal drug delivery system. The nanoparticles exhibited crystalline structures and displayed sizes of 20 ± 3 nm (Fe3O4) and 11 ± 2 nm (MnFe2O4). The Fe3O4 nanoparticles showed a higher saturation magnetization and heating efficiency compared with the MnFe2O4 nanoparticles. Functionalization with citrate and bovine serum albumin was found to improve the stability and modified surface properties. The nanoparticles were encapsulated in nanogels, and provided high drug encapsulation efficiencies (~70%) using doxorubicin as a model drug. The nanogels exhibited sustained drug release, with enhanced release under near-infrared (NIR) laser irradiation and acidic pH. The nanogels containing BSA-functionalized nanoparticles displayed improved sustained drug release at physiological pH, and the release kinetics followed a diffusion-controlled mechanism. These results demonstrate the potential of synthesized nanoparticles and nanogels for controlled drug delivery, offering opportunities for targeted and on-demand release in biomedical applications.

High-Strength Magnetic Hydrogels with Photoweldability Made by Stepwise Assembly of Magnetic-Nanoparticle-Integrated Aramid Nanofiber Composites

Hydrogels capable of transforming in response to a magnetic field hold great promise for applications in soft actuators and biomedical robots. However, achieving high mechanical strength and good manufacturability in magnetic hydrogels remains challenging. Here, inspired by natural load-bearing soft tissues, a class of composite magnetic hydrogels is developed with tissue-mimetic mechanical properties and photothermal welding/healing capability. In these hydrogels, a hybrid network involving aramid nanofibers, Fe₃O₄ nanoparticles, and poly(vinyl alcohol) is accomplished by a stepwise assembly of the functional components. The engineered interactions between nanoscale constituents enable facile materials processing and confer a combination of excellent mechanical properties, magnetism, water content, and porosity. Furthermore, the photothermal property of Fe₃O₄ nanoparticles organized around the nanofiber network allows near-infrared welding of the hydrogels, providing a versatile means to fabricate heterogeneous structures with custom designs. Complex modes of magnetic actuation are made possible with the manufactured heterogeneous hydrogel structures, suggesting opportunities for further applications in implantable soft robots, drug delivery systems, human-machine interactions, and other technologies.

Biocompatible Short-Peptides Fibrin Co-assembled Hydrogels

Fibrin hydrogels made by self-assembly of fibrinogen obtained from human plasma have shown excellent biocompatible and biodegradable properties and are widely used in regenerative medicine. The fibrinogen self-assembly process can be triggered under physiological conditions by the action of thrombin, allowing the injection of pregel mixtures that have been used as cell carriers, wound-healing systems, and bio-adhesives. However, access to fibrinogen from human plasma is expensive and fibrin gels have limited mechanical properties, which make them unsuitable for certain applications. One solution to these problems is to obtain composite gels made of fibrin and other polymeric compounds that improve their mechanical properties and usage. Herein, we prepared composite hydrogels made by the self-assembly of fibrinogen together with Fmoc-FF (Fmoc-diphenylalanine) and Fmoc-RGD (Fmoc-arginine-glycine-aspartic acid). We have shown that the mixture of these three peptides co-assembles and gives rise to a unique type of supramolecular fiber, whose morphology and mechanical properties can be modulated. We have carried out a complete characterization of these materials from chemical, physical, and biological points of view. Composite gels have improved mechanical properties compared to pure fibrin gels, as well as showing excellent biocompatibility ex vivo. In vivo experiments have shown that these gels do not cause any type of inflammatory response or tissue damage and are completely resorbed in short time, which would enable their use as vehicles for cell, drug, or growth factor release.

Preparation of a 3D printable high-performance GelMA hydrogel loading with magnetic cobalt ferrite nanoparticles

Osteosarcoma remains a worldwide concern due to the poor effectiveness of available therapies in the clinic. Therefore, it is necessary to find a safe and effective therapy to realize the complete resection of osteosarcoma and reconstruction of the bone defect. Magnetic hyperthermia based on magnetic nanoparticles can kill tumor cells by raising the temperature without causing the side effects of conventional cancer treatments. This research aims to design a high-performance magnetic hydrogel composed of gelatin methacrylate and highly magnetic cobalt ferrite (CFO) nanoparticles for osteosarcoma treatment. Specifically, CFO is surface functionalized with methacrylate groups (MeCFO). The surface modified CFO has good biocompatibility and stable solution dispersion ability. Afterward, MeCFO nanoparticles are incorporated into GelMA to fabricate a three-dimensional (3D) printable MeCFO/GelMA magnetic hydrogel and then photocross-linked by UV radiation. MeCFO/GelMA hydrogel has high porosity and swelling ability, indicating that the hydrogel possesses more space and good hydrophily for cell survival. The rheological results showed that the hydrogel has shear thinning property, which is suitable as a bioprinting ink to produce desired structures by a 3D printer. Furthermore, 50 μg/mL MeCFO not only decreases the cell activity of osteosarcoma cells but also promotes the osteogenic differentiation of mBMSCs. The results of the CCK-8 assay and live/dead staining showed that MeCFO/GelMA hydrogel had good cytocompatibility. These results indicated that MeCFO/GelMA hydrogel with potential antitumor and bone reconstruction functions is a promising therapeutic strategy after osteosarcoma resection.

Technological advances in fibrin for tissue engineering

Fibrin is a promising natural polymer that is widely used for diverse applications, such as hemostatic glue, carrier for drug and cell delivery, and matrix for tissue engineering. Despite the significant advances in the use of fibrin for bioengineering and biomedical applications, some of its characteristics must be improved for suitability for general use. For example, fibrin hydrogels tend to shrink and degrade quickly after polymerization, particularly when they contain embedded cells. In addition, their poor mechanical properties and batch-to-batch variability affect their handling, long-term stability, standardization, and reliability. One of the most widely used approaches to improve their properties has been modification of the structure and composition of fibrin hydrogels. In this review, recent advances in composite fibrin scaffolds, chemically modified fibrin hydrogels, interpenetrated polymer network (IPN) hydrogels composed of fibrin and other synthetic or natural polymers are critically reviewed, focusing on their use for tissue engineering.

Plasmonic lipogels: driving co-assembly of composites with peptide-based gels for controlled drug release

Supramolecular short peptide-based gels are promising materials for the controlled release of drugs (e.g. chemotherapeutic drugs) owing to the biocompatibility and similarity to cell matrix. However, the drug encapsulation and control over its release, mainly the hydrophilic drugs, can be a cumbersome task. This can be overcome through encapsulation/compartmentalization of drugs in liposomes, which can also enable spatiotemporal control and enhanced drug release through a trigger, such as photothermia. Having this in mind, we explored the assembly of silica-coated gold nanoparticles and liposomes (storage units) with dehydropeptide-based hydrogels as a proof-of-concept to afford peptide-based NIR light-responsive lipogels. Several liposomes compositions were assessed that displayed influence on the final assembly properties by combining with silica-coated gold nanorods (∼106 nm). Gold nanospheres (∼11 nm) were used to study the preparation method, which revealed the importance of initially combine liposomes with nanoparticles and then the gelator solution to achieve a closer proximity of the nanoparticles to the liposomes. The control over a hydrophilic model drug, 5(6)-carboxyfluorescein, was only achieved by its encapsulation in liposomes, in which the presence of silica-coated nanorods further enabled the use of photothermia to induce the liposomes phase transition and stimulate the drug release. Further, both composites, the liposomes and silica-coated gold nanorods, induced a lower elastic modulus, but also provided an enhanced gelation kinetics. Hereby, this work advances fabrication strategies for the development of short peptide-based hydrogels towards on-demand, sustained and controlled release of hydrophilic drugs through photothermia under NIR light irradiation.

Synthesis of magnetic/pH dual responsive dextran hydrogels as stimuli-sensitive drug carriers

Hydrogels loaded with magnetic nanoparticles have been widely researched recently as biomaterials, due to their good biocompatibility and unique magnetic characteristics. In this study, water-soluble superparamagnetic iron oxide nanoparticles (Fe₃O₄) prepared by coprecipitation were physically doped into the dextran hydrogels which were formed via Schiff base reactions between ethylenediamine and oxidized dextran. The combination of magnetic nanoparticles and chemical cross-linked hydrogels leads to magnetic/pH dual-sensitive hydrogels which can be used as stimuli-responsive carrier. Magnetic properties, swelling, and rheology behaviors of the resulted magnetic hydrogels were strongly affected by the Fe₃O₄ nanoparticle content. Moreover, doxorubicin (DOX⋅HCl) was embedded into the magnetic hydrogels and pH/magnetic sensitive release profiles were identified. The release mechanism analysis indicated that the release behaviors of DOX⋅HCl were controlled by the diffusion, swelling, and erosion processes simultaneously. The prepared hydrogel/Fe₃O₄ composites with dual magnetic/pH stimuli-responsiveness hold the promise to be used in various applications such as drug release.

Tuning the drug multimodal release through a co-assembly strategy based on magnetic gels

Self-assembled short peptide-based gels are highly promising drug delivery systems. However, implementing a stimulus often requires screening different structures to obtain gels with suitable properties, and drugs might not be well encapsulated and/or cause undesirable effects on the gel's properties. To overcome this challenge, a new design approach is presented to modulate the release of doxorubicin as a model chemotherapeutic drug through the interplay of (di)phenylalanine-coated magnetic nanoparticles, PEGylated liposomes and doxorubicin co-assembly in dehydropeptide-based gels. The composites enable an enhancement of the gelation kinetics in a concentration-dependent manner, mainly through the use of PEGylated liposomes. The effect of the co-assembly of phenylalanine-coated nanoparticles with the hydrogel displays a concentration and size dependence. Finally, the integration of liposomes as doxorubicin storage units and of nanoparticles as composites that co-assemble with the gel matrix enables the tuneability of both passive and active doxorubicin release through a thermal, and a low-frequency alternating magnetic field-based trigger. In addition to the modulation of the gel properties, the functionalization with (di)phenylalanine improves the cytocompatibility of the nanoparticles. Hereby, this work paves a way for the development of peptide-based supramolecular systems for on-demand and controlled release of drugs.

Electroconductive and injectable hydrogels based on gelatin and PEDOT:PSS for mini-invasive approaches in nervous tissue regeneration

This work describes the development of electroconductive hydrogels as injectable matrices for neural tissue regeneration by exploiting a biocompatible conductive polymer – i.e. poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) – combined with a biomimetic...

A Comparison Study on the Magneto-Responsive Properties and Swelling Behaviors of a Polyacrylamide-Based Hydrogel Incorporating with Magnetic Particles

This work investigates the mechanical properties, microstructures, and water-swelling behavior of a novel hydrogel filled with magnetic particles. The nanoparticles of magnetite (Fe₃O₄) and the micro-particles of carbonyl iron (CI) were selected and filled into a polyacrylamide (PAAM) hydrogel matrix to create two types of magnetic hydrogels. The isotropy and anisotropy of magnetic hydrogels are also presented in this study. The isotropic samples were cured without applying a magnetic field (MF), and the anisotropic samples were cured by applying an MF in the direction perpendicular to the thickness of the samples. The effects of the size, content, and inner structures of magnetic particles on the magneto-responsive and swelling properties of magnetic hydrogels were investigated. It was found that the magnetorheological (MR) effect of anisotropic samples was apparently higher than that of isotropic samples, and the hydrogels with CI exhibited a noticeable MR effect than those with Fe₃O₄. The storage modulus can be enhanced by increasing the filler content and size, forming an anisotropic structure, and applying an external MF. In addition, the magnetic hydrogels also have a swelling ability that can be tuned by varying the content and size of the particle fillers.

Injectable Magnetic-Responsive Short-Peptide Supramolecular Hydrogels: Ex Vivo and In Vivo Evaluation

The inclusion of magnetic nanoparticles (MNP) in a hydrogel matrix to produce magnetic hydrogels has broadened the scope of these materials in biomedical research. Embedded MNP offer the possibility to modulate the physical properties of the hydrogel remotely and on demand by applying an external magnetic field. Moreover, they enable permanent changes in the mechanical properties of the hydrogel, as well as alterations in the micro- and macroporosity of its three-dimensional (3D) structure, with the associated potential to induce anisotropy. In this work, the behavior of biocompatible and biodegradable hydrogels made with Fmoc-diphenylalanine (Fmoc-FF) (Fmoc = fluorenylmethoxycarbonyl) and Fmoc-arginine-glycine-aspartic acid (Fmoc-RGD) short peptides to which MNP were incorporated was studied in detail with physicochemical, mechanical, and biological methods. The resulting hybrid hydrogels showed enhance mechanical properties and withstood injection without phase disruption. In mice, the hydrogels showed faster and improved self-healing properties compared to their nonmagnetic counterparts. Thanks to these superior physical properties and stability during culture, they can be used as 3D scaffolds for cell growth. Additionally, magnetic short-peptide hydrogels showed good biocompatibility and the absence of toxicity, which together with their enhanced mechanical stability and excellent injectability make them ideal biomaterials for in vivo biomedical applications with minimally invasive surgery. This study presents a new approach to improving the physical and mechanical properties of supramolecular hydrogels by incorporating MNP, which confer structural reinforcement and stability, remote actuation by magnetic fields, and better injectability. Our approach is a potential catalyst for expanding the biomedical applications of supramolecular short-peptide hydrogels.

The role of thermal diffusion, particle clusters, hydrodynamic and magnetic forces on the flow behaviour of magneto-polymer composites

In this paper, we study the shear-induced flow of magneto-polymer composites, consisting of dispersions of magnetic particles in solutions of polymers, as a competition between the colloidal forces amid particles and their bulk transport induced by the hydrodynamic forces. For this aim, we analyse the role of different experimental parameters. Firstly, by using only solutions of a well-known anionic polymer (sodium alginate), we provoke a moderate hindering of particle movement, but keeping the liquid-like state of the samples. On the contrary, a gel-like behaviour is conferred to the samples when a cationic polymer (chitosan) is additionally added, which further reduces the particle movement. We analyse the effect of an applied magnetic field, which is opposed to particle transport by hydrodynamic forces, by inducing magnetic attraction between the particles. We perform the analysis under both stationary and oscillatory shear. We show that by using dimensionless numbers the differences between samples and experimental conditions are emphasized. In all cases, as expected, the transport of particles driven by bulk hydrodynamic forces dominates at high values of the shear rate. This article is part of the theme issue 'Transport phenomena in complex systems (part 1)'.

Natural Polymeric Scaffolds for Tissue Engineering Applications

Natural polymeric scaffolds can be used for tissue engineering applications such as cell delivery and cell-free supporting of native tissues. This is because of their desirable properties such as; high biocompatibility, tunable mechanical strength and conductivity, large surface area, porous- and extracellular matrix (ECM)-mimicked structures. Specifically, their less toxicity and biocompatibility makes them suitable for several tissue engineering applications. For these reasons, several biopolymeric scaffolds are currently being explored for numerous tissue engineering applications. To date, research on the nature, chemistry, and properties of nanocomposite biopolymers are been reported, while the need for a comprehensive research note on more tissue engineering application of these biopolymers remains. As a result, this present study comprehensively reviews the development of common natural biopolymers as scaffolds for tissue engineering applications such as cartilage tissue engineering, cornea repairs, osteochondral defect repairs, and nerve regeneration. More so, the implications of research findings for further studies are presented, while the impact of research advances on future research and other specific recommendations are added as well.

Dynamic Tuning of Viscoelastic Hydrogels with Carbonyl Iron Microparticles Reveals the Rapid Response of Cells to Three-Dimensional Substrate Mechanics

Current methods to dynamically tune three-dimensional hydrogel mechanics require specific chemistries and substrates that make modest, slow, and often irreversible changes in their mechanical properties, exclude the use of protein-based scaffolds, or alter the hydrogel microstructure and pore size. Here, we rapidly and reversibly alter the mechanical properties of hydrogels consisting of extracellular matrix proteins and proteoglycans by adding carbonyl iron microparticles (MPs) and applying external magnetic fields. This approach drastically alters hydrogel mechanics: rheology reveals that application of a 4000 Oe magnetic field to a 5 mg/mL collagen hydrogel containing 10 wt % MPs increases the storage modulus from approximately 1.5 to 30 kPa. Cell morphology experiments show that cells embedded within these hydrogels rapidly sense the magnetically induced changes in ECM stiffness. Ca²⁺ transients are altered within seconds of stiffening or subsequent softening, and slower but still dynamic changes occur in YAP nuclear translocation in response to time-dependent application of a magnetic field. The near instantaneous change in hydrogel mechanics provides new insight into the effect of changing extracellular stiffness on both acute and chronic changes in diverse cell types embedded in protein-based scaffolds. Due to its flexibility, this method is broadly applicable to future studies interrogating cell mechanotransduction in three-dimensional substrates.

Review on the advancements of magnetic gels: towards multifunctional magnetic liposome-hydrogel composites for biomedical applications

Magnetic gels have been gaining great attention in nanomedicine, as they combine features of hydrogels and magnetic nanoparticles into a single system. The incorporation of liposomes in magnetic gels further leads to a more robust multifunctional system enabling more functions and spatiotemporal control required for biomedical applications, which includes on-demand drug release. In this review, magnetic gels components are initially introduced, as well as an overview of advancements on the development, tuneability, manipulation and application of these materials. After a discussion of the advantages of combining hydrogels with liposomes, the properties, fabrication strategies and applications of magnetic liposome-hydrogel composites (magnetic lipogels or magnetolipogels) are reviewed. Overall, the progress of magnetic gels towards smart multifunctional materials are emphasized, considering the contributions for future developments.

Supramolecular ultra-short carboxybenzyl-protected dehydropeptide-based hydrogels for drug delivery

Self-assembled peptide-based hydrogels are promising materials for biomedical research owing to biocompatibility and similarity to the extracellular matrix, amenable synthesis and functionalization and structural tailoring of the rheological properties. Wider developments of self-assembled peptide-based hydrogels in biomedical research and clinical translation are hampered by limited commercial availability allied to prohibitive costs. In this work a focused library of Cbz-protected dehydrodipeptides Cbz-L-Xaa-Z-ΔPhe-OH (Xaa= Met, Phe, Tyr, Ala, Gly) was synthesised and evaluated as minimalist hydrogels. The Cbz-L-Met-Z-ΔPhe-OH and Cbz-L-Phe-Z-ΔPhe-OH hydrogelators were comprehensively evaluated regarding molecular aggregation and self-assembly, gelation, biocompatibility and as drug carriers for delivery of the natural compound curcumin and the clinically important antitumor drug doxorubicin. Drug release profiles and FRET studies of drug transport into small unilamellar vesicles (as biomembrane models) demonstrated that the Cbz-protected dehydropeptide hydrogels are effective nanocarriers for drug delivery. The expedite and scalable synthesis (in 3 steps), using commercially available reagents and amenable reaction conditions, makes Cbz-protected dehydrodipeptide hydrogels, widely available at affordable cost to the research community.

Impact of Citrate and Lipid-Functionalized Magnetic Nanoparticles in Dehydropeptide Supramolecular Magnetogels: Properties, Design and Drug Release

Currently, the nanoparticle functionalization effect on supramolecular peptide-based hydrogels remains undescribed, but is expected to affect the hydrogels' self-assembly and final magnetic gel properties. Herein, two different functionalized nanoparticles: citrate-stabilized (14.4 ± 2.6 nm) and lipid-coated (8.9 ± 2.1 nm) magnetic nanoparticles, were used for the formation of dehydropeptide-based supramolecular magnetogels consisting of the ultra-short hydrogelator Cbz-L-Met-Z-ΔPhe-OH, with an assessment of their effect over gel properties. The lipid-coated nanoparticles were distributed along the hydrogel fibers, while citrate-stabilized nanoparticles were aggregated upon gelation, which resulted into a heating efficiency improvement and decrease, respectively. Further, the lipid-coated nanoparticles did not affect drug encapsulation and displayed improved drug release reproducibility compared to citrate-stabilized nanoparticles, despite the latter attaining a stronger AMF-trigger. This report points out that adsorption of nanoparticles to hydrogel fibers, which display domains that improve or do not affect drug encapsulation, can be explored as a means to optimize the development of supramolecular magnetogels to advance theranostic applications.

In vivo time-course biocompatibility assessment of biomagnetic nanoparticles-based biomaterials for tissue engineering applications

Novel artificial tissues with potential usefulness in local-based therapies have been generated by tissue engineering using magnetic-responsive nanoparticles (MNPs). In this study, we performed a comprehensive in vivo characterization of bioengineered magnetic fibrin-agarose tissue-like biomaterials. First, in vitro analyses were performed and the cytocompatibility of MNPs was demonstrated. Then, bioartificial tissues were generated and subcutaneously implanted in Wistar rats and their biodistribution, biocompatibility and functionality were analysed at the morphological, histological, haematological and biochemical levels as compared to injected MNPs. Magnetic Resonance Image (MRI), histology and magnetometry confirmed the presence of MNPs restricted to the grafting area after 12 weeks. Histologically, we found a local initial inflammatory response that decreased with time. Structural, ultrastructural, haematological and biochemical analyses of vital organs showed absence of damage or failure. This study demonstrated that the novel magnetic tissue-like biomaterials with improved biomechanical properties fulfil the biosafety and biocompatibility requirements for future clinical use and support the use of these biomaterials as an alternative delivery route for magnetic nanoparticles.

In situ magnetorheological SANS setup at Institut Laue-Langevin

A magnetorheological sample environment is presented that allows for in situ magnetic field and shear flow during small-angle neutron scattering (SANS) measurements and is now available at the Institut Laue-Langevin (ILL). The setup allows performing simultaneous magnetorheological measurements together with the investigation of structural and magnetic changes on the nanometer length scale underlying the rheological response of ferrofluids. We describe the setup consisting of a commercial rheometer and a custom-made set of Helmholtz coils and show exemplarily data on the field and shear flow alignment of a dispersion of hematite nanospindles in water.

Revealing importance of particles' surface functionalization on the properties of magnetic alginate hydrogels

Iron/silica core-shell microparticles (IMPs) were functionalized by different functional groups including amine, glycidoxy, phenyl, and thiocyanate. Many of the IMPs modifications are reported for the first time. The resulting surface chemistry turned out to affect the properties of magnetic alginate hydrogels fabricated from sodium alginate and dispersed IMPs. Differences in magnetorheological properties of the obtained magnetic hydrogels can be at least partially attributed to the interactions between alginate and surface functionalities of IMPs. Density Functional Theory (DFT) calculations were carried out to get detailed insight into those interactions in order to link them with the observed macroscopic properties of the obtained hydrogels. For example, amine groups on the IMPs surface resulted in well-formed hydrogels while the presence of thiocyanate or phenyl groups - in poorly formed ones. This observation can be used for tuning the properties of various carbohydrate-based hydrogels.

Evaluation of Fibrin-Agarose Tissue-Like Hydrogels Biocompatibility for Tissue Engineering Applications

Generation of biocompatible and biomimetic tissue-like biomaterials is crucial to ensure the success of engineered substitutes in tissue repair. Natural biomaterials able to mimic the structure and composition of native extracellular matrices typically show better results than synthetic biomaterials. The aim of this study was to perform an in vivo time-course biocompatibility analysis of fibrin-agarose tissue-like hydrogels at the histological, imagenological, hematological, and biochemical levels. Tissue-like hydrogels were produced by a controlled biofabrication process allowing the generation of biomechanically and structurally stable hydrogels. The hydrogels were implanted subcutaneously in 25 male Wistar rats and evaluated after 1, 5, 9, and 12 weeks of in vivo follow-up. At each period of time, animals were analyzed using magnetic resonance imaging (MRI), hematological analyses, and histology of the local area in which the biomaterials were implanted, along with major vital organs (liver, kidney, spleen, and regional lymph nodes). MRI results showed no local or distal alterations during the whole study period. Hematology and biochemistry showed some fluctuation in blood cells values and in some biochemical markers over the time. However, these parameters were progressively normalized in the framework of the homeostasis process. Histological, histochemical, and ultrastructural analyses showed that implantation of fibrin-agarose scaffolds was followed by a progressive process of cell invasion, synthesis of components of the extracellular matrix (mainly, collagen) and neovascularization. Implanted biomaterials were successfully biodegraded and biointegrated at 12 weeks without any associated histopathological alteration in the implanted zone or distal vital organs. In summary, our in vivo study suggests that fibrin-agarose tissue-like hydrogels could have potential clinical usefulness in engineering applications in terms of biosafety and biocompatibility.

Rheology of magnetic colloids containing clusters of particle platelets and polymer nanofibres

Magnetic hydrogels (ferrogels) are soft materials with a wide range of applications, especially in biomedicine because (i) they can be provided with the required biocompatibility; (ii) their heterogeneous structure allows their use as scaffolds for tissue engineering; (iii) their mechanical properties can be modified by changing different design parameters or by the action of magnetic fields. These characteristics confer them unique properties for acting as patterns that mimic the architecture of biological systems. In addition, and (iv) given their high porosity and aqueous content, ferrogels can be loaded with drugs and guided towards specific targets for local (non-systemic) pharmaceutical treatments. The ferrogels prepared in this work contain magnetic particles obtained by precipitation of magnetite nanoparticles onto the porous surface of bentonite platelets. Then, the particles were functionalized by adsorption of alginate molecules and dispersed in an aqueous solution of sodium alginate. Finally, the gelation was promoted by cross-linking the alginate molecules with Ca²⁺ ions. The viscoelastic properties of the ferrogels were measured in the absence/presence of external magnetic fields, showing that these ferrogels exhibited a strong enough magnetorheological effect. This behaviour is explained considering the field-induced strengthening of the heterogeneous (particle-polymer) network generated inside the ferrogel. This article is part of the theme issue 'Patterns in soft and biological matters'.

Poly(2-oxazoline)-based magnetic hydrogels: Synthesis, performance and cytotoxicity

Research on the subject of smart biomaterials has become a cornerstone of tissue engineering and regenerative medicine. Herein, the authors report on developing magnetic hydrogels that combine high biocompatibility and remarkable activity in magnetic fields. We fabricated magnetic hydrogels based on poly(2-ethyl-2-oxazoline) (POx) via living ring-opening cationic polymerization with in-situ embedding of the carbonyl iron (CI) particles. Investigation was made as to the effect exerted by the concentration of CI on magnetic, viscoelastic/magnetorheological properties, the degree of equilibrium swelling, and cytotoxicity. The hydrogels exhibited an open pore structure, as evidenced by computed tomography (CT) imaging. Susceptibility measurements revealed the concentration-dependent field-induced particle restructuration indicating elongation/contraction of the material, thereby determining the potential for magneto-mechanical stimulation of the cells. The POx-based magnetic hydrogels were amphiphilic in character, showing decrease in their capability to hold liquid alongside increase in CI concentration. Viscoelastic measurements suggested that interaction occurred between the particles and matrix based on inconsistency between the experimental storage modulus and the Krieger-Dougherty model. The synthesized materials exhibited excellent biocompatibility toward the 3T3 fibroblast cell line in tests of extract toxicity and direct contact cytotoxicity (ISO standards). The unique combination of properties exhibited by the material - magneto-mechanical activity and biocompatibility - could prove favorable in fields such as biomedicine and biomechanics.

Role of Nanoparticle-Polymer Interactions on the Development of Double-Network Hydrogel Nanocomposites with High Mechanical Strength

Extensive experimental and theoretical research over the past several decades has pursued strategies to develop hydrogels with high mechanical strength. Our study investigated the effect of combining two approaches, addition of nanoparticles and crosslinking two different polymers (to create double-network hydrogels), on the mechanical properties of hydrogels. Our studies revealed that these orthogonal approaches may be combined to synthesize hydrogel composites with enhanced mechanical properties. However, the enhancement in double network hydrogel elastic modulus due to incorporation of nanoparticles is limited by the ability of the nanoparticles to strongly interact with the polymers in the network. Moreover, double-network hydrogel nanocomposites prepared using lower monomer concentrations showed higher enhancements in elastic moduli compared to those prepared using high monomer concentrations, thus indicating that the concentration of hydrogel monomers used for the preparation of the nanocomposites had a significant effect on the extent of nanoparticle-mediated enhancements. Collectively, these results demonstrate that the hypotheses previously developed to understand the role of nanoparticles on the mechanical properties of hydrogel nanocomposites may be extended to double-network hydrogel systems and guide the development of next-generation hydrogels with extraordinary mechanical properties through a combination of different approaches.

In vitro characterization of a novel magnetic fibrin-agarose hydrogel for cartilage tissue engineering

The encapsulation of cells into biopolymer matrices enables the preparation of engineered substitute tissues. Here we report the generation of novel 3D magnetic biomaterials by encapsulation of magnetic nanoparticles and human hyaline chondrocytes within fibrin-agarose hydrogels, with potential use as articular hyaline cartilage-like tissues. By rheological measurements we observed that, (i) the incorporation of magnetic nanoparticles resulted in increased values of the storage and loss moduli for the different times of cell culture; and (ii) the incorporation of human hyaline chondrocytes into nonmagnetic and magnetic fibrin-agarose biomaterials produced a control of their swelling capacity in comparison with acellular nonmagnetic and magnetic fibrin-agarose biomaterials. Interestingly, the in vitro viability and proliferation results showed that the inclusion of magnetic nanoparticles did not affect the cytocompatibility of the biomaterials. What is more, immunohistochemistry showed that the inclusion of magnetic nanoparticles did not negatively affect the expression of type II collagen of the human hyaline chondrocytes. Summarizing, our results suggest that the generation of engineered hyaline cartilage-like tissues by using magnetic fibrin-agarose hydrogels is feasible. The resulting artificial tissues combine a stronger and stable mechanical response, with promising in vitro cytocompatibility. Further research would be required to elucidate if for longer culture times additional features typical of the extracellular matrix of cartilage could be expressed by human hyaline chondrocytes within magnetic fibrin-agarose hydrogels.

Field-induced surface deformation of magnetoactive elastomers with anisometric fillers: a single-particle model

Surface relief of magnetoactive elastomers (MAEs) based on soft polymer matrices filled with anisometric magnetically hard fillers is studied theoretically in magnetic fields applied perpendicular to the MAE surface. A single-particle 2D cell model describing the rotation of one individual elliptical particle in a near-surface MAE layer is developed. The equilibrium rotation angle of particles is defined by a balance between Zeeman, magnetic anisotropy and elastic (generated in the polymer matrix) energy increments. The Stoner-Wohlfarth model is used to describe magnetic properties of the filler particles while the elastic energy as a function of the particle rotation angle is evaluated numerically using FEM simulations. A representative surface MAE system is constructed via superposition of single-particle cells with field-driven magnetic particles, and surface relief characteristics are derived for various sets of geometric and statistical parameters. Limitations of the proposed approach have been discussed.

Dehydropeptide-based plasmonic magnetogels: a supramolecular composite nanosystem for multimodal cancer therapy

Supramolecular hydrogels are highly promising candidates as biomedical materials owing to their wide array of properties, which can be tailored and modulated. Additionally, their combination with plasmonic/magnetic nanoparticles to form plasmonic magnetogels further improves their potential in biomedical applications through the combination of complementary strategies, such as photothermia, magnetic hyperthermia, photodynamic therapy and magnetic-guided drug delivery. Here, a new dehydropeptide hydrogelator, Npx-l-Met-Z-ΔPhe-OH, was developed and combined with two different plasmonic/magnetic nanoparticle architectures, i.e., core/shell manganese ferrite/gold nanoparticles and gold-decorated manganese ferrite nanoparticles with ca. 55 nm and 45 nm sizes, respectively. The magnetogels were characterized via HR-TEM, FTIR spectroscopy, circular dichroism and rheological assays. The gels were tested as nanocarriers for a model antitumor drug, the natural compound curcumin. The incorporation of the drug in the magnetogel matrices was confirmed through fluorescence-based techniques (FRET, fluorescence anisotropy and quenching). The curcumin release profiles were studied with and without the excitation of the gold plasmon band. The transport of curcumin from the magnetogels towards biomembrane models (small unilamellar vesicles) was assessed via FRET between the fluorescent drug and the lipid probe Nile Red. The developed magnetogels showed promising results for photothermia and photo-triggered drug release. The magnetogels bearing gold-decorated nanoparticles showed the best photothermia properties, while the ones containing core/shell nanoparticles had the best photoinduced curcumin release.

Evaluation methods as quality control in the generation of decellularized peripheral nerve allografts

Nowadays, the high incidence of peripheral nerve injuries and the low success ratio of surgical treatments are driving research to the generation of novel alternatives to repair critical nerve defects. In this sense, tissue engineering has emerged as a possible alternative with special attention to decellularization techniques. Tissue decellularization offers the possibility to obtain a cell-free, natural extracellular matrix (ECM), characterized by an adequate 3D organization and proper molecular composition to repair different tissues or organs, including peripheral nerves. One major problem, however, is that there are no standard quality control methods to evaluate decellularized tissues. Therefore, in this review, a brief description of current strategies for peripheral nerve repair is given, followed by an overview of different decellularization methods used for peripheral nerves. Furthermore, we extensively discuss the available and currently used methods to demonstrate the success of tissue decellularization in terms of the cell removal, preservation of essential ECM molecules and maintenance or modification of biomechanical properties. Finally, orientative guidelines for the evaluation of decellularized peripheral nerve allografts are proposed.

Ferrogels Ultrasonography for Biomedical Applications

Ferrogels (FG) are magnetic composites that are widely used in the area of biomedical engineering and biosensing. In this work, ferrogels with different concentrations of magnetic nanoparticles (MNPs) were synthesized by the radical polymerization of acrylamide in stabilized aqueous ferrofluid. FG samples were prepared in various shapes that are suitable for different characterization techniques. Thin cylindrical samples were used to simulate the case of targeted drug delivery test through blood vessels. Samples of larger size that were in the shape of cylindrical plates were used for the evaluation of the FG applicability as substitutes for damaged structures, such as bone or cartilage tissues. Regardless of the shape of the samples and the conditions of their location, the boundaries of FG were confidently visualized over the entire range of concentrations of MNPs while using medical ultrasound. The amplitude of the reflected echo signal was higher for the higher concentration of MNPs in the gel. This result was not related to the influence of the MNPs on the intensity of the reflected echo signal directly, since the wavelength of the ultrasonic effect used is much larger than the particle size. Qualitative theoretical model for the understanding of the experimental results was proposed while taking into account the concept that at the acoustic oscillations of the hydrogel, the macromolecular net, and water in the gel porous structure experience the viscous Stocks-like interaction.

Mechanical properties of magnetic gels containing rod-like composite particles

Magnetic gels (ferrogels) are heterogeneous systems structured at the nanoscale that contains magnetic particles dispersed in three-dimensional networks of polymer chains. In the present work, the magnetic particles were synthesized with a core-shell structure, consisting of sepiolite particles covered by magnetite nanoparticles. These composite particles had a rod-like shape with a high aspect ratio. The obtained sepiolite-magnetite particles showed a high enough susceptibility and saturation magnetization. The magneto-rheological (MR) properties, and the intensity of the MR effect, of aqueous suspensions of the synthesized particles were studied. The particles, functionalized by adsorption of alginate molecules, were imbedded in alginate hydrogels to get homogeneous soft materials. The particles were linked to the polymer chains as the knots in a network and dominated in a great extent the mechanical properties of the materials. After determining the optimal compositions of the ferrogels, their viscoelastic properties were measured in the absence/presence of magnetic fields. The results pointed out that the MR effect provided by the clay-magnetite particles was considerably more intense than those achieved in ferrogels that contain spherical magnetic microparticles. Therefore, the imbedding of rod-shaped magnetic particles in hydrogels allows controlling the mechanical properties in a wider range than in conventional ferrogels. This article is part of the theme issue 'Heterogeneous materials: metastable and non-ergodic internal structures'.

Anisotropic magnetic hydrogels: design, structure and mechanical properties

Anisotropy is an intrinsic feature of most of the human tissues (e.g. muscle, skin or cartilage). Because of this, there has been an intense effort in the search of methods for the induction of permanent anisotropy in hydrogels intended for biomedical applications. The dispersion of magnetic particles or beads in the hydrogel precursor solution prior to cross-linking, in combination with applied magnetic fields, which gives rise to columnar structures, is one of the most recently proposed approaches for this goal. We have gone even further and, in this paper, we show that it is possible to use magnetic particles as actuators for the alignment of the polymer chains in order to obtain anisotropic hydrogels. Furthermore, we characterize the microstructural arrangement and mechanical properties of the resulting hydrogels. This article is part of a theme issue 'Heterogeneous materials: metastable and non-ergodic internal structures'.

Inherently magnetic hydrogel for data storage based on the magneto-optical Kerr effect

In this data explosion age, a large amount of data is generated every day. Such a fast data growth has aroused great interest in the field of data storage. Conventional data storage materials are mainly composed of hard and brittle materials but they may break in the case of mechanical operations, causing irreversible data loss. In this work, efforts have been devoted to fabricating a flexible and stretchable double network hydrogel for data storage based on the magneto-optical Kerr effect. The hydrogel possesses a storage modulus of over 10⁴ Pa and remains unbroken under a strain of 3000%. The surface of the hydrogel is patterned with diamagnetic parts and paramagnetic parts alternately. When placed under a magnetic field, the surface of the hydrogel reflects the incident laser beam and changes the polarization plane of the reflected light. The outstanding flexibility and inherent magnetic properties of this hydrogel lay the groundwork for data storage and guarantee data safety.

Magnetogels: Prospects and Main Challenges in Biomedical Applications

Drug delivery nanosystems have been thriving in recent years as a promising application in therapeutics, seeking to solve the lack of specificity of conventional chemotherapy targeting and add further features such as enhanced magnetic resonance imaging, biosensing and hyperthermia. The combination of magnetic nanoparticles and hydrogels introduces a new generation of nanosystems, the magnetogels, which combine the advantages of both nanomaterials, apart from showing interesting properties unobtainable when both systems are separated. The presence of magnetic nanoparticles allows the control and targeting of the nanosystem to a specific location by an externally applied magnetic field gradient. Moreover, the application of an alternating magnetic field (AMF) not only allows therapy through hyperthermia, but also enhances drug delivery and chemotherapeutic desired effects, which combined with the hydrogel specificity, confer a high therapeutic efficiency. Therefore, the present review summarizes the magnetogels properties and critically discusses their current and recent biomedical applications, apart from an outlook on future goals and perspectives.

Shear Elasticity of Magnetic Gels with Internal Structures

We present the results of the theoretical modeling of the elastic shear properties of a magnetic gel, consisting of soft matrix and embedded, fine magnetizable particles, which are united in linear chain-like structures. We suppose that the composite is placed in a magnetic field, perpendicular to the direction of the sample shear. Our results show that the field can significantly enhance the mechanical rigidity of the soft composite. Theoretical results are in quantitative agreement with the experiments.

Generation of genipin cross-linked fibrin-agarose hydrogel tissue-like models for tissue engineering applications

The generation of biomimetic and biocompatible artificial tissues is the basic research objective for tissue engineering (TE). In this sense, the biofabrication of scaffolds that resemble the tissues' extracellular matrix is an essential aim in this field. Uncompressed and nanostructured fibrin-agarose hydrogels (FAH and NFAH, respectively) have emerged as promising scaffolds in TE, but their structure and biomechanical properties must be improved in order to broaden their TE applications. Here, we generated and characterized novel membrane-like models with increased structural and biomechanical properties based on the chemical cross-linking of FAH and NFAH with genipin (GP at 0.1%, 0.25%, 0.5% and 0.75%). Furthermore, the scaffolds were subjected to rheological (G, G', G″ modulus), ultrastructural and ex vivo biocompatibility analyses. Results showed that all GP concentrations increased the stiffness (G) and especially the elasticity (G') of FAH and NFAH. Ultrastructural analyses demonstrated that GP and nanostructuration of FAH allowed us to control the porosity of FAH. In addition, biological studies revealed that higher concentration of GP (0.75%) started to compromise the cell function and viability. Finally, this study demonstrated the possibility to generate natural and biocompatible FAH and NFAH with improved structural and biomechanical properties by using 0.1%-0.5% of GP. However, further in vivo studies are needed in order to demonstrate the biocompatibility, biodegradability and regeneration capability of these cross-linked scaffolds.