Controlled Lyophilization

Lyophilization, also called "freeze-drying", is a process by which the material is rapidly frozen and dehydrated under high vacuum [57, 58]. Controlled lyophilization techniques in which the rate of freezing and thawing can be tightly controlled, is a useful platform technology for engineering porous 3D scaffolds [59, 60]. The pore size and porosity of the ensuing scaffolds will depend primarily on the composition/concentration of the scaffold materials and the different parameters of the lyophilization protocol. Complex ECM extracts, such as Matrigel™ and other hydrogels, contain numerous differentiation cues, which are either not present in synthetic scaffolds or destroyed from natural proteins through conventional scaffold preparation techniques. Lyophilization, and to some extent also electrospinning, are rather gentle methods for creating 3D scaffolds that retain the complexity of the ingredients and the functionality of natural ECM proteins. Moreover, lyophilized ECM extracts can be used either directly as a porous scaffold or further processed by electrospinning to yield nanofibrous scaffolds, which retain bioactive proteins and growth factors necessary for tissue-specific cell proliferation and differentiation. Figure 1.2 illustrates SEM micrographs of lyophilized gelatin (Fig. 1.2A) and Matrigel™ (Fig. 1.2B) scaffolds, indicating the highly connected regular porous structure, which is particularly well suited for engineering vital organs, such as the liver.

Fig. 1.2 Typical images of natural scaffolds for tissue engineering. Scanning electron microscopy micrographs of (A) lyophilized gelatin and (B) Matrigel™ scaffolds.

10 | 1 Intelligent Biomatrices and Engineered Tissue Constructs 1.3.3

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