Development of Thermosensitive Hydrogels with Tailor-Made Geometries to Modulate Cell Harvesting of Non-Flat Cell Cultures

Considering the complexity in terms of design that characterizes the different tissues of the human body, it is necessary to study and develop more precise therapies. In this sense, this article presents the possibility of fabricating photocurable thermosensitive hydrogels with free geometry and based on N-Vinyl Caprolactam (VCL) with the aim of modulating the adhesion of non-planar cell cultures. The fabrication process is based on the use as a mold of two-layer thick water-soluble polyvinyl alcohol (PVA) previously printed by Extrusion Material (MatEx). From this technology it has been possible to obtain hydrogels with different 3D geometries and different crosslinking percentages (2, 4 and 6 mol%). Studies have shown that networks reduce their thermosensitivity not only when the percentage of crosslinking in the formulation increases, but also when the thickness of the hydrogel obtained increases. Based on this reduction in thermosensitivity, the less crosslinked (2 mol%) hydrogels have been evaluated to carry out a novel direct application in which hydrogels with curved geometry have allowed cell adhesion and proliferation at 37 °C with the endothelial cell line C166-GFP; likewise, non-aggressive cell detachment was observed when the hydrogel temperature was reduced to values of 20 °C. Therefore, the present manuscript shows a novel application for the synthesis of free-form thermosensitive hydrogels that allows modulation of non-planar cell cultures.

Hydrogels with dual sensitivity to temperature and pH in physiologically relevant ranges as supports for versatile controlled cell detachment

Thermosensitive hydrogels based on the N-vinyl caprolactam (VCL), capable of allowing for cell adhesion and proliferation, as well as non-aggressive detachment by controlled temperature drop, were functionalized with 23 % or lower molar percentages of the cationizable hydrophobic unit 2-(diisopropylamino) ethyl methacrylate (DPAEMA), to obtain networks with dual sensitivity to temperature and pH. The swelling analysis of the systems has shown a transition pK (pKb) close to physiological values, dependent on the temperature of the medium (pKb of 6.6 and 6.9 when the temperature of the medium is above and below the transition temperature VPTT, respectively) and little dependence on the degree of functionalization of DPAEMA. In addition, at temperatures below the transition temperature (VPTT), the systems have shown large swelling variations as a function of the pH (i.e. below and above the pKb), exhibiting greater absorption capacity at pHs below pKb, where the DPAEMA units are cationized. Cytocompatibility and transplant capacity have been evaluated using the C166-GFP endothelial cell line. None of the thermosensitive hydrogels with variable DPAEMA content showed a delay with respect to the control without DPAEMA neither in terms of adhesion nor in proliferation. However, by increasing the percentage of DPAEMA functionalization -and decreasing thermosensitivity-, a correlative decrease in mitochondrial activity was obtained in the transplant, with significant differences for the hydrogels with DPAEMA molar percentage of 3 % or higher. Taking advantage of the proximity of the pKb to the physiological value, we have evaluated the cellular response and the capacity for transplantation after lowering the pH to 6.5, below pKb. A direct relationship of the DPAEMA functionalization degree on the detachment efficiency was observed, since the hydrogels with the highest molar load of DPAEMA showed higher mitochondrial metabolic activity after cell detachment.

Fabrication of 3D cylindrical thermosensitive hydrogels as supports for cell culture and detachment of tubular cell sheets

Pseudo interpenetrating vinyl-caprolactam (VCL) based thermosensitive tubular hydrogels with a volume phase transition temperature, VPTT, around 35 °C, have been prepared by combining two different crosslinkers, a di-methacrylate (C1) and a di-vinyl urea (C2). The molar ratio between the two crosslinkers (for a global crosslinker molar percentage of 1.9) has shown to play a key role on the properties of the hydrogel. Increasing the amount of di-vinyl urea, leads to transparent but rather fragile materials and to a lower extent of thermosensitivity, that is, to a lower variation in the hydrogel swelling upon temperature change. However, tubes prepared with a selected crosslinker molar ratio C1/C2 of 65/35 provided a compromise between transparency, thermosensitivity and maneuverability and were, thus, evaluated as supports for cell culture using premyoblastic cells. These hydrogels, used as supports, allow for surface adhesion and cell proliferation until confluence, and eventually an efficient monolayer detachment (and transplant to a 3D-printed polylactic acid (PLA) support) through a controlled drop in temperature. As a result, this method permits to obtain tubular tissue constructs with potential applications in tissue engineering such as in the elaboration of vascular grafts.

One-Shot Preparation of Polyacrylamide/Poly(sodium styrenesulfonate) Double-Network Hydrogels for Rapid Optical Tissue Clearing

In this study, we propose a convenient method for the synthesis of double-network gels by the one-shot radical polymerization for their application to rapid optical tissue clearing. Double-network gels were produced during the radical polymerization of acrylamide (AAm) and sodium styrenesulfonate (SS) in the presence of N,N'-methylenebisacrylamide and sodium divinylbenzenesulfonate as the cross-linkers by simultaneous addition, that is, one-shot polymerization accompanying the delay of polymerization for a second network monomer. We analyzed the polymerization process based on the consumption rates of each monomer during the reactions in the absence of the cross-linkers in order to estimate the repeating unit structure of the resulting polymers. We then fabricated the AAm/SS gels by the polymerization of AAm and SS in the presence of the cross-linkers. We analyzed the swelling, viscoelastic, and mechanical properties of the produced gels to investigate their network structure. Finally, we demonstrated the validity of the double-network gels for the application to rapid optical tissue clearing.

Ionic Conductivity, Diffusion Coefficients, and Degree of Dissociation in Lithium Electrolytes, Ionic Liquids, and Hydrogel Polyelectrolytes

The conductive and diffusional behavior of electrolytes in media with different dielectric and viscoelastic properties is investigated. A revised model to separate the contribution of dissociated and nondissociated species to the diffusion coefficients determined with NMR is proposed. Impedance spectroscopy is used to measure the ionic conductivity of lithium salts in aqueous medium, ionic liquids in aprotic solvents, and hydrogel polyelectrolytes. The diffusion coefficients of the species of interest in those systems are determined with multinuclear pulsed-gradient spin-echo (PGSE) NMR. The results are analyzed using the revised model. It is shown that the degree of ionization could be determined directly from measurements of ionic conductivity and diffusion coefficients in very different types of electrolytes and in a wide range of concentrations. Furthermore, these findings support the original Arrhenius hypothesis about electrolytes and show that the assumption of a complete dissociation is not required to describe their conductive behavior. The reduced conductivity observed in hydrogels, at or near swelling equilibrium, compared to that in solutions could be attributed mainly to the hindered ionic mobility caused by the network structure.

Facile one-pot exfoliation and integration of 2D layered materials by dispersion in a photocurable polymer precursor

Efficient exfoliation of graphene and related materials (GRM) and fast and inexpensive integration/assembly are crucial to fulfil their full potential. A high degree of exfoliation in organic media can be achieved with high boiling point liquids that usually leave residues after drying, which is a handicap for many applications. Here, the effective exfoliation and dispersion of GRM in a vinyl monomer, which is subsequently converted to a functional polymer by photopolymerization, is reported. Nanocomposite membranes and three-dimensional objects are produced by the photo-curing process and stereolithography 3D printing, respectively.

Cell Adhesion and Proliferation on Sulfonated and Non-Modified Chitosan Films

Three types of chitosan-based films have been prepared and evaluated: a non-modified chitosan film bearing cationizable aliphatic amines and two films made of N-sulfopropyl chitosan derivatives bearing both aliphatic amines and negative sulfonate groups at different ratios. Cell adhesion and proliferation on chitosan films of C2C12 pre-myoblastic cells and B16 cells as tumoral model have been tested. A differential cell behavior has been observed on chitosan films due to their different surface modification. B16 cells have shown lower vinculin expression when cultured on sulfonated chitosan films. This study shows how the interaction among cells and material surface can be modulated by physicochemical characteristics of the biomaterial surface, altering tumoral cell adhesion and proliferation processes.

Hydrogels with Modulated Ionic Load for Mammalian Cell Harvesting with Reduced Bacterial Adhesion

In this manuscript, we describe the fabrication of hydrogel supports for mammalian cell handling that can simultaneously prevent materials from microbial contamination and therefore allow storage in aqueous media. For that purpose, hydrogels based on the antifouling polymer polyvinylpyrrolidone (PVP) were functionalized with different ionic groups (anionic, cationic, or two types of zwitterions). In order to prevent bacterial adhesion in the long-term, we took advantage of the synergistic effect of inherently antifouling PVP and additional antifouling moieties incorporated within the hydrogel structure. We evaluated, in a separated series of experiments, both the capability of the materials to act as supports for the growth of mammalian cell monolayers for transplantation (using C-166-GFP endothelial cell line), as well their antifouling properties against Staphylococcus aureus, were studied. All of the hydrogels are structurally pseudodouble networks with high swelling (around 90%) and similar mechanical properties (in the low range for hydrogel materials with Young modulus below 1250 kPa). With some differences, all the charged hydrogels were capable of hosting mouse endothelial cell line C166-GFP to confluence, as well as a monolayer detachment and transplantation through simple mechanical agitation. On the contrary, the uncharged hydrogel was not capable to detach a full monolayer for transplantation. Bacterial adhesion and proliferation was highly sensitive to the functionality (type of charge and density). In particular, we evidenced that monomers bearing zwitterionic sulfobetaine groups, those negatively charged as well as "electro neutral" hydrogels fabricated from stoichiometric amounts of positive and negative units, exhibit excellent antifouling properties both at initial adhesion times and during longer periods up to 72 h.

Chemical and Topographical Modification of Polycarbonate Surfaces through Diffusion/Photocuring Processes of Hydrogel Precursors Based on Vinylpyrrolidone

Facile procedures capable of simultaneously conferring hydrophilicity and tailored topography to surfaces of hydrophobic supports, such as polycarbonate (PC), are very attractive but rare. In this work, we describe a simple methodology to wrinkle PC surfaces after a process of (a) contacting with a photopolymerizable vinylic solution, (b) UV curing of such solutions, and (c) detachment of the formed polymer network, upon swelling in ethanol. The influence of different parameters such as contact lag time between the PC surface and the polymerizable solution, the monomer concentration and type of solvents, as well as the cross-linking degree on the formation of wrinkles, has been studied. The dimensions of the wrinkles can be tailored to some extent by altering the different parameters. Surface chemistry has been analyzed by contact angle measurements and by confocal Raman microscopy. The results are consistent with a chemical alteration of the surface and the formation of an outer hydrogel layer, which is interpenetrated into the PC structure. A mechanism of monomer diffusion and PC swelling that produces surface instabilities and wrinkling is proposed.

Dual physically crosslinked double network hydrogels with high toughness and self-healing properties

Toughness and self-healing properties are desirable characteristics in engineered hydrogels used for many practical applications. However, it is still challenging to develop hydrogels exhibiting both of these attractive properties in a single material. In this work, we present the fabrication of fully physically-linked Agar/PAAc-Fe³⁺ DN gels. These hydrogels exhibited dual physical crosslinking through a hydrogen bonded crosslinked agar network firstly, and a physically linked PAAc-Fe³⁺ network via Fe³⁺ coordination interactions secondly. Due to this dual physical crosslinking, the fabricated Agar/PAAc-Fe³⁺ DN gels exhibited very favorable mechanical properties (tensile strength 320.7 kPa, work of extension 1520.2 kJ m^-3, elongation at break 1130%), fast self-recovery properties in Fe³⁺ solution (100% recovery within 30 min), in 50 °C conditions (100% recovery within 15 min), and under ambient conditions (100% recovery of the initial properties within 60 min), as well as impressive self-healing properties under ambient conditions. All of the data indicate that both the hydrogen bonds in the first network and the ionic coordination interactions in the second network act as reversible sacrificial bonds to dissipate energy, thus conferring high mechanical and recovery properties to the prepared Agar/PAAc-Fe³⁺ DN gels.

Fabrication and Application of Photocrosslinked, Nanometer-Scale, Physically Adsorbed Films for Tissue Culture Regeneration

This study describes the development and cell culture application of nanometer thick photocrosslinkable thermoresponsive polymer films prepared by physical adsorption. Two thermoresponsive polymers, poly(N-isopropylacrylamide (NIPAm)-co-acrylamidebenzophenone (AcBzPh)) and poly(NIPAm-co-AcBzPh-co-N-tertbutylacrylamide) are investigated. Films are prepared both above and below the polymers' lower critical solution temperatures (LCSTs) and cross-linked, to determine the effect, adsorption preparation temperature has on the resultant film. The films prepared at temperatures below the LCST are smoother, thinner, and more hydrophilic than those prepared above. Human pulmonary microvascular endothelial cell (HPMEC) adhesion and proliferation are superior on the films produced below the polymers LCST compared to those produced above. Cells sheets are detached by simply lowering the ambient temperature to below the LCST. Transmission electron, scanning electron, and light microscopies indicate that the detached HPMEC sheets maintain their integrity.

Bio-fabrication and physiological self-release of tissue equivalents using smart peptide amphiphile templates

In this study we applied a smart biomaterial formed from a self-assembling, multi-functional synthetic peptide amphiphile (PA) to coat substrates with various surface chemistries. The combination of PA coating and alignment-inducing functionalised substrates provided a template to instruct human corneal stromal fibroblasts to adhere, become aligned and then bio-fabricate a highly-ordered, multi-layered, three-dimensional tissue by depositing an aligned, native-like extracellular matrix. The newly-formed corneal tissue equivalent was subsequently able to eliminate the adhesive properties of the template and govern its own complete release via the action of endogenous proteases. Tissues recovered through this method were structurally stable, easily handled, and carrier-free. Furthermore, topographical and mechanical analysis by atomic force microscopy showed that tissue equivalents formed on the alignment-inducing PA template had highly-ordered, compact collagen deposition, with a two-fold higher elastic modulus compared to the less compact tissues produced on the non-alignment template, the PA-coated glass. We suggest that this technology represents a new paradigm in tissue engineering and regenerative medicine, whereby all processes for the bio-fabrication and subsequent self-release of natural, bio-prosthetic human tissues depend solely on simple template-tissue feedback interactions.

Self-healable, tough and highly stretchable ionic nanocomposite physical hydrogels

We present a facile strategy to synthesize self-healable tough and highly stretchable hydrogels. Our design rationale for the creation of ionic cross-linked hydrogels is to graft an acrylic acid monomer on the surface of vinyl hybrid silica nanoparticles (VSNPs) for the growth of poly(acrylic) acid (PAA), and the obtained VSNP-PAA nanobrush can be used as a gelator. Physical cross-linking through hydrogen bonding and ferric ion-mediated ionic interactions between PAA polymer chains of the gelators yielded ionic nanocomposite physical hydrogels with excellent and balanced mechanical properties (tensile strength 860 kPa, elongation at break ∼2300%), and the ability to self-repair (tensile strength ∼560 kPa, elongation at break ∼1800%). The toughness and stretchability arise from the reversible cross-linking interactions between the polymer chains that help dissipate energy through stress (deformation) triggered dynamic processes. These unique properties will enable greater application of these hydrogel materials, especially in tissue engineering.

Fundamentals of double network hydrogels

Double network (DN) hydrogels as promising soft-and-tough materials intrinsically possess extraordinary mechanical strength and toughness due to their unique contrasting network structures, strong interpenetrating network entanglement, and efficient energy dissipation.

Highly stretchable and super tough nanocomposite physical hydrogels facilitated by the coupling of intermolecular hydrogen bonds and analogous chemical crosslinking of nanoparticles

Nanocomposite physical hydrogels fabricated by a one-step polymerization show ultra-extensibility and toughness due to an effective strengthening mechanism.