Hydroxyapatite coating of poly(2-hydroxyethyl methacrylate) hydrogel by biomimetic method

The recently proposed biomimetic method has been performed on samples of poly(2-hydroxyethyl methacrylate) p(HEMA) hydrogel in order to improve their interface with bone. Following the biomimetic process it was possible to obtain a bioactive hydroxyapatite coating on the p(HEMA) substrates. FTIR analysis showed that characteristic peaks of p(HEMA) progressively disappear, owing to the formation of a surface coating. In fact new peaks appear corresponding to the ones of P-O stretching (1116 and 1035 cm–1) and P-O bending (580 cm–1) vibration modes, thus suggesting that the method is effective in promoting the formation of a surface phosphate layer. The formation of hydroxyapatite crystals is confirmed by SEM and EDS results. The adhesive strength was measured and turned out to be higher than the one reported for PMMA substrate. The experimental results suggest that, as reported in the literature for other supports, the silicate ions released from the glass in the first stage bind themselves to the polymeric support.     

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.     

Apatite formation on poly(2-hydroxyethyl methacrylate)-silica hybrids prepared by sol-gel process

Hybrids of poly(2-hydroxyethyl methacrylate) (PHEMA), a polymer that has been employed in a wide variety of biomedical applications, and silica-gel, which exhibits a well-known bioactivity, were produced. The obtained hybrids were characterized and their in vitro ability to induce the formation of a calcium phosphate layer on the surface was evaluated. The surface area of hybrids decreased with increasing amounts of PHEMA so that hybrids with more than ~40% PHEMA are virtually non-porous. All hybrids induced the formation of a calcium phosphate layer on their surfaces when soaked into simulated body fluid. The induction time and the morphology of the apatite layer varied according to the polymer content.     

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.     

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.     

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.     

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.     

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.     

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.     

Properties of flexible, transparent barium titanate nanoparticle/poly(2-hydroxyethyl methacrylate) hybrid

A transparent BaTiO₃ particle/poly(2-hydroxyethyl methacrylate) (PHEMA) hybrid was synthesized from a Ba–Ti double alkoxide modified with an organic ligand, and the refractive index and ferroelectric properties of the hybrid were studied. The transparent hybrid was flexible and could be shaped not only in the form of thin films on substrates but also in the form of flexible self-standing films. BaTiO₃ particles around 5 nm in diameter were dispersed uniformly in the polymer matrix. The refractive indexes of the hybrid films increased with decreasing wavelength and were dependent on the volume fractions of the BaTiO₃ and polymer phases. The hybrid film synthesized at a molar ratio of BaTiO₃:2-vinlyloxyethanol:H₂O:PHEMA = 1:8:30:5 gave the best result, with a refractive index of 1.55 at 589 nm and an Abbe number of 37.5. A transparent hybrid film on platinized silicone substrates exhibited a polarization–electric field hysteresis loop.     

Enhancing the grafting of poly(2-hydroxyethyl methacrylate) on silica nanoparticles (SiO2-g-PHEMA) by the sequential UV-induced graft polymerization with a multiple-UV irradiation

Grafting of poly(2-hydroxyethyl methacrylate) on silica nanoparticles was accomplished via the sequential UV-induced graft polymerization. Under UV-irradiation, the silica was functionalized with the surface initiator, benzophenone (BP) and subsequently graft-polymerized with 2-hydroxyethyl methacrylate (HEMA). The grafting on the silica particles was confirmed by DSC analysis which revealed a shift of the glass transition temperature (T_g) of grafted PHEMA to higher temperature than T_g of ungrafted PHEMA. A significant improvement in the grafting efficiency and the grafting percentage was achieved when a sequential grafting approach was taken, employing multiple UV exposures. Using this approach, the efficient chain extension from the grafted-PHEMA was possible without producing significant amounts of ungrafted PHEMA when low HEMA concentrations were used during each UV-exposure.     

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.     

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.     

Evaluation of biocompatibility of the copolymer of 2-hydroxyethyl methacrylate with 2-(methylsulfanyl)ethyl methacrylate

This study compares subcutaneous and intracerebral biocompatibilty of two hydrogels: copolymer of 2-hydroxyethyl methacrylate with 2-(methylsulfanyl)ethyl methacrylate and poly(2-hydroxyethyl methacrylate) as reference polymer. The experimental copolymer was more biologically inert than poly(2-hydroxyethyl methacrylate) in both the studied parameters, hence the former material is a suitable candidate for biomedical application.     

Rheological properties of poly(2-hydroxyethyl methacrylate) (pHEMA) as a function of water content and deformation frequency

Poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels have been used, or suggested for use, in a wide range of biomedical applications. In many of these applications, the mechanical properties of the gel are important for its proper functioning. These properties are influenced by a number of factors, including water content. In this study the storage and loss shear moduli were measured as a function of frequency for gels with water contents ranging from 22% to 48% at a temperature of 37 °C. At low frequencies and high water contents, deformation frequency had little effect. However, at higher frequencies and lower water contents, both moduli increased markedly with increasing frequency. This can be explained by the gels approaching a glass transition. The curves describing the behavior of each gel were combined to form a master curve, using a method analogous to the time–temperature superposition principle. This master curve can be used to predict the shear moduli for gels with a wide range of water contents and loading frequencies. For example, for a gel with a water content of 47.8% (as a percentage of the mass of gel), the curve provides shear moduli values over a frequency range of 10^–2–10⁴ Hz.     

Synthesis of BaTiO3 nanoparticle/poly(2-hydroxyethyl methacrylate) hybrid nanofibers via electrospinning

BaTiO₃ nanoparticle/poly(2-hydroxyethyl methacrylate) (PHEMA) hybrid nanofibers were fabricated from an in situ synthesized BaTiO₃ nanoparticle/polymer hybrid by electrospinning. The bulk hybrid for nanofibers was synthesized through the in situ hydrolysis of Ba–Ti alkoxide modified with 2-vinyloxyethanol and subsequent copolymerization with HEMA monomer. IR and ¹³C NMR spectra showed the formation of polymer matrix. The molecular weights of BaTiO₃ nanoparticle/PHEMA hybrid for spinning were 1.3 × 10⁵ for 20 equiv. PHEMA and 5.7 × 10⁵ for 30 equiv. PHEMA. The crystallite size of BaTiO₃ particles in the hybrid was 4.5 nm according to the Scherrer equation. The diameter of BaTiO₃ nanoparticle/PHEMA hybrid nanofibers ranged from 500 nm to 1 μm. A field stress–strain curve was observed for the BaTiO₃ nanoparticle/PHEMA hybrid nanofiber.     

The effect of laser radiation on the conductivity of poly(diacetylene)

It is demonstrated that an increase in the dark conductivity of poly(diacetylene) (PDA) upon laser irradiation in oxygen-containing media is determined by the formation of thin conducting surface layers. Pulsed laser irradiation at an energy of 0.04 J, a wavelength of 530 nm, and a pulse duration of 20 ns increases the conductivity of the surface PDA layers by four orders of magnitude. It is supposed that the laser-induced degradation and the subsequent stabilization of the surface PDA layers are mainly determined by photooxidation.     

Microlenses fabricated by ultraviolet excimer laser irradiation of poly(methyl methacrylate) followed by styrene diffusion

A new technique of microlens array fabrication based on the use of excimer laser radiation is described. Poly(methyl methacrylate) (PMMA) substrates are treated with many low-energy KrF laser pulses and exposed to styrene vapor. The irradiated material swells, producing spherical microlenses that are stabilized by UV polymerization. The chemistry of this process and the optical quality of the lenses are discussed.