Imaging the Structure of Macroporous Hydrogels by Two-Photon Fluorescence Microscopy

Cryostructuring of Polymeric Systems. 50.† Cryogels and Cryotropic Gel-Formation: Terms and Definitions

A variety of cryogenically-structured polymeric materials are of significant scientific and applied interest in various areas. However, in spite of considerable attention to these materials and intensive elaboration of their new examples, as well as the impressive growth in the number of the publications and patents on this topic over the past two decades, a marked variability of the used terminology and definitions is frequently met with in the papers, reviews, theses, patents, conference presentations, advertising materials and so forth. Therefore, the aim of this brief communication is to specify the basic terms and definitions in the particular field of macromolecular science.

Optical Projection Tomography Technique for Image Texture and Mass Transport Studies in Hydrogels Based on Gellan Gum

The microstructure and permeability are crucial factors for the development of hydrogels for tissue engineering, since they influence cell nutrition, penetration, and proliferation. The currently available imaging methods able to characterize hydrogels have many limitations. They often require sample drying and other destructive processing, which can change hydrogel structure, or they have limited imaging penetration depth. In this work, we show for the first time an alternative nondestructive method, based on optical projection tomography (OPT) imaging, to characterize hydrated hydrogels without the need of sample processing. As proof of concept, we used gellan gum (GG) hydrogels obtained by several cross-linking methods. Transmission mode OPT was used to analyze image microtextures, and emission mode OPT to study mass transport. Differences in hydrogel structure related to different types of cross-linking and between modified and native GG were found through the acquired Haralick's image texture features followed by multiple discriminant analysis (MDA). In mass transport studies, the mobility of FITC-dextran (MW 20, 150, 2000 kDa) was analyzed through the macroscopic hydrogel. The FITC-dextran velocities were found to be inversely proportional to the size of the dextran as expected. Furthermore, the threshold size in which the transport is affected by the hydrogel mesh was found to be 150 kDa (Stokes' radii between 69 and 95 Å). On the other hand, the mass transport study allowed us to define an index of homogeneity to assess the cross-linking distribution, structure inside the hydrogel, and repeatability of hydrogel production. As a conclusion, we showed that the set of OPT imaging based material characterization methods presented here are useful for screening many characteristics of hydrogel compositions in relatively short time in an inexpensive manner, providing tools for improving the process of designing hydrogels for tissue engineering and drugs/cells delivery applications.

Imaging strategies for tissue engineering applications

Tissue engineering has evolved with multifaceted research being conducted using advanced technologies, and it is progressing toward clinical applications. As tissue engineering technology significantly advances, it proceeds toward increasing sophistication, including nanoscale strategies for material construction and synergetic methods for combining with cells, growth factors, or other macromolecules. Therefore, to assess advanced tissue-engineered constructs, tissue engineers need versatile imaging methods capable of monitoring not only morphological but also functional and molecular information. However, there is no single imaging modality that is suitable for all tissue-engineered constructs. Each imaging method has its own range of applications and provides information based on the specific properties of the imaging technique. Therefore, according to the requirements of the tissue engineering studies, the most appropriate tool should be selected among a variety of imaging modalities. The goal of this review article is to describe available biomedical imaging methods to assess tissue engineering applications and to provide tissue engineers with criteria and insights for determining the best imaging strategies. Commonly used biomedical imaging modalities, including X-ray and computed tomography, positron emission tomography and single photon emission computed tomography, magnetic resonance imaging, ultrasound imaging, optical imaging, and emerging techniques and multimodal imaging, will be discussed, focusing on the latest trends of their applications in recent tissue engineering studies.

Thermoresponsive biodegradable HEMA-lactate-Dextran-co-NIPA cryogels for controlled release of simvastatin

Abstract NIPA and HEMA-lactate-Dextran-based biodegradable and thermoresponsive cryogels were synthesized at different compositions by cryogelation. Chemical and morphological properties of the HEMA-lactate-Dextran-co-NIPA cryogel matrices were demonstrated by FTIR, SEM, and ESEM. Thermoresponsivity of the prepared cryogels was investigated by DSC, imaging NMR, and swelling studies. For possible use of the cryogels in potential bone tissue engineering applications, either hydrophobic simvastatin was embedded, or hydrophilic simvastatin was incorporated in the cryogels. Release profiles of simvastatin delivering cryogel scaffolds depending on their composition, hydrophobicity or hydrophilicity of loaded simvastatin and the medium temperature were demonstrated.

Thermosensitive macroporous cryogels functionalized with bioactive chitosan/bemiparin nanoparticles

Thermosensitive macroporous scaffolds of poly(N-isopropylacrylamide) (polyNIPA) loaded with chitosan/bemiparin nanoparticles are prepared by the free radical polymerization in cryogenic conditions. Chitosan/bemiparin nanoparticles of 102 ± 6.5 nm diameter are prepared by complex coacervation and loaded into polyNIPA cryogels. SEM image reveal the highly porous structure of cryogels and the integration of nanoparticles into the macroporous system. Volume phase transition temperature (VPT) and total freezing water content of cryogels are established by differential scanning calorimetry, and their porosity is determined by image-NMR. Swelling of cryogels (above and below the VPT) is highly dependent on nanoparticles concentration. In vitro release profile of bemiparin from cryogel is highly modulated by the presence of chitosan. Bemiparin released from nanoparticles preserves its biological activity, as shown by the BaF32 cell proliferation assay. Cryogels are not cytotoxic for the human fibroblast cells and present excellent properties for application on tissue engineering and controlled release of heparin.

Enzyme-catalyzed crosslinking in a partly frozen state: a new way to produce supermacroporous protein structures

In this study a new way to produce supermacroporous protein structures was investigated. Enzyme-mediated crosslinking of gelatin or casein was performed in a partly frozen state, which yielded stable, protein-based cryogels. The reaction kinetics for the formation of cryogels were found to be fairly slow, most likely due to the low temperature (-12 °C) used or due to an increased viscosity owing to the cryo-concentration taking place. The produced cryogels were characterized with regards to their physical properties and in vitro degradation. Furthermore, cryogels produced from gelatin and casein were evaluated as potential scaffolds by fibroblast cultivation to confirm their in vitro biocompatibility. Gelatin- and casein-based scaffolds both supported cell proliferation and migration through the scaffold.

3D ingrowth of bovine articular chondrocytes in biodegradable cryogel scaffolds for cartilage tissue engineering

A feasibility study was undertaken to examine the potential of biodegradable HEMA-lactate-dextran (HEMA-LLA-D)-based cryogels as scaffolds for cartilage tissue engineering. This was a preliminary in vitro study giving essential information on the biocompatibility of cryogels with cartilage cells. HEMA-lactate (HEMA-LLA) and HEMA-LLA-D were synthesized and characterized by different techniques. Cryogel scaffolds with supermacroporous structures were produced by cryogenic treatment of these macromers. Chondrocytes obtained from bovine articular cartilage were seeded onto cylindrical cryogels and cultured. The samples were examined by several microcopical techniques for cell viability and morphological analyses were performed at two culture points. Histological study of the constructs revealed the cells' growth on the surface and within the scaffolds. Confocal microscopical images demonstrated that the majority of live vs. dead cells had been attached to and integrated with the pores of the scaffold. SEM analysis showed round to oval-shaped chondrocytic cells interconnected with each other by communicating junctions. The chondrocytes rapidly proliferated in the cryogels, manifesting that they fully covered the scaffold surface after 9 days and almost filled the spaces in the pores of the scaffold after 15 days of culture. Chondrocytes secreted significant amount of extracellular matrix in the scaffolds and exhibited highly interconnective morphology. Light and transmission electron microscopy revealed groups of active cartilage cells closely apposed to the cryogel. We concluded that cryogel scaffolds could be excellent candidates for cartilage tissue regeneration with their extraordinary properties, including soft, elastic nature, highly open interconnected pore structure and very rapid, controllable swellability.

Modulating the porosity of cryogels by influencing the nonfrozen liquid phase through the addition of inert solutes

The freezing of monomeric mixtures is known to concentrate solutes in a nonfrozen phase in the area surrounding the ice crystals. The concentration of such solutes is determined by the freezing temperature. Although salts or solvents do not directly react in the polymerization reaction, they do change the composition and properties of the nonfrozen phase. In this study, we investigated the influence of the addition of various salts and solvents on the structure of macroporous hydrogels formed in a semifrozen state through aqueous free-radical polymerization. The change in composition of the nonfrozen phase was studied using NMR to monitor the freezing of water, and the structural changes of the gels were observed using scanning electron microscopy. It was found that the addition of methanol or acetone caused the formation of reaction-induced phase separation polymerization due to cryoconcentration, which caused a significant increase of methanol or acetone in the nonfrozen phase. This resulted in a material with bimodal pore size distribution with pores of 10-80 μm in diameter caused by cryogelation, and with pores in the polymeric matrix with a diameter of less than 1 μm due to the reaction-induced phase separation. Addition of salts to the monomeric mixture resulted in a structure with only pores of 10-80 μm in diameter due to cryogelation. Increasing the amount of salts added resulted in the formation of thicker pore walls and thus a slight reduction in pore size compared to a sample with no added solute. The possibility of changing the structure and properties of the gels by adding different solutes could open up new applications for these materials, for example, chromatography applications.