Designing degradable hydrogels is complicated by the structural and temporal complexities of the gel and evolving tissue. A major challenge is to create scaffolds with sufficient mechanical properties to restore initial function while simultaneously controlling temporal changes in the gel structure to facilitate tissue formation. Poly(ethylene glycol) was used in this work, to form biodegradable poly(ethylene glycol)-based hydrogels with hydrolyzable poly-L-lactide segments in the backbone. Non-degradable poly(ethylene glycol) was also introduced in the formulation to obtain control of the degradation profile that encompasses cell growth and new tissue formation. The dependence on polymer composition was observed by higher degradation profiles and decreased mechanical properties as the content of degradable segments was increased in the formulation. Based on in vitro tests, no toxicity of extracts or biomaterial in direct contact with human adipose tissue stem cells was observed, and the ultraviolet light treatment did not affect the proliferation capacity of the cells.

Introduction: Biological apatites, the major inorganic component of bone mineral, differ from pure hydroxyapatite (HA) in stoichiometry, composition, crystallinity and other physical and mechanical properties that explain their special behaviour in bone remodelling cycles. Simulated or synthetic body fluids (SBF) are prepared in accordance with the chemical analysis of human body fluid. They are metastable buffered solutions supersaturated towards apatite crystals. When supersaturation degree increases (because of the addition of calcium and phosphate salts), mineralization is induced. Once apatite is nucleated, it can grow spontaneously incorporating ions from the SBF (Na, Mg, CO3, etc.) into the structure (so differing from the formula Ca10(PO4)6(OH)2 and 1.67 of Ca/P molar ratio of synthetic ones) and such precipitation is similar to biological mineralization [1]. The term nanogel refers to spherical, nanometric, crosslinked polymer matrix that forms colloidally stable dispersions. Due to their small sizes and high surface-volume ratio, they have fast swelling– deswelling properties [2]. They can be charged with drugs and used as controlled release systems [3,4]. Hydroxyapatite precursor salts (calcium chloride and sodium hydrogen phosphate) can be encapsulated into nanogels to the aim of controlling the hydroxyapatite reaction kinetics. Moreover, nanogels could act as nucleation sites of hydroxyapatite. In this work advantages from the precipitation in SBF and from nanometric hydrogel properties have been combined. When heated to physiologic temperature, above the lower critical solution temperature (LCST) of the poly(N-Isopropylacrylamide-co-acrilic acid) nanogels, both Ca and P ions-loaded nanogels become hydrophobic and release their contents into the SBF solution, where the ions react to form hydroxyapatite mineral. Materials and methods: Poly(NIPAAm-co-AA) nanogels were synthesized by precipitation radical polymerization in water as previously described [5]. After dialysis and freeze-drying nanometric hydrogels of sizes close to 100nm were obtained. Nanogels were dispersed in highly concentrated aqueous solutions of CaCl2 and Na2HPO4 and after several hours they were recovered by ultrafiltration and freeze-drying. Then, the SBF solution was prepared and the load and controlled release of sodium and phosphate ions was measured. It is known that a Ca/P molar ratio of 1.80 added to SBF guaranties a good biomimetic hydroxyapatite, for this reason, it is necessary to control the behaviour of nanogels in the presence of these ions. Different concentrations of salts and nanogels were used to study the load of the salts into the nanogels (data not shown). The amount of salt loaded into the nanogels was characterized by ICP-AES (Inductively coupled plasma atomic emission spectroscopy, axial, model VISTA-MPX from VARIAN). For the study of the controlled release, loaded nanogels were introduced in dialysis bags with deionized water at 37oC instead of SBF in order to avoid any precipitation and to determine the Ca or Pi released without the influence of any other salt. The controlled release of Ca or Pi was analyzed at different time points by ICP. For the synthesis of biomimetic hydroxyapatite powders in SBF, both Ca-loaded (1000 mg/ml; 400:3) and Pi-loaded nanogels (90 mg/ml; 12:1), in a ratio of 2:1, were placed in SBF solution for five days at 37oC. Powders were recovered by centrifugation, washed and dried, and characterized by XRD (D8Advance, Bruker), FTIR spectroscopy and ICP-AES. Results and discussion: Results demonstrated that the higher the amount of salt in the solution, the bigger amount the nanogels can load, but this loaded amount does not depend on the amount of nanogels. It could be due to electrostatic interactions that only let the nanogels load with salt until a limit, when the limit is reached, no more nanogels absorb salt. Figure 1 shows the accumulated concentration of Ca or Pi released from the nanogels to water along the time. The release almost reached equilibrium at the fourth day. Concentration of Ca or Pi released depends on the amount of loaded nanogels and initial solution concentration used for the load. The bigger the amount of loaded nanogels, the larger the amount of salt released. Ca/P molar ratios between the Ca and Pi released were estimated from the curves and, as the ideal Ca/P molar ratio to form a biomimetic hydroxyapatite in SBF must be close to 1.80, the combined release from 100 mg of Ca-nanogel and 200 mg of Pi-nanogel was selected for the study of the hydroxyapatite synthesis. XRD spectra of the hydroxyapatite powders recovered after five days incubation of salt-loaded nanogels in SBF solution had got peaks that corresponded to the standard for stoichiometric HA (International Centre of Diffraction Data (ICDD), JCPDS 09-0432) confirming the apatitic nature of the sample. Figure 2 shows FTIR spectra of the powders obtained. The broad band beyond 3000 cm−1 corresponds to the OH groups. The little band at 1650 cm−1 corresponds to adsorbed water. Phosphate groups are seen in the region between 550 and 600 cm−1 and the strong band close to 1000 cm−1. Carbonate vibration bands are observed at 1400–1550 cm−1 confirming the carbonate apatite nature. The percentage of elements found in the sample determined by ICP-AES ( %Na 0.37, %K 0.15, %Mg 0.32, %S 0.04) confirmed the substitutional inclusion of different amounts of ions from the SBF solution in the apatitic structure as in natural hard tissues.

Polymer scaffolds play an important role in tissue engineering applications. Poly(ethylene glycol) based hydrogels have received a lot of attention in this field because of their high biocompatibility and ease of processing. However, in many cases they do not exhibit proper tissue invasion and nutrient transport because of their dense structure. In the present work, several approaches were developed and compared to each other to produce interconnected macroporous poly(ethylene glycol) hydrogels by including different types of porogens in the photocrosslinking reaction. The swelling capacity of the resulting hydrogels was analyzed and compared to non-porous hydrogel samples. Moreover, the obtained materials were characterized by means of mechanical properties and porosity using rheometry, scanning electron microscopy, and mercury intrusion porosimetry. Results showed that interconnected and uniform pores were obtained when a porogen template was used during hydrogel fabrication by photocrosslinking. On the other side, when the porogen particles were dispersed into the macromer solution before matrix photocrosslinking the interconnexion was negligible. The templates must be dissolved before the hydrogel's cell-seeding in vitro, while the dispersed porogen can be used in situ in the in vitro seeding tests.

A new family of multifunctional scaffolds, incorporating selected biopolymer coatings on basic Bioglass® derived foams has been developed. The polymer coatings were investigated as carrier of vancomycin which is a suitable drug to impart antibiotic function to the scaffolds. It has been proved that coating with PLGA (poly(lactic-co-glycolic acid)) with dispersed vancomycin-loaded microgels provides a rapid delivery of drug to give antibacterial effects at the wound site and a further sustained release to aid mid to long-term healing. Furthermore, the microgels also improved the bioactivity of the scaffolds by acting as nucleation sites for the formation of HA crystals in simulated body fluid.

Stimuli-sensitive micro gels of poly(N-isopropylacrylamide-co-acrylic acid) (designated as P(NIPAAm-co-AA)) were prepared through precipitation polymerization. Their capacity to load and release different drugs under different conditions, including physiological, in a controlled manner was analyzed. Two drugs were assayed and compared: dexamethasone and vancomycin. The prepared microgel particles show good thermosensitivity. In addition, the amount of cross-linker used in the preparation of the microgels does not greatly influence the drug-release capability of P(NIPAAm-co-AA)), but the amount of drug used to load the microgels did result in bigger amounts of drug released afterwards. These results imply potential application of prepared stimuli-sensitive microgel dispersions as drug-delivery systems and tissue engineering materials.