Blanca Vázquez

New partially degradable and bioactive acrylic bone cements based on starch blends and ceramic fillers

This work reports the development of new partially biodegradable acrylic bone cements based on corn starch/cellulose acetate blends (SCA), prepared by the free radical polymerization of methyl methacrylate and acrylic acid at low temperature. Amounts of biocompatible, osteoconductive and osteophilic mineral component such as hydroxylapatite (sintered and non-sintered), were incorporated in different percentages to confer a bone-bonding character to the bone cements in this type of applications. All cement formulations were characterized by 1H NMR spectroscopy. Curing parameters and mechanical properties were determined finding formulations which complete the ASTM legislation. Hydration degree, degradation studies, as well as bioactivity tests were performed in all prepared formulations. The developed systems show a range of properties that might allow for their application as self-curing bone cements, exhibiting several advantages with respect to other commercially available bone cements.

Water sorption of flexible networks based on 2- hydroxyethyl methacrylate-triethylenglycol dimethacrylate copolymers

Abstract Cross-linked films of copolymer of 2-hydroxyethyl methacrylate (HEMA) and triethylenglycol dimethacrylate (TEGDMA) were prepared by free radical polymerization initiated at 60°C. The swelling behaviour of films prepared with 2, 5 and 10 mol% of TEGDMA was followed gravimetrically and the diffusion coefficients were determined according to the Fickian law at several temperatures in the interval 25–50°C. The apparent activation energy was independent of the composition of the networks, giving a value of 15.5 kJ mol−1 which corresponds to a typical diffusion process. The freezing point, the enthalpy of fusion and the amount of non-freezable water were determined by differential scanning calorimetry (DSC) on hydrated samples with different hydration degree. There is a critical hydration interval (W = 20–30 wt%) in which it is possible to distinguish the different states of water in the cross-linked hydrogel. The variation of the enthalpy ΔHf with the hydration degree gives a maximum amount of non-freezable water of about 23 wt%, independent of the TEGDMA content of copolymer systems.

New starch-based thermoplastic hydrogels for use as bone cements or drug-delivery carriers

The development of new biodegradable hydrogels, based on corn starch/cellulose acetate blends, produced by free-radical polymerization with methyl methacrylate monomer (MMA) and/or an acrylic acid monomer (AA), is reported. The polymerization was initiated by a redox system consisting of a benzoyl peroxide and 4-dimethlyaminobenzyl alcohol at low temperature. These hydrogels may constitute an alternative to the materials currently used as bone cements or drug-delivery carriers. Swelling studies were carried out, as a function of pH and temperature, in buffered solutions. The xerogels were further characterized by Fourier transform–infrared spectroscopy. Tensile and compression tests, and dynamic mechanical thermal analysis were used to assess the mechanical performance of the developed materials. The fracture surfaces were observed by scanning electron microscopy. The developed materials are sensitive to the pH, showing a clear reversible transition in a relatively narrow interval of pH, which is just in the range of physiological conditions. These properties make the materials developed in this study very promising for biomedical applications. Fickian-type diffusion is the mechanism predominant in these systems, except for the composition with a higher concentration of AA, that corresponds to the most desirable kinetical behavior for controlled release (case II-transport mechanism). Furthermore, the results obtained in the mechanical tests are in the range of those reported for typical PMMA bone cements, showing that it is possible to develop partially degradable cements with an adequate mechanical behavior.

Role of amine activators on the curing parameters, properties and toxicity of acrylic bone cements

A review about the accelerating effect of tertiary aromatic amines used as activator in the benzoyl peroxide/amine system for the curing of acrylic resins is presented. The kinetics, mechanism and activation energy of the reaction are considered, together with some toxicity, residuals and leaching data concerned with biomedical applications of this system, e.g. denture resins or acrylic bone cements. Furthermore, some results relating the effect of the temperature of the surroundings on the curing parameters of the cements prepared with three amines (N,N-dimethyl-4-toluidine, N,N-dimethylbenzyl alcohol and N,N-dimethylbenzyl methacrylate) are shown. The results indicate that the temperature has a significant effect on the curing parameters, and must be considered in the evaluation of new activators. The relevance of these results lies with the importance of thermal trauma generally associated with the implantation of acrylic bone cements.

Application of tertiary amines with reduced toxicity to the curing process of acrylic bone cements

4-Dimethylaminobenzyl alcohol (DMOH) and 4-dimethylaminobenzyl methacrylate (DMMO) were used as the activators in the benzoyl peroxide initiated redox polymerization for the preparation of acrylic bone cement based on poly(methylmethacrylate) beads of different particle size. The residual monomer content of the cured cements was about 2 wt %, independent of the redox system used in the polymerization, indicating that the activating effect of the tertiary aromatic amines DMOH or DMMO was sufficient to reach a polymerization conversion similar to that obtained with the benzoyl peroxide (BPO) N,N-dimethyl-4-toluidine (DMT) system. The BPO/DMOH and BPO/DMMO redox systems provided exotherms of decreasing peak temperature and increasing setting time, and the cured materials presented higher average molecular weight and similar glass transition temperatures in comparison with those obtained when DMT was used as the activator. In addition, these activators are three times less toxic than the classical DMT.

Analysis of the leaching and toxicity of new amine activators for the curing of acrylic bone cements and composites

The comparative reactivity of new tertiary amine activators with the basic chemical structure of N,N-dimethyl-4-toluidine, but reduced toxicity, is analysed. The leaching of the amine compounds from cured cements was studied by analysis of the concentration of the corresponding amine in a physiological saline solution after 3 months of immersion, giving lower values for the new amine compounds as compared to N,N-dimethyl-4-toluidine. The acute toxicity was determined by intravenous injection of saline solutions of the corresponding chlorhydrates in mice and the cytotoxicity by the evolution of specific culture media. The results obtained demonstrate a lower acute toxicity and cytotoxicity of the new activators, even with a noticeable antiseptic action, which makes these materials very interesting from a practical point of view as activators of the curing process of acrylic bone cements for orthopaedic surgery and dentistry.

Radiopaque acrylic cements prepared with a new acrylic derivative of iodo-quinoline.

A novel iodine-containing methacrylate, 2,5-diiodo-8-quinolyl methacrylate, has been synthesized and used in the preparation of acrylic radiopaque cements. The effect of incorporation of this monomer to the self-curing resins, on the curing parameters, swelling behaviour and mechanical properties was studied. The incorporation of the radiopaque compound 2,5-diiodo-8-hydroxyquinoline to the solid phase was also carried out for comparative experiments. A decrease in the peak temperature and an increase in the setting time was observed with the addition of the radiopaque monomer, however, the curing parameters did not appreciably change with the addition of the radiopaque compound to the solid phase. Swelling of the modified cements was in the same range as that of the radiolucent cement; however, the diffusion coefficients calculated according to the Ficks law were higher for the iodine-containing materials. The addition of 5 wt% of the iodine-containing methacrylate provided a significant increase in the tensile properties with respect to either control radiolucent formulations or BaSO4-containing formulations. Biocompatibility of the modified cements was studied by implantation of rods of the cements into rats and histological analysis of the surrounding tissue.

Oxidized dextrins as alternative crosslinking agents for polysaccharides: application to hydrogels of agarose-chitosan.

Hydrogel networks that combine suitable physical and biomechanical characteristics for tissue engineering scaffolds are in demand. The aim of this work was the development of hydrogel networks based on agarose and chitosan using oxidized dextrins as low cytotoxicity crosslinking agents, paying special attention to the study of the influence of the polysaccharide composition and oxidation degree of the dextrins in the final characteristics of the network. The results show that the formation of an interpenetrating or a semi-interpenetrating polymer network was mainly dependent on a minimum agarose content and degree of oxidation of dextrin. Spectroscopic, thermal and swelling analysis revealed good compatibility with an absence of phase separation of polysaccharides at agarose:chitosan proportions of 50:50 and 25:75. The analysis of atomic force microscopy images showed the formation of a fibrillar microstructure whose distribution within the crosslinked chitosan depended mainly on the crosslinker. All materials exhibited the viscoelastic behaviour typical of gels, with a constant storage modulus independent of frequency for all compositions. The stiffness was strongly influenced by the degree of oxidation of the crosslinker. Cellular response to the hydrogels was studied with cells of different strains, and cell adhesion and proliferation was correlated with the homogeneity of the samples and their elastic properties. Some hydrogel formulations seemed to be candidates for tissue engineering applications such as wound healing or soft tissue regeneration.

Intrinsically antibacterial materials based on polymeric derivatives of eugenol for biomedical applications

Infections are the most common cause of biomaterial implant failure representing a constant challenge to the more widespread application of medical implants. This study reports on the preparation and characterization of novel hydrophilic copolymeric systems provided with antibacterial properties coming from eugenol residues anchored to the macromolecular chains. Thus, high conversion copolymers were prepared from the hydrophilic monomer 2-hydroxyethyl methacrylate (HEMA) and different eugenol monomeric derivatives, eugenyl methacrylate (EgMA) and ethoxyeugenyl methacrylate (EEgMA), by bulk polymerization reaction. Thermal evaluation revealed glass transition temperature values in the range 95-58 degrees C following the order HEMA-co-EgMA > PHEMA > HEMA-co-EEgMA and a clear increase in thermal stability with the presence of any eugenyl monomer in the system. In vitro wettability studies showed a reduction of water sorption capacity and surface free energy values with increasing the content of eugenol residues in the copolymer. The antimicrobial activity of copolymeric discs was evaluated by determining their capacity to reduce or inhibit colony formation by different bacterial species. All eugenyl containing materials showed bacteria growth inhibition, this one being higher for the EEgMA derivative copolymers.

Modified acrylic bone cement with high amounts of ethoxytriethyleneglycol methacrylate.

One cause of arthroplasty failure is the brittle mechanical behavior of bone cements. However, the improvement of cement formulations must also be accompanied by the maintenance of a wide variety of characteristics. New bone cements were obtained by the substitution of high percentages, up to 60% (v/v), of methyl methacrylate (MMA) by a higher molecular weight and more hydrophilic monomer, ethoxytriethyleneglycol methacrylate (TEG). The essential advantages of these materials were the decrease of maximum temperature together with a decrease in the residual monomer content with respect to conventional cement formulations. The water absorption process obeyed diffusion laws and the equilibrium water content increased by the introduction of higher percentages of the hydrophilic component. This characteristic had an appreciable effect on the viscoelastic behavior analyzed by DMTA. These modified bone cements had reduced polymerization shrinkage and similar levels of porosity. Tensile test revealed that the introduction of TEGMA gave rise to an important modification of the mechanical behavior, with a noticeable increase in the fracture strain. This fact was also confirmed by means of the analysis of the fracture surfaces by SEM.