Barbieri Thesis - BioMedical Materials program (BMM)

Chapter 4 – Control of mechanical and degradation properties in composites
a concentration of 0.1 g dL –1 and determined the intrinsic viscosity () using an
Ubbelohde (ASTM) viscometer (0C, PSL–Rheotek, Burnham on Crouch, United
Kingdom) at 25±0.1°C. The intrinsic viscosity of the polymers was calculated with the
Solomon–Ciută equation [356] as follows:
rel = tpol / tchlor
sp = rel – 1
= [sqrt(2 · (sp – ln(rel) ))] / ĉ
where tpol and tchlor are the measured time for the solution of polymer and of pure
chloroform to flow in the viscometer respectively, and ĉ is the polymer concentration
in chloroform. The effective content by weight (%wt.) of apatite (uapatite) and polymer
(upolymer) in the extruded composites was determined by burning the polymer out from
the composites in a sinter oven (C19, Nabertherm, Lilienthal, Germany) at 900±5°C
for two hours. We studied the surface chemistry of the composites (bars) with XRD
and CR as described in §4.2.1. Cross sections of the extruded materials were
observed with scanning electron microscopy (Philips XL 30 ESEM–FEG, Philips,
Eindhoven, the Netherlands) in the backscattered mode (BSEM) to evaluate the
distribution of apatite particles in the entire material. After making ultra–thin sections
of the three materials (thickness ~50 nm) using a diamond knife (ultra AFM knife,
Diatome AG, Biel, Switzerland), the distribution of apatite in the materials was
observed also using transmission electron microscopy (TEM, Tecnai–200FEG, FEI
Europe, Eindhoven, the Netherlands). Dynamic contact angle measurements were
performed to evaluate the surface hydrophilicity and drop spreading rate on the
surface of the composites. At room temperature a drop of 0.6 l of distilled water was
placed on the surface of the bars. Pictures were taken with a digital camera
(PowerShot SX200 IS, Canon Nederland NV, Amstelveen, the Netherlands)
immediately after the drop placement and then every 5 seconds for 9 minutes.
Measurements of the contact angle from the pictures were done with the software
ImageJ (v1.43u, NIH, USA) using the freely distributed plugin Drop–Shape Analysis
working on the snake–based approach. [335]
4.2.3. In vitro degradation of the composites
A saline physiological solution (SPS) was prepared by dissolving sodium chloride
(NaCl, Merck, c=8 g L –1 ) and 4–(2–hydroxyethyl)–1–iperazineethane–sulfonic acid
(HEPES, Sigma–Aldrich, c=11.92 g L –1 ) in distilled water. The pH of the solution was
adjusted to 7.3 with 2M NaOH (Merck) at 37°C. Sterile granules of each composite
(0.5±0.01 g) were carefully weighed before use (m0) and soaked in 200 mL SPS (in
triplicate) at 37±1°C for 3 months under a 3–week refreshing regime. Every three
weeks, the pH of the degrading solution was recorded with a pH–meter (Orion 4 Star,
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