Tunable self-assembly of genetically engineered silk--elastin-like protein polymers

Abstract

Silk--elastin-like protein polymers (SELPs), consisting of the repeating units of silk and elastin blocks, combine a set of outstanding physical and biological properties of silk and elastin. Because of the unique properties, SELPs have been widely fabricated into various materials for the applications in drug delivery and tissue engineering. However, little is known about the fundamental self-assembly characteristics of these remarkable polymers. Here we propose a two-step self-assembly process of SELPs in aqueous solution for the first time and report the importance of the ratio of silk-to-elastin blocks in a SELP's repeating unit on the assembly of the SELP. Through precise tuning of the ratio of silk to elastin, various structures including nanoparticles, hydrogels, and nanofibers could be generated either reversibly or irreversibly. This assembly process might provide opportunities to generate innovative smart materials for biosensors, tissue engineering, and drug delivery. Furthermore, the newly developed SELPs in this study may be potentially useful as biomaterials for controlled drug delivery and biomedical engineering.

Figure 1

Characterization of purified silk-elastin-like protein polymers (SELPs). (a) The repeats of the monomer genes and molecular weight (Mw) along with amino acid numbers of the encoded SELPs. (b) Coomassie-stained 4–12% SDS-PAGE gel analysis of purified SELPs. (c) MALDI-TOF spectrum of SELPs.

Figure 2

a) Constructs of silk-elastin-like proteins that contain varying ratios of the silk to elastin blocks in each monomer repeat. b) Representative SEM images of the spherical micellar-like particles derived from self-assembly of SE8Y, S2E8Y, and S4E8Y in aqueous solutions. c) DLS size distribution profiles for the SE8Y, S2E8Y, and S4E8Y solutions at 20°C.

Figure 3

Turbidity profiles of the silk-elastin-like protein solutions at 0.5 mg/mL as a function of temperature. Turbidity profiles were obtained by monitoring optical density at 300 nm as the aqueous solutions were heated (solid lines) and cooled (dash lines) at a rate of 3°C/min. Arrows indicate the locations of the transition.

Figure 4

Differential scanning calorimetry (DSC) of protein polymers SE8Y (blue line), S2E8Y (dark red line) and S4E8Y (green line), showing heat flow as a function of temperature. The concentrations of all samples were 1 mg/mL with a heating (solid lines) and cooling (dash lines) rate of 1°C/min.

Figure 5

Temperature-dependent CD spectra of a) SE8Y, b) S2E8Y and c) S4E8Y. Scans were performed at increasing (solid lines) and then decreasing temperatures (dash lines) with 10 min equilibration at each temperature: 4°C (green line), 37°C (pink line), 60°C (black line), and 95°C (blue line).

Figure 6

a) Representative AFM images of the nanostructures derived from self-assembly of SE8Y, S2E8Y and S4E8Y in aqueous solutions upon heating to 60°C and b) cooling back to 20°C. DLS size distribution profiles for c) SE8Y and d) S2E8Y particles formed upon heating from 20°C to 60°C and cooling back (dash lines).

Scheme 1

Proposed two-step self-assembly of silk-elastin-like protein polymers. Step 1: spontaneous formation of micellar-like particles; step 2: thermal responsive self-assembly of the particles into reversible coacervates or irreversible gel states.