V. K. Popov

Supercritical fluid mixing: preparation of thermally sensitive polymer composites containing bioactive materials

We report the use of supercritical carbon dioxide (scCO2) to create a diverse range of polymeric composites incorporating thermal and solvent labile guest materials such as proteins; no additional co-solvents are required; the entire process can be carried out at near ambient conditions; polymer morphology is controllable; high loadings of guest species can be achieved and the protein function is retained.

Supercritical carbon dioxide: putting the fizz into biomaterials

This paper describes recent progress made in the use of high pressure or supercritical fluids to process polymers into three-dimensional tissue engineering scaffolds. Three current examples are highlighted: foaming of acrylates for use in cartilage tissue engineering; plasticization and encapsulation of bioactive species into biodegradable polyesters for bone tissue engineering; and a novel laser sintering process used to fabricate three-dimensional biodegradable polyester structures from particles prepared via a supercritical route.

Osteoblast growth on titanium foils coated with hydroxyapatite by pulsed laser ablation.

Pulsed laser ablation is a new method for deposition of thin layers of hydroxyapatite (HA) on to biomaterial surfaces. In this paper, we report activity and morphology of osteoblasts grown on HA surfaces fabricated using different laser conditions. Two sets of films were deposited from dense HA targets, at three different laser fluences: 3, 6 and 9 Jcm(-2). One set of the surfaces was annealed at 575 degrees C to increase the crystallinity of the deposited films. Primary human osteoblasts were seeded onto the material surfaces and cytoskeletal actin organisation was examined using confocal laser scanning microscopy. The annealed surfaces supported greater cell attachment and more defined cytoskeletal actin organisation. Cell activity, measured using the alamar Blue assay, was also found to be significantly higher on the annealed samples. In addition, our results show distinct trends that correlate with the laser fluence used for deposition. The cell activity increases with increasing fluence. This pattern was repeated for alkaline phosphatase production by the cells. Differences in cell spreading were apparent which were correlated with the fluence used to deposit the HA. The optimum surface for initial attachment and spreading of osteoblasts was one of the HA films deposited using 9 J cm(-2) laser fluence and subsequently annealed at 575 degrees C.

Physical, chemical, and biological characterization of pulsed laser deposited and plasma sputtered hydroxyapatite thin films on titanium alloy

The physical, chemical, and biological properties of pulsed laser deposited (PLD) and plasma sputtered (PS) hydroxyapatite (HA) coatings were compared. Human osteoblast-like cell responses to these coatings in vitro were assayed for proliferation and phenotypic expression. PS coatings formed smooth and continuous thin films that followed the contours of the substrate surface. PLD coatings consisted of numerous spheroidal micro- and macroparticles. The crystallinity of all coatings was quantified by comparison with the HA target used for both the PS and PLD processes. The XRD and FTIR results indicated that unannealed PLD coatings deposited at room temperature had X-ray spectra consistent with an amorphous structure and were found to dissolve after only a few hours in saline solution. Annealing at 400 degrees C increased the crystallinity (87-98%), which resulted in improved stability and cell activity. The PS coatings showed greater chemical stability than the unannealed PLD coatings and contained an approximate 15% crystalline phase, increasing to 65% postannealing. Cell proliferation and alkaline phosphatase production were significantly higher on unannealed PS specimens than the other coating treatments. There may be benefits in engineering the presence of a minor percentage of a microcrystalline phase in an amorphous or nanometer scale polycrystalline HA structure.

Putting the fizz into chemistry: applications of supercritical carbon dioxide in tissue engineering, drug delivery and synthesis of novel block copolymers.

This paper describes the recent progress at Nottingham towards the exploitation of the unique properties of scCO(2) (supercritical carbon dioxide) for the preparation of polymeric scaffolds for tissue engineering applications and new devices for controlled drug delivery, as well as the synthesis of novel block copolymers by the combination of eROP (enzymatic ring opening polymerization) and controlled polymerization methods for the potential use as drug carriers.

Biocompatibility and osteogenic potential of human fetal femur-derived cells on surface selective laser sintered scaffolds

For optimal bone regeneration, scaffolds need to fit anatomically into the requisite bone defects and, ideally, augment cell growth and differentiation. In this study we evaluated novel computationally designed surface selective laser sintering (SSLS) scaffolds for their biocompatibility as templates, in vitro and in vivo, for human fetal femur-derived cell viability, growth and osteogenesis. Fetal femur-derived cells were successfully cultured on SSLS-poly(d,l)-lactic acid (SSLS-PLA) scaffolds expressing alkaline phosphatase activity after 7days. Cell proliferation, ingrowth, Alcian blue/Sirius red and type I collagen positive staining of matrix deposition were observed for fetal femur-derived cells cultured on SSLS-PLA scaffolds in vitro and in vivo. SSLS-PLA scaffolds and SSLS-PLA scaffolds seeded with fetal femur-derived cells implanted into a murine critical-sized femur segmental defect model aided the regeneration of the bone defect. SSLS techniques allow fabrication of biocompatible/biodegradable scaffolds, computationally designed to fit any defect, providing a template for cell osteogenesis in vitro and in vivo.

Macroparticle distribution and chemical composition of laser deposited apatite coatings

We have studied the effect of pulsed laser ablation conditions on the deposition of biocompatible apatite coatings on Ti and Ti–6Al–4V alloy at room temperature. We have made detailed analyses of the spatial distribution of the macroparticles (MP) and of the Ca/P ratio in the coatings. We find that (i) two types of MP are observed, differing in size, shape, and stoichiometry, and (ii) the size distribution of the MP has a maximum depending on the laser fluence and gas pressure in the deposition chamber. Manipulation of the laser deposition conditions allows fine control over both morphology and stoichiometry of coatings. Experimental results are explained on the basis of a theoretical model which includes the analysis of cluster‐type ablation mechanisms due to the high pressures of gas evolved in thermal decomposition of the target material under laser irradiation.

The morphological stability in supercritical fluid chemical deposition of films near the critical point

Abstract In this paper, the results of experimental and theoretical studies of the chemical deposition of copper films from metalorganic compounds dissolved in supercritical C 2 F 6 are reported. The optimal conditions for the growth of highly adherent Cu films with good surface morphology have been determined. A theoretical analysis of the kinetics, the stability of the growth interface together with the transport phenomena inside the supercritical cell shows that the morphological stability is determined by the interplay of three factors. These are the bulk diffusion near the interface, the thermally activated kinetics, and the heat transfer across the deposited layer. It is shown that the morphological stability of the grown film is ensured by an enhanced turbulent convection occurring if the operation pressure and temperature are close enough to the critical point.

Atomic force microscopic study of the surface morphology of apatite films deposited by pulsed laser ablation

Atomic force microscopy (AFM) has been used to study the surface morphology of apatite films deposited on metallic and polyethylene substrates by laser ablation using KrF and transversely excited atmospheric CO2 lasers. The films are found to consist of a smooth apatite coating with macroparticles scattered on the surface. A wide variety of macroparticles, differing in size, shape and roughness, were found and analysed employing the high spatial resolution of AFM (< 1 nm). We have investigated the correlation between the apatite film morphology and the deposition conditions. Of particular importance are laser fluence, gas pressure, the nature of the target and the substrate temperature. We have explained these dependencies on the basis of a theoretical model which includes evaporation and a cluster-type laser ablation mechanism.

The fibre optic reflectometer: A new and simple probe for refractive index and phase separation measurements in gases, liquids and supercritical fluids

A fibre optic reflectometer (FOR) has been developed to monitor refractive index (n) and density (ρ) changes in gases, liquids and supercritical fluids (SCFs). The key operating principle is the measurement of the intensity of light from a light emitting diode, LED, which is reflected from the end of a fibre immersed in the medium. The amount of reflected light depends on the difference between the refractive indices of the fibre and the medium. The refractive index of the medium is related in turn to its density. The FOR is particularly sensitive to phase separation, a key aspect of supercritical fluid systems, because of the density discontinuity which accompanies the separation. This device allows the collection of data with a high spatial (9 μm-fibre diameter) and time (∼0.02 s) resolution. The small size of the fibre optic probe means that it can be put into operating reactors and even catalyst beds. Experiments are described with pure CO2 and CHF3, and mixtures CO2 + cyclohexane and CO2 + MeOH over wide pressure and temperatures ranges. The measurements are in excellent agreement with published data for these systems.