The remarkable features of silk fibroin (SF) from the silkworm (Bombyx mori) have fueled its application as a candidate biomaterial for tissue regeneration and repair. For an ideal scaffold, the rate of degradation should be synchronized to match the rate of new tissue formation, and tuning this rate is essential, as diverse tissues differ in terms of regeneration period. In this Review, we discuss the factors influencing the degradability of SF, which can vary from days to several months, depending on the state of the raw material, the scaffold preparation process, morphological features, and host factors. This knowledge facilitates strategies for tuning the SF degradation rate, including manipulation of molecular weight, crystalline level, and crosslinking degree. Since these strategies have a great influence on the mechanical properties, the superiority of SF has to be sacrificed to satisfy the requirements for degradation rate. We further explore additional strategies, including the incorporation of degradation-promoting supplements such as blending with another polymer (e.g., gelatin) and the incorporation of enzyme-sensitive peptides. The information in this Review will likely aid scientists working with SF materials for the regeneration of diverse tissues.

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Efficient and inducible recombinase-mediated DNA excision is an optimal technology for automatically deleting unwanted DNA sequences, including selection marker genes. However, this methodology has yet to be established in transgenic silkworms. To achieve efficient and inducible FLP recombinase-mediated DNA excision in transgenic silkworms, one transgenic target strain (TTS) containing an FRT-flanked silkworm cytoplasmic actin 3 gene promoter (A3)-enhanced green fluorescent protein (EGFP) expression cassette, as well as two different types of FLP recombinase expression helper strains were generated. Then, the FLP recombinase was introduced into the TTS silkworms by pre-blastoderm microinjection and sexual hybridization. Successful recombinase-mediated deletion of the A3-EGFP expression cassette was observed in the offspring of the TTS, and the excision efficiencies of the FLP expression vector and FLP mRNA pre-blastoderm microinjection were 2.38 and 13.3 %, respectively. The excision efficiencies resulting from hybridization between the TTS and the helper strain that contained a heat shock protein 70 (Hsp70)-FLP expression cassette ranged from 32.14 to 36.67 % after heat shock treatment, while the excision efficiencies resulting from hybridization between the TTS and the helper strain containing the A3-FLP expression cassette ranged from 97.01 to 100 %. These results demonstrate that the FLP/FRT system can be used to achieve highly efficient and inducible post-integration excision of unwanted DNA sequences in transgenic silkworms in vivo. Our present study will facilitate the development and application of the FLP/FRT system for the functional analysis of unknown genes, and establish the safety of transgenic technologies in the silkworm and other lepidopteran species.

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