Hybrid materials for biomedical applications

The increased longevity of humans and the demand for a better quality of life have led to a continuous search for new implant materials. Scientific development coupled with a growing multidisciplinarity between materials science and life sciences has given rise to new approaches such as regenerative medicine and tissue engineering. The search for a material with mechanical properties close to those of human bone produced a new family of hybrid materials that take advantage of the synergy between inorganic silica (SiO4) domains, based on sol-gel bioactive glass compositions, and organic polydimethylsiloxane, PDMS ((CH3)2.SiO2)n, domains. Several studies have shown that hybrid materials based on the system PDMS-SiO2 constitute a promising group of biomaterials with several potential applications from bone tissue regeneration to brain tissue recovery, passing by bioactive coatings and drug delivery systems. The objective of the present work was to prepare hybrid materials for biomedical applications based on the PDMS-SiO2 system and to achieve a better understanding of the relationship among the sol-gel processing conditions, the chemical structures, the microstructure and the macroscopic properties. For that, different characterization techniques were used: Fourier transform infrared spectrometry, liquid and solid state nuclear magnetic resonance techniques, X-ray diffraction, small-angle X-ray scattering, smallangle neutron scattering, surface area analysis by Brunauer–Emmett–Teller method, scanning electron microscopy and transmission electron microscopy. Surface roughness and wettability were analyzed by 3D optical profilometry and by contact angle measurements respectively. Bioactivity was evaluated in vitro by immersion of the materials in Kokubos’s simulated body fluid and posterior surface analysis by different techniques as well as supernatant liquid analysis by inductively coupled plasma spectroscopy. Biocompatibility was assessed using MG63 osteoblastic cells. PDMS-SiO2-CaO materials were first prepared using nitrate as a calcium source. To avoid the presence of nitrate residues in the final product due to its potential toxicity, a heat-treatment step (above 400 °C) is required. In order to enhance the thermal stability of the materials subjected to high temperatures titanium was added to the hybrid system, and a material containing calcium, with no traces of nitrate and the preservation of a significant amount of methyl groups was successfully obtained. The difficulty in eliminating all nitrates from bulk PDMS-SiO2-CaO samples obtained by sol-gel synthesis and subsequent heat-treatment created a new goal which was the search for alternative sources of calcium. New calcium sources were evaluated in order to substitute the nitrate and calcium acetate was chosen due to its good solubility in water. Preparation solgel protocols were tested and homogeneous monolithic samples were obtained. Besides their ability to improve the bioactivity, titanium and zirconium influence the structural and microstructural features of the SiO2-TiO2 and SiO2-ZrO2 binary systems, and also of the PDMS-TiO2 and PDMS-ZrO2 systems. Detailed studies with different sol-gel conditions allowed the understanding of the roles of titanium and zirconium as additives in the PDMS-SiO2 system. It was concluded that titanium and zirconium influence the kinetics of the sol-gel process due to their different alkoxide reactivity leading to hybrid xerogels with dissimilar characteristics and morphologies. Titanium isopropoxide, less reactive than zirconium propoxide, was chosen as source of titanium, used as an additive to the system PDMS-SiO2-CaO. Two different sol-gel preparation routes were followed, using the same base composition and calcium acetate as calcium source. Different microstructures with high hydrophobicit were obtained and both proved to be biocompatible after tested with MG63 osteoblastic cells. Finally, the role of strontium (typically known in bioglasses to promote bone formation and reduce bone resorption) was studied in the PDMS-SiO2-CaOTiO2 hybrid system. A biocompatible material, tested with MG63 osteoblastic cells, was obtained with the ability to release strontium within the values reported as suitable for bone tissue regeneration.

Evaluation of circulating endothelial progenitor cells by multicolor flow cytometry in chronic kidney disease patients

Endothelial dysfunction and impaired endothelial regenerative capacity play a key role in the pathogenesis of cardiovascular disease, which is one of the major causes of mortality in chronic kidney disease (CKD) patients. Circulating endothelial cells (CEC) may be an indicator of vascular damage, while circulating endothelial progenitor cells (EPC) may be a biomarker for vascular repair. However, the simultaneously evaluation of CEC and EPC circulating levels and its relation were not previously examined in CKD population. A blood sample (18ml) of healthy subjects (n=10), early CKD (n=10) and advanced CKD patients (n=10) was used for the isolation of early and late EPCs, CECs, and hematopoietic cells, identified by flow cytometry (BD FACSCanto™ II system) using a combination of fluorochrome-conjugated primary antibodies: CD31-PE, CD45-APC Cy7, CD34-FITC, CD117-PerCp Cy5.5, CD133-APC, CD146-Pacific Blue, and CD309-PECy7. Exclusion of dead cells was done according to a fixable viability dye staining. This eightcolor staining flow cytometry optimized protocol allowed us to accurate simultaneously identify EPCs, CECs and hematopoietic cells. In addition, it was also possible to distinguish the two subpopulations of EPCs, early and late EPCs subpopulation, by CD45intCD31+CD34+CD117-CD133+CD309-CD146- and CD45intCD31+CD34+CD117-CD133-CD309+CD146- multiple labeling, respectively. Moreover, the identification of CECs and hematopoietic cells was performed by CD45-CD31+CD34-/lowCD117-CD133-CD309-CD146+ and CD34+CD117+, respectively. The levels of CECs were non-significantly increased in early CKD (312.06 ± 91.34) and advanced CKD patients (191.43±49.86) in comparison with control group (103.23±24.13). By contrast, the levels of circulating early EPCs were significantly reduced in advanced CKD population (17.03±3.23) in comparison with early CKD (32.31±4.97), p=0.04 and control group (36.25 ± 6.16), p=0.03. In addition the levels of late EPCs were significantly reduced in both advanced (6.60±1.89), p=0.01, and early CKD groups (8.42±2.58), p=0.01 compared with control group (91.54±29.06). These results were accompanied by a dramatically reduction in the recruitment, differentiation and regenerative capacity indexes in CKD population. Taken together, these results suggest an imbalance in the process of endothelial repairment in CKD population, and further propose that the indexes of recruitment, differentiation and regenerative capacity of EPCs, may help to select the patients to benefit from guiding intervention strategies to improve cardiovascular health by inducing vascular protection.