Thermal analysis of poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels

The influence of water on the physical properties of a hydrogel is important for understanding natural tissues and in designing synthetic materials to replace them. In this study, poly (2-hydroxyethyl methacrylate) (pHEMA) was used as a model system to understand how water interacts with the polymer of a hydrogel. Thermal analysis methods (thermogravimetric analysis coupled to mass spectrometry and differential scanning calorimetry) were used to determine: (i) the total water content of pHEMA gels; (ii) how this water was lost during heating; (iii) the relationship between water content of the gel and its glass transition temperature; and (iv) the behavior of the water in the gel on cooling. Previous researchers have invoked various models to describe the organization of water in a hydrogel. In this study, the simplest model which could explain all of the results from the different thermal analysis techniques was one which consisted of three classes of water: (i) hydration water in close proximity to the polymer; (ii) interstitial water in regions or cavities surrounded by polymer chains; and (iii) bulk water.     

Initiated chemical vapor deposition of poly(2-hydroxyethyl methacrylate) hydrogels

Initiated chemical vapor deposition (iCVD), a low temperature variant of hot-wire chemical vapor deposition (HWCVD) is a solvent-free polymerization technique. It was used to synthesize thick, free-standing films of the hydrogel poly(2-hydroxyethyl methacrylate) (PHEMA). In this work, we show that the iCVD technique can yield PHEMA which is free from residual entrained monomer, has low non-specific protein adsorption and is capable of supporting good cell adhesion and proliferation.     

Preferential interactions of calcium ions in poly(2-hydroxyethyl methacrylate) hydrogels

An investigation of the preferential interaction of calcium ions with oxygen atoms in poly(2-hydroxyethyl methacrylate) (PHEMA)-based hydrogels has been carried out. The formation of polymer–Ca complexes was achieved by exposing powdered or fully hydrated samples with 5 mM, 0.1–0.5 M, or saturated CaCl₂ solutions for certain periods of time. The characteristics of the polymer–Ca complexes were deduced from the effect of the solute on the equilibrium water content, and from NMR, atomic absorption and infrared spectroscopies. The absence of significant changes in the NMR chemical shift and infrared vibrational wavenumbers for the various functional groups confirmed that polymer complexation with Ca²⁺ ions involves only weak interactions, possibly electrostatic or ion–dipole interactions. Among the three types of oxygen atoms in PHEMA, hydroxyl oxygen atoms seem to be the most sensitive to the presence of Ca²⁺ ions. Complexation at the ester oxygen atoms was also evidenced by a new band in the infrared spectra at 1,550 cm^-1. On the other hand, there were no indications that the hydrophobic domains in the backbone and the methyl groups at the side chain of PHEMA interact significantly with Ca²⁺ ions.     

Morphological evaluation of bioartificial hydrogels as potential tissue engineering scaffolds

Poly(vinyl alcohol) hydrogels prepared by freeze-thawing procedure represent synthetic systems widely investigated as non-biodegradable scaffolds for tissue regeneration. In order to improve the biocompatibility properties of pure poly(vinyl alcohol) (PVA) hydrogels, blends of PVA with different biological macromolecules, such hyaluronic acid, dextran, and gelatin were prepared and used to produce bioartificial hydrogels. The porosity characteristics of these hydrogels were investigated by scanning electron microscopy and mercury intrusion porosimetry. The morphology of bioartificial hydrogels was evaluated and compared with that of pure PVA hydrogels. In particular the effect exerted by each biological component on pore size and distribution was investigated. The obtained results indicate that when a natural macromolecule is added to PVA the internal structure of the material changes. A small amount of biopolymer induces the structural elements of PVA matrix to take on a well evident lamellar appearance and an apparent preferential orientation. Comparing the results of SEM and mercury intrusion porosimetry it was concluded that hydrogels containing 20% of biological component have the most regular structure and at the same time the lowest total porosity. On the contrary samples with the highest content of natural polymer (40%) show the less regular structure and the highest total porosity.     

Adsorption of a blood protein on to hydrophilic sponges based on poly(2-hydroxyethyl methacrylate)

Spongy materials of poly(2-hydroxyethyl methacrylate) were synthesized and the adsorption of bovine serum albumin was carried out onto their surfaces. The sponges were characterized by IR spectral analysis, and water sorption property. It was noticed that the chemical architecture of the sponge has a pronounced impact on both the water sorption capacity and adsorption affinity of the sponge surfaces. The adsorption was also studied kinetically and the effect of pH was also investigated. The synthesized sponges were evaluated for antithrombogenic property by performing blood-clot formation tests.     

Surface modification of hydrogels based on poly(2-hydroxyethyl methacrylate) with extracellular matrix proteins

Infrared attenuated total reflection spectroscopy was used for in situ observation of the deposition of collagen I on poly(2-hydroxyethyl methacrylate-co-methacrylic acid, 2.9%) hydrogels and subsequent attachment of laminin or fibronectin on the collagen surface. While there was no adsorption of collagen dissolved in an acid solution on the hydrogel surface, it deposited on the surface at pH 6.5. The collagen layers with attached laminin or fibronectin were stable on hydrogel surface in physiological solution. The modification with collagen and particularly with collagen and laminin or fibronectin allowed the adhesion and growth of mesenchymal stromal cells and astrocytes on the hydrogel surface.     

Host reaction to poly(2-hydroxyethyl methacrylate) scaffolds in a small spinal cord injury model

Tissue engineered scaffolds and matrices have been investigated over the past decade for their potential in spinal cord repair. They provide a 3-D substrate that can be permissive for nerve regeneration yet have other roles including neuroprotection, altering the inflammatory cascade and mechanically stabilizing spinal cord tissue after injury. In this study we investigated very small lesions (approx. 0.25 μL in volume) of the dorsal column into which a phase-separated poly(2-hydroxyethyl methacrylate) hydrogel scaffold is implanted. Using fluorescent immunohistochemistry to quantify glial scarring, the poly(2-hydroxyethyl methacrylate) scaffold group showed reduced intensity compared to lesion controls for GFAP and the chondroitin sulfate proteoglycan neurocan after 6 days. However, the scaffold and tissue was also pushed dorsally after 6 days while the scaffold was not integrated into the spinal cord after 28 days. Overall, this small-lesion spinal cord injury model provided information on the host tissue reaction of a TE scaffold while reducing animal discomfort and care.     

Synthesis and characterization of hydrogels based on poly(2-hydroxyethyl methacrylate) for drug delivery under UV irradiation

The present study aims to create a controlled release system through the preparation and characterization of hydrogels based on 2-hydroxyl ethyl methacrylate (HEMA). In order to investigate the influence of photo-initiators on the drug release behavior of the resulting hydrogels, three different photo-initiators [2,2-dimethoxy-2-phenyl-acetophenone] (Irgacure 651), 1-hydroxycyclohexyl phenyl ketone (Irgacure 184) and 2-hydroxy-4′-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959) were used. In addition, hydroxyapatite (HAp) was employed to modify HEMA hydrogels. The synthesis of hydrogels was confirmed by characterization through Fourier transform infrared spectroscopy, nuclear magnetic resonance (¹³C NMR) spectroscopy and digital microscope. The responsive behaviors were investigated by recording swelling ratios under different conditions. In vitro drug release studies were performed for donepezil hydrochloride-loaded hydrogels at pH 1.2, 6.8 and 7.4. The results indicated that hydrogels synthesized using Irgacure 2959 released the maximum amount of donepezil hydrochloride. Moreover, the release rate decreased in the presence of HAp.     

Micro- and nanoscale modification of poly(2-hydroxyethyl methacrylate) hydrogels by AFM lithography and nanoparticle incorporation

Poly(2-hydroxyethyl methacrylate) (PHEMA) hydrogels are widely used as biomaterials. Due to their unique combination of biocompatibility and good mechanical properties, they have potential as scaffolds for tissue engineering applications. To this purpose, topographic and chemical patterning at the nano- to the mesoscale is crucial in order to favor and to characterize cell adhesion and proliferation. Here we report the characterization of as-prepared and patterned PHEMA hydrogels, produced by conventional radical polymerization in water and dimethylformamide. We have obtained chemical and morphological micro- and nanoscale patterning by atomic force microscopy based lithography. We also demonstrate that it is possible to incorporate carbon nanoparticles in the hydrogel matrix by supersonic cluster beam deposition.     

壳聚糖支架在组织工程中的应用

综述了壳聚糖作为细胞生长载体在软骨组织工程，骨组织工程和皮肤组织工程等方面的应用进展，表明壳聚糖有望成为优异的组织工程支架材料。     

Three-dimensional nonwoven scaffolds from a novel biodegradable poly(ester amide) for tissue engineering applications

Biodegradable polyesters are established biomaterials in medicine due to their chemical characteristics and options for material processing. A main problem, however, is the release of acid degradation products during biodegradation with severe local pH-drops and inflammatory reactions. Polyesteramides, in contrast, show a less prominent pH-drop during degradation. In this study, we developed a simple, reproducible synthesis of the poly(ester amide) (PEA) type C starting from ε-caprolactame, 1,4-butanediol, and adipic acid in a one-batch two-step reaction and conducted the manufacturing of PEA-derived 3D textile scaffolds applicable for tissue engineering purposes. The thermal and mechanical properties of PEA-type C were analysed and the structural conformity of different batches was confirmed by NMR spectroscopy and size exclusion chromatography. The polymer was formed into nonwovens by textile manufacturing. Cytotoxicity tests and X-ray photoelectron spectroscopy (XPS) were used to analyze the effect of scaffold extraction before cell seeding. The manufactured carriers were seeded with human preadipocytes and examined for cellular proliferation and differentiation. The production of PEA type C successfully occurred via simultaneous ring-opening polymerization of ε-caprolactame and polycondensation with 1,4-butanediol and adipic acid at 250 °C under high-vacuum. Soxhlet extraction allowed optimal cleaning of nonwoven scaffolds. Extracted PEA-derived matrices were capable of allowing good adherence, proliferation, and differentiation of preadipocytes. These results are encouraging and guidance towards an optimally prepared nonwoven carrier applicable for clinical use. K. Hemmrich and J. Salber have contributed equally.     

Porous protein-based scaffolds prepared through freezing as potential scaffolds for tissue engineering

Successful tissue engineering with the aid of a polymer scaffold offers the possibility to produce a larger construct and to mould the shape after the defect. We investigated the use of cryogelation to form protein-based scaffolds through different types of formation mechanisms; enzymatic crosslinking, chemical crosslinking, and non-covalent interactions. Casein was found to best suited for enzymatic crosslinking, gelatin for chemical crosslinking, and ovalbumin for non-covalent interactions. Fibroblasts and myoblasts were used to evaluate the cryogels for tissue engineering purposes. The stability of the cryogels over time in culture differed depending on formation mechanism. Casein cryogels showed best potential to be used in skeletal tissue engineering, whereas gelatin cryogels would be more suitable for compliable soft tissues even though it also seemed to support a myogenic phenotype. Ovalbumin cryogels would be better suited for elastic tissues with faster regeneration properties due to its faster degradation time. Overall, the cryogelation technique offers a fast, cheap and reproducible way of creating porous scaffolds from proteins without the use of toxic compounds.     

Development of poly (lactic-co-glycolic acid)-collagen scaffolds for tissue engineering

Collagen as an important extra-cellular matrix (ECM) in many tissues is weakly antigenic and the structure of collagen sponges is highly porous with interconnected pores effective for cell infiltration and mass transfer of oxygen and nutrients. Its application as a scaffold is limited by poor mechanical strength and rapid biodegradation. In this paper, we attempt to graft hydrolyzed PLGA fiber surfaces with collagen by N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide (EDC) in combination with N-hydroxysuccinimide (NHS), and then embed these collagen-grafted PLGA fibers in collagen sponge to form a hybrid PLGA-collagen scaffold. For further stability, we cross-linked the collagen in the scaffold and used it in rat liver cell cultivation. The scaffold was characterized by mechanical micro-tester, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Results showed that (1) the scaffolds exhibited isotropic and interconnected porous structure; (2) the compression modulus of this scaffold was enhanced 50 fold compared to the collagen scaffolds. The cell attachment and cytotoxicity of this scaffold were studied. Cell attachment was improved remarkably and the cytotoxicity of the hybrid PLGA-collagen scaffold was lower than that of the un-grafted PLGA-collagen scaffolds using alamarBlue™ assay normalized to the DNA content in each scaffold. This new hybrid scaffold has potential applications for tissue engineering.     

Synthesis and characterisation of poly(2-hydroxyethyl methacrylate) polyelectrolyte complexes

Copolymerisation of charged and neutral monomers is a well-known methodology to introduce charged moieties in a polymeric chain to obtain polyelectrolytes. New polyelectrolyte complexes have been synthesised by radical copolymerisation of neutral methacrylic monomer 2-hydroxyethyl methacrylate (HEMA) with cationic 2-methacryloyloxyethyltrimethyl ammonium chloride and anionic 2-acrylamido-2-methylpropane-sulphonic acid monomers in 10:1:1 and 10:1:2 stechiometric ratio. Chemical structure of the synthesised terpolymers was confirmed by FT-IR spectroscopy, moreover, X-ray photoelectron spectroscopy showed the presence of a cationic charge excess on the 10:1:2 terpolymer surface with respect to 10:1:1 terpolymer. Swelling studies for 10:1:2 terpolymers showed a high water content in the swollen state and a "smart behaviour" upon changes in external stimuli such as pH, while, 10:1:1 terpolymer presented the behaviour of a neutral polymer. Mechanical and differential scanning calorimetry analysis confirmed that terpolymer networks were stabilised by ionic co-operative interactions. Infact, the inclusion of oppositely ionic charges in the polymeric network of p(HEMA) represent a way to achieve higher elastic modulus as they stabilise the terpolymer networks. Cytotoxicity and cytocompatibility studies demonstrated that all materials were not toxic, moreover, the presence of a cationic charge excess on 10:1:2 terpolymer surface was able to promote fibroblast adhesion.     

Chitosan/poly(dl,lactide-co-glycolide) scaffolds for tissue engineering

Chitosan/poly(dl-lactide-co-glycolide) (Ch/dl PLG) composite scaffolds were fabricated by freeze-drying lyophilization, and were evaluated and compared for use as a bone regeneration scaffold through measurements of the compression mechanical properties of the porous scaffolds. Also, In vitro cell culture of Sprague?CDawley rat??s osteoblasts were used to evaluate the phenotype expression of cells in the scaffolds, characterizing the cellular adhesion, proliferation and alkaline phosphatase activity. The gene expression of osteocalcin, sialoprotein, alkaline phosphatase, Type I collagen and TGF??1 were confirmed in the samples; moreover, it was confirmed, the mineralization by IR spectra and EDS analysis. Our results thus show that Ch/dl PLG scaffolds are suitable for biological applications.     

Porous poly (lactic-co-glycolide) microsphere sintered scaffolds for tissue repair applications

In this paper, a new route to preparing porous poly (lactic-co-glycolide) (PLGA) scaffolds for bone tissue repair applications was developed. Novel porous PLGA scaffolds were fabricated via microsphere sintered technique and gas forming technique. Ammonium bicarbonate was used to regulate porosity of these porous scaffolds. Porosity of the scaffolds, and cell attachment, viability and proliferation on the scaffolds were evaluated. The results indicated that PLGA porous scaffolds were with the porosity from around 30% to 95% by regulating ammonium bicarbonate content from 0 to 10%. We also found that PLGA porous microsphere scaffolds benefited cell attachment and viability. Taken together, the achieved porous scaffolds have controlled porosity and also support mesenchymal stem cell proliferation, which could serve as potential scaffolds for bone repair applications.     

Evaluation of electrospun fibrous scaffolds of poly(dl-lactide) and poly(ethylene glycol) for skin tissue engineering

This study is derived from the innate concerns of electrospun poly(DL-lactide) (PDLLA) fibers as tissue engineering scaffolds: hydrophobic surface, surface erosion and dimensional shrinkage, which are not favorable to trigger the initial adhesion and further growth and population of cells. Blending electrospinning of PDLLA and poly(ethylene glycol) (PEG) with different PEG contents was evaluated for optimal tissue engineering scaffolds. The surface hydrophilicity was improved, and the degradation patterns of PDLLA/PEG mats changed from surface erosion to bulk degradation with the increase in PEG contents. The dimensional shrinkage was alleviated through the formation of crystal regions of PEG in the fiber matrix. The PDLLA/PEG fibrous mats were slightly weakened with the increase in the PEG contents, but a significant decrease in the tensile strength could be found for those with PEG contents of over 40%. Human dermal fibroblasts (HDFs) interacted and integrated well with the surrounding fibers containing 20 and 30% PEG, which provided significantly better environment for biological activities of HDFs than electrospun PDLLA mats. It indicated that electrospun mats containing 30% PEG exhibited the most balanced properties, including moderately hydrophilic surface, minimal dimensional changes, adaptable bulk biodegradation pattern and enhancement of cell penetration and growth within fibrous mats.     

New hydrogels based on maleilated collagen with potential applications in tissue engineering

New hydrogels based on maleic anhydride (MA) modified collagen were prepared with the aim of overcoming the high degradation rate displayed by collagen that is not otherwise chemically crosslinked. Semi-interpenetrated matrices were obtained by free radical polymerization of maleilated collagen (CM) and 2-hydroxyethyl methacrylate (HEMA) in the presence of ammonium persulfate (APS) and N,N,N′,N′-tetramethylethylenediamine (TEMED) as initiating system. The resulting matrices (CMH) had a sharp decrease in degradation, when compared to pure collagen. FTIR and H¹ NMR spectroscopies were used to confirm the incorporation of MA on the collagen peptide chains. The final composition of CMH was found to be strongly dependent by the concentration of maleilated collagen. The morphology of the hydrogels was studied by Scanning electron microscopy (SEM) and the macro-gel structure was confirmed. Water uptake of the synthetised hydrogels is influenced by both composition and the porosity of the matrices.     

Alginate hydrogels crosslinked with different strontium-calcium ratios as injectable scaffolds for bone tissue engineering

The easy loss of crosslinking ions in alginate can result in a structural collapse of the physiological environment, thereby losing its characteristics as a bone scaffold. Meanwhile, alginate lacks osteoconductive properties, which are necessary for ideal bone scaffolds. In this study, strontium (Sr) in combination with calcium (Ca) at different ratios were used as a crosslinking agent for the alginate to investigate the effect of Ca–Sr ratio on the physicochemical properties and biological preformation of alginate hydrogel. Here, Ca and Sr in different weight ratios (4:0, 3:1, 2:2, 1:3, and 0:4) were employed as crosslinking agents. The physicochemical properties of hydrogels, including pore size, elastic modulus, degradation rate and swelling ratio, could be effectively tuned by controlling the amount of Sr. The ion release experiment revealed a burst release of Sr²⁺ in the first day after crosslinking. However, after 3 days, the amount of Sr²⁺ release had significantly declined and was proportional to the total strontium initially introduced into the alginate. Meanwhile, the live/dead results exhibited higher cell viability for alginate with 2:2 Ca–Sr weight ratio. The alginate with 2:2 Ca–Sr ratio not only improved osteoblastic attachment, but also up-regulated the alkaline phosphatase activity, the expression of osteogenic marker genes, and relative growth factors. These findings indicate that alginate with 2:2 Ca–Sr ratio might be a promising scaffold for bone tissue engineering.

Graphical abstract