Influence of the Processes Parameters on the Properties of The ...

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Chapter 5. Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering (A)-dP/dt = 1 bar/s d e = 264μm (B)-dP/dt = 20 bar/s d e = 100μm PLGA 85:15 scaffold manufactured at, T sat = 35°C ; t sat = 60 min ; P sat = 100 bar Figure 5.25: Micrographs revealing <strong>the</strong> effect <strong>of</strong> dP/dt and dT/dt on pore size. Pore size decreases with increasing <strong>the</strong> dP/dt. A rapid drop in temperature implies faster vitrification <strong>of</strong> polymer that blocks <strong>the</strong> growth <strong>of</strong> pores and <strong>the</strong> average pore diameter decreases. It remains one important phenomenon which deserves a proper consideration. As mentioned previously in section 2.3, when <strong>the</strong> same dP/dt is applied from two different saturation pressures, a difference occurs between <strong>the</strong> final temperatures measured at <strong>the</strong> gas output. This can be explained by <strong>the</strong> increasing heating effect. Indeed, <strong>the</strong> same rate <strong>of</strong> depressurization from different saturation pressures, does not give <strong>the</strong> same depressurization time. Thus, <strong>the</strong> depressurization time from a high saturation pressure is greater and it causes an increase <strong>of</strong> <strong>the</strong> heating time <strong>of</strong> <strong>the</strong> pressure chamber. Hence, <strong>the</strong> vitrification <strong>of</strong> <strong>the</strong> scaffolds is encouraged by <strong>the</strong> sudden temperature drop from smaller saturation pressures. This can be <strong>the</strong> reason why <strong>the</strong> effect <strong>of</strong> <strong>the</strong> saturation pressure is found to be smaller than <strong>the</strong> effect <strong>of</strong> <strong>the</strong> depressurization rate in <strong>the</strong> analysis <strong>of</strong> <strong>the</strong> Doehlert’ design. On <strong>the</strong> o<strong>the</strong>r hand, since <strong>the</strong> desorption-diffusion coefficients are lower with lower saturation pressures than higher saturation pressures (cf. Table 5.8), <strong>the</strong> polymer processed with lower saturation pressures vitrifies later. 4.7 Effect <strong>of</strong> <strong>the</strong> Geometry <strong>of</strong> <strong>the</strong> Pressure Chamber We have to remember that <strong>the</strong> PLGA 50:50 exposed to high pressurized CO 2 , changes its state from glassy to rubbery. When <strong>the</strong> depressurization occurs from high pressurized saturation <strong>of</strong> <strong>the</strong> polymer, <strong>the</strong> polymer swells as <strong>the</strong> CO 2 desorbs and expands as a result <strong>of</strong> <strong>the</strong> phase separation. Figure 5.26: Representation <strong>of</strong> <strong>the</strong> effect <strong>of</strong> <strong>the</strong> geometry <strong>of</strong> <strong>the</strong> pressure chamber on <strong>the</strong> porous structure. - 140 -
Chapter 5. Characterization <strong>of</strong> Scaffolds for Connective Tissue Engineering Actually, this swelling is <strong>the</strong> result <strong>of</strong> <strong>the</strong> pore growth. When a slow depressurization happens, <strong>the</strong> polymer swells slowly and since <strong>the</strong> temperature drop is also slow, <strong>the</strong> vitrification happens later. Thus, when <strong>the</strong> polymer swells enough to touch <strong>the</strong> edges <strong>of</strong> <strong>the</strong> pressure chamber, <strong>the</strong> edges are behaving like a volume constraint and <strong>the</strong> pores which are closer to <strong>the</strong> edges is growing more than <strong>the</strong> pore which are in <strong>the</strong> centre <strong>of</strong> <strong>the</strong> scaffold, indeed, due to <strong>the</strong> blocking <strong>of</strong> desorption <strong>of</strong> <strong>the</strong> CO 2 by <strong>the</strong> wall edges (cf. Figure 5.26). The CO 2 inside <strong>the</strong> pores expands as <strong>the</strong> pressure and <strong>the</strong> temperature decreases which results in <strong>the</strong> growth <strong>of</strong> <strong>the</strong>se pores. This behaviour is mostly related to <strong>the</strong> much plasticized state <strong>of</strong> <strong>the</strong> polymer. For this reason, we have changed our experimental setup. We have removed <strong>the</strong> Teflon isolation which was restricting <strong>the</strong> diameter <strong>of</strong> <strong>the</strong> working area inside <strong>the</strong> chamber. We have replaced <strong>the</strong> volume by small glass balls with 3 mm <strong>of</strong> diameter, and we have placed a metal grill with holes on <strong>the</strong>se glass balls. As a result, we have gained approximately 50% more area which was sufficiently enough to prevent <strong>the</strong> volume constraint. 4.8 Interconnectivity and Coalescence Behaviour <strong>of</strong> <strong>the</strong> Scaffolds Our image analysis on micrographs and calculations shows <strong>the</strong> decrease <strong>of</strong> <strong>the</strong> average pore density per unit volume, with <strong>the</strong> increasing pore size. This trend is in agreement with <strong>the</strong> literature work [Barry et al., 2006]. The coalescence phenomenon is <strong>the</strong> master bone <strong>of</strong> <strong>the</strong> interconnectivity <strong>of</strong> <strong>the</strong> pores. During <strong>the</strong> growth, two pores join to create one, and reduce <strong>the</strong> pore density. This decrease in <strong>the</strong> pore density confirms <strong>the</strong> bubble coalescence <strong>the</strong>ory proposed in <strong>the</strong> literature [Rodeheaver and Colton, 2001]. Since <strong>the</strong> pore density is decreasing with <strong>the</strong> increasing depressurization time (foaming time), longer foaming times results in more pores to coalesce. An example to this phenomenon is presented in Figure 5.27-(A). A dP/dt <strong>of</strong> 0.056 bar/s has been applied to create this scaffold and it has been observed that <strong>the</strong>re is only one giant pore. An o<strong>the</strong>r example in Figure 5.27-(B), dP/dt <strong>of</strong> 1 bar/s was applied to create scaffold but a collapse <strong>of</strong> large pores was observed in SEM micrograph. On <strong>the</strong> o<strong>the</strong>r hand, we must admit, it is not easy to interpret <strong>the</strong> interconnectivity behaviour only by SEM micrographs. Thus, fur<strong>the</strong>r investigation is necessary in order to quantify <strong>the</strong> degree <strong>of</strong> interconnectivity. Procedures like mercury porosimetry or µ- tomography can be considered for such analysis. (A)- PLGA 50:50 (B)- PLGA 85:15 P sat = 100 bar, T sat = 36.5°C, t sat = 60 min. and dP/dt = 0.056 bar /s. P sat = 200 bar, T sat = 45°C, t sat = 20 min. and dP/dt = 1 bar /s. Figure 5.27: Micrographs depicting coalescence and collapse <strong>of</strong> pores. - 141 -
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Institut National Polytechnique de
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Publications and Conferences The wo
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Nomenclature and Abbreviations Abbr
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ρ p Pellet density Splitting (Bra
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Chapter 2 .........................
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