Page 152 - Tailoring Electrospinning Techniques for Regenerative Medicine - Marc Simonet

CHAPTER 7
few GPa.22–24 In chapter 4 we used synthetic materials covering this Young’s modulus range, with the PCL having a Young’s modulus of a few hundred MPa and the PLA with a few GPa. Many native tissues exhibit two mechanical phases of stress strain response. A low stress linear elastic phase at low strains, where the wavy collagen fibrils are straightened/unfolded and a phase where the collagen fibrils are actually stretched. Since electrospun fibers are generally straight, sca olds can only be matched to one of the phases as shown with the extended stress-strain curves from chapter 4 (Figure 7.4).
Both LTE spun sca olds exhibit similar mechanics to the elastic phase up to 50% elongation whereas the conventionally spun sca olds show an imminent steep stress reaction with a modulus comparable to the collagen phase response, but without the typical strain hardening of native tissue (Figure 7.4b and d). As visible in Figure 7.4a and Figure 7.4c opposite to the HSV, none of these sca olds show a strain hardening at elevated stain rate nor should they be considered as fully elastic in the lower strain range.
A possible direct way to achieve the mechanical compliance might be to mimic the wavy structure of collagen fibrils Figure 7.5a as shown in Figure 7.5b by side by side electrospinning.26,27 While this technique has been known for quite a while, there is only a very limited number of studies for ligaments elucidating this technique to mimic native tissue mechanics.28–30
An additional benefit of these wavy fibrous
structures is the, for cell ingrowth beneficial, increased pore size, since these fibers cannot be as densely packed as electrospun straight fibers.
8.3.3 Combining technologies for complex hybrid structures
With the advances in knowledge and techniques, the regenerative medicine field moves towards more complex sca old structures. The desire to implement various functionalities, complying with mechanical demands and mimicking the tissue to be replaced, requires more and more complex and spatially ordered sca old structures (Figure 7.6).
Many additive manufacturing processes like fused deposition modeling (FDM) can create sca olds with a high degree of control on fiber deposition. A big advantage of these direct controlled 3D printing processes is their spatial control in building up a sca old, allowing to engineer patterned and mechanically strong structures. For regenerative medicine, the highest potential for additive manufacturing comprises hybrid sca olds, using the strength of di erent manufacturing techniques.31–34 Figure 7.7 shows a hybrid sca old, where PCL struts provide the hierarchical sca old structure and strength, electrospun fibers o er a more ECM like environment and the cells are deposited at defined locations.
While defined structures, as e.g. the one shown in Figure 7.7a can also be built with other techniques,
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