Electrodeposited Assembly of Additive-Free Silk Fibroin Coating from Pre-Assembled Nanospheres for Drug Delivery

Abstract

Electrophoretically deposited (EPD) polymer-based coatings have been extensively reported as reservoirs in medical devices for delivery of therapeutic agents, but control over drug release remains a challenge. Here, a simple but uncommon assembly strategy for EPD polymer coatings was proposed to improve drug release without introducing any additives except the EPD matrix polymer precursor. The added value of the proposed strategy was demonstrated by developing a novel EPD silk fibroin (SF) coating assembled from pre-assembled SF nanospheres for an application model, that is, preventing infections around percutaneous orthopedic implants via local delivery of antibiotics. The EPD mechanism of this nanosphere coating involved oxidation of water near the substrate to neutralize SF nanospheres, resulting in irreversible deposition. The deposition process and mass could be easily controlled using the applied EPD parameters. In comparison with the EPD SF coating assembled in a conventional way (directly obtained from SF molecule solutions), this novel coating had a similar adhesion strength but exhibited a more hydrophobic nanotopography to induce better fibroblastic response. Moreover, the use of nanospheres as building blocks enabled 1.38 and 21 times enhancement on the antibiotic release amount and time (of 95% maximum dug release), respectively, while retaining drug effectiveness and showing undetectable cytotoxicity. This unexpected release kinetics was found attributable to the electrostatic and hydrophobic interactions between the drug and nanospheres and a negligible initial dissolution effect on the nanosphere coating. These results illustrate the promising potential of the pre-assembled strategy on EPD polymer coatings for superior control over drug delivery.

Keywords: coating; drug delivery; electrodeposition; pre-assembly; silk fibroin.

Figure 1

EPD assembly mechanism of SFN coating. (a) Pre-assembly vs (b) conventional assembly of SF EPD coatings. (c) Scanning electron micrographs of SFNs. (d) ζ-Potential and particle size of SFNs as a function of pH. (e) Digital photographs of SFNs showing the stability of SFNs aqueous solution as a function of pH. Error bars represent one standard deviation.

Figure 2

EPD parametric influence on SFN coating thickness. Thickness of the SFN coating as a function of EPD processing parameters, including (a) suspension concentration, (b) electric field, and (c) deposition time. Error bars represent one standard deviation.

Figure 3

Material characterization SFN coating. (a) Scanning electron micrographs of coatings. (b) FTIR absorbance spectra of the amide I region (between 1695 and 1595 cm^–1) showing the conformational changes during the preparation process of coatings. WA: after water annealing. AD: after air drying. E-gel: electrogel. sol: solution. sus: suspension. (c) Surface roughness of coatings. (d) Surface wettability determined by water contact angle measurements and representative images of water droplets. (e) Remaining mass of coatings immersed in PBS after 1, 3, 7, and 14 days. (f) Adhesion strength of coatings measured by lap shear tensile testing. Error bars represent one standard deviation (*p < 0.05).

Figure 4

Cellular response to SFN coating. (a) Cell spreading after 24 h shown as immunofluorescent images of vinculin (purple), F-actin (white), and nucleus (blue), corresponding heatmap, and scanning electron micrographs. Quantitative analysis of (b) FA area per cell, (c) CSI, and (d) cell area. (e) Cell proliferation measured by CCK-8. Relative mRNA expression level of (f) vinculin and (g) RhoA. For each box plot, the box boundaries represent the 25–75% quartiles, and the whiskers represent the minimum and maximum value. Error bars represent one standard deviation (*p < 0.05).

Scheme 1. Schematic Illustrating Mechanisms of Drug Loading and Release from (a) Pre-Assembled SFNV Coatings vs (b) Conventional SFMV Coatings

Figure 5

Drug loading onto SFN coating. (a) Encapsulation efficiency and loading content of SF nanospheres as a function of the weight ratio. (b) Particle size and (c) ζ-potential and of SF nanospheres and drug-loaded SF nanospheres. (d) Scanning electron micrographs of SFNV and SFMV coatings. (e) FTIR spectra showing pure vancomycin powder, SFM, SFV, SFN, and SFNV coatings. Error bars represent one standard deviation (*p < 0.05).

Figure 6

Drug Release from SFN coating. (a) Vancomycin release profiles shown as the cumulative release amount, and the dashed line indicating the maximum release amount. (b) Vancomycin release profiles shown as a cumulative release percentage and the dashed line indicating 95% maximum release amount with arrows indicating when it arrives. (c) MIC tests showing the antibacterial bioactivity of SFNV and SFMV coatings at different time points. (d) Cytocompatibility of SFNV and SFMV coatings. Vancomycin release kinetics from SFN coatings in media of different (e) ionic strength and (f) detergent concentrations. Error bars represent one standard deviation (*p < 0.05).