Biologic Biomaterials: Silk - Biomaterials - page 163

for integrin-mediated cell adhesion [60-62], a sequence found in the silk fibroin sequence from the wild
silkworms, but not the domestic silkworms [56].
Regenerated silk fibroin has been used as a coating material for cell culture and tissue engineering
[63-67]. Coating poly(d,l-lactic acid) films with regenerated silk fibroin improved interactions between
osteoblasts and the polymer films [66,67]. The surface of 2D and 3D polyurethane scaffolds were coated
by dipping in 3-4% (w/w) silk fibroin solutions from B. mori , resulting in a stable silk fibroin coating
with a thickness of 200-600 nm [63]. The effect of silk fibroin coatings on 2D poly(carbonate)-urethane
substrates on attachment, proliferation, metabolism, and ECM synthesis of four strains of human fibro-
blasts was also evaluated [64]. Improved cell attachment, which resulted in a 2.5-fold increase in total
cell numbers by day 30 in culture, was found on the silk fibroin films. Concurrently, the silk fibroin
coating significantly affected the metabolism of fibroblasts, inducing higher glucose uptake and lower
glutamine consumption in the initial stages of cultivation. The coating also enhanced the extracellular
assembly of collagen type I (Col-I).
Fibroblasts seeded on silk fibroin-coated substrates did not secrete appreciable levels of cytokines
such as IL-1-, TNF--, or TGF--1, all of which are implicated in inflammation and tissue repair during
wound healing. However, IL-6 secretion, another important cytokine involved in inflammation reac-
tions and wound healing, was enhanced by the silk fibroin coating after 2 weeks. Using similar meth-
odology, the response of human fibroblasts seeded on silk-fibroin-coated 3D polyurethane scaffolds was
assessed [65], and cell attachment, proliferation, and cellular metabolism were also found, as above.
However, in comparison to the 2D substrates, expression of IL-6 with silk fibroin-coated 3D scaffolds
was not significantly affected, nor was the extracellular assembly. These studies provided an experimen-
tal basis for silk fibroin as a coating material for tissue-engineering scaffolds. An all-aqueous stepwise
(layer-by-layer) deposition technique was used to assemble nanoscale thin-film silk fibroin coatings on
a number of substrates, and the response of human bone marrow mesenchymal stem cells (hMSCs) to
the coatings was assessed [68]. Mechanistically, the main driving force were hydrophobic interactions
and partial electrostatic interactions for the deposition and stabilization of the silk fibroin on the solid
substrate surfaces, thus both hydrophilic and hydrophobic materials could be coated. The thickness of
the multilayered film coatings was linearly correlated with the number of layers, each of which had a
controlled thickness in the range of a few to tens of nanometers depending on the concentration of silk
fibroin used in the process and the level of salt. The silk fibroin underwent a structural transition from a
mixture of random coil and α-helices (silk I) to organized β-sheets (silk II structure) and supported the
attachment, proliferation, and differentiation of the hMSCs. This simple, yet versatile, technique has the
potential to be used to generate silk fibroin films with controlled morphological and structural features
for clinical applications such as drug delivery and tissue engineering.
For drug delivery, nanolayer coatings of silk fibroin containing model small-molecule drugs and pro-
teins, such as rhodamine B and azoalbumin, were studied in Ref. [69], as were heparin, paclitaxel, and
clopidogrel for vascular systems [70]. Cell attachment and viability with human aortic endothelial cells
and human coronary artery smooth muscle cells on the drug-incorporated silk coatings demonstrated
that paclitaxel and clopidogrel inhibited smooth muscle cell proliferation and retarded endothelial cell
proliferation [70]. The silk multilayers with heparin promoted human aortic endothelial cell prolifera-
tion while inhibiting human coronary artery smooth muscle cell proliferation, which was a desired
outcome for the prevention of restenosis [70]. Solid adenosine powder reservoirs coated with silk fibroin
were investigated for local and sustained delivery of the anticonvulsant adenosine from the encapsulated
reservoirs [71]. These reports demonstrated that silk coatings are effective for drug-eluting coatings.
7.4.3 Surface-Decorated Silk Fibroin Films
Surface modifications with Arg-Gly-Asp (RGD) or specific growth factors/integrin binding sites have
also been pursued with silks. RGD coupling, via carbodiimide chemistry, to silk fibroin films and fibers
for attachment, spreading, proliferation, and differentiation of human Saos-2 osteoblasts, fibroblasts,