Chemical Synthesis of Silk-Mimetic Polymers

Abstract

Silk is a naturally occurring high-performance material that can surpass man-made polymers in toughness and strength. The remarkable mechanical properties of silk result from the primary sequence of silk fibroin, which bears semblance to a linear segmented copolymer with alternating rigid ("crystalline") and flexible ("amorphous") blocks. Silk-mimetic polymers are therefore of great emerging interest, as they can potentially exhibit the advantageous features of natural silk while possessing synthetic flexibility as well as non-natural compositions. This review describes the relationships between primary sequence and material properties in natural silk fibroin and furthermore discusses chemical approaches towards the synthesis of silk-mimetic polymers. In particular, step-growth polymerization, controlled radical polymerization, and copolymerization with naturally derived silk fibroin are presented as strategies for synthesizing silk-mimetic polymers with varying molecular weights and degrees of sequence control. Strategies for improving macromolecular solubility during polymerization are also highlighted. Lastly, the relationships between synthetic approach, supramolecular structure, and bulk material properties are explored in this review, with the aim of providing an informative perspective on the challenges facing chemical synthesis of silk-mimetic polymers with desirable properties.

Keywords: bioinspired materials; macromolecular self-assembly; silk-mimetic polymers.

Figure 1

Tensile behavior of silk compared to various natural materials, including merino wool, silkworm silk (shown as “silk”), spider silk, and synthetic materials, including Kevlar 29, polyethylene terephthalate (PET), and nylon 6. The y-axis shows tensile stress (force per unit area) in GPa, and the x-axis shows tensile strain (extension normalized by initial length) as a dimensionless number. Copyright 2004 American Chemical Society, reproduced with permission from [44].

Figure 2

Mature silk threads consist of double silk fibroin cores coated by sericin (for silkworm silk) or a single spidroin core coated by lipids and glycoproteins (for spider silk). Importantly, silk fibroin and spidroin form nanofibrillar subunits within the cores.

Figure 3

(a) Schematic of silk fibroin primary sequence, showing a repetitive core domain consisting of alternating rigid and flexible blocks. Self-assembly of silk fibroin results in the formation of nanocrystals, which consist of “stacked” β-sheets with hydrophobic interactions between amino acid side chains extending orthogonal from the sheet plane, embedded in a hydrophilic amorphous matrix. (b) Silk fibroin from different species exhibit varying primary sequences, with corresponding differences in mechanical properties. A. diadematus ADF-4 dragline spidroin consists of alternating flexible “amorphous” (A) and rigid “crystalline” (C) blocks, where C is 6–9 alanines and the A-C length is typically 33–45 amino acids (aa) long. B. mori silk fibroin consists of A-C repeats ranging from 80–639 amino acids where C is rich in glycine-X motifs. E. variegata silk fibroin consists of A-C repeats approximately 160 amino acids long, where C and A blocks are divided into 2 distinct sections. C₁ specifically consists of a (A)₉E(A)₁₂ sequence, while C₂ and A₂ are glycine-rich. A₁ is a short linker rich in valine.

Figure 4

Schematic of step-growth polymerization using homobifunctional (left) or heterobifunctional (right) crystalline and amorphous prepolymers to synthesize silk-mimetic copolymers.

Figure 5

(a) Synthetic strategy used by Sogah and coworkers [96] to generate silk-mimetic polymers based on chain extension of an oligoalanine-PEG-oligoalanine triblock. Self-assembly of silk-mimetic polymers into β-sheet rich crystalline domains as shown by (b) solid-state FTIR and (c) powder XRD. FTIR spectra were obtained for P2 and P3, which are silk-mimetic ALA-PEG polymers were the ALA degree of polymerizations are approximately 4 and 6, respectively. P2 and P3 film samples were made by drop-casting from a 40% w/v solution dissolved in hexafluoroisopropanol. Resolution-enhanced spectra are shown (dotted lines a and c), as well as second derivative spectrums (solid lines b and d), showing the presence of anti-parallel β-sheets based on bands at 1630 and 1692 cm⁻¹. Powder XRD of P2 (solid line a) and P3 (solid line b) shows d spacings of 5.28, 4.41, and 3.77 Å, which are consistent with anti-parallel β-sheet spacings reported for N. clavipes silk. (b) and (c) are Copyright 2001 American Chemical Society, reproduced with permission from [96].

Figure 6

Synthesis of silk-mimetic polymers by Sogah and coworkers [97] with an aromatic hairpin to template parallel β-sheets.

Figure 7

(a) Chemical structure of polyisoprene and oligoalanine prepolymers used by Shao and coworkers [101] to synthesize silk-mimetic polymers based on step-growth polymerization. (b) Self-assembly of these silk-mimetic polymers (PEP-PI2200) into β-sheet structures as demonstrated by characteristic sharp Bragg peaks in WAXD compared to silk-mimetic polymers with a longer polyisoprene chain (PEP-PI5000) or polyisoprene alone (PI5000). (c) Formation “plum blossom” micellar structures by self-assembly of these silk-mimetic polymers. (b) and (c) are Copyright 2006 American Chemical Society, reproduced with permission from [101].

Figure 8

(a) Structural schematic of natural spider silk, which contains β-crystalline domains (green blocks) that act as heat-stable netpoints and hydrogen bonds (red dimer block) that act as moisture-sensitive netpoints, compared to (b) a synthetic bioinspired shape-memory polymer, which contains heat-stable silk-mimetic β-crystalline domains formed by oligoalanine and crystalline PCL domains (red rectangles) that act as heat-labile crosslinks. (c) The chemical structures of the PCL and oligoalanine-poly(propylene glycol) (PPG) segments of this bioinspired polymer are shown. While natural spider silk can undergo contraction and shape recovery when exposed to water or high humidity, these synthetic shape-memory polymers are temperature-sensitive, retaining a stretched state if cooled under tension and returning to their original shape upon re-heating. Copyright 2013 WILEY-VCH Verlag GmbH & Co, adapted with permission from [112].

Figure 9

(a) Chemoenzymatic synthesis of polypeptide prepolymers for step-growth polymerization of silk-mimetic polymers. First, amino acid esters are used as monomers for papain-catalyzed polymerization of rigid polyalanine and soft glycine-rich polypeptides. Polycondensation is then used to synthesize silk-mimetic multiblock polypeptides. (b) Self-assembly of these polymers results in silk-like supramolecular structure, including antiparallel β-sheets and nanofiber morphologies visible by AFM. Copyright 2017 American Chemical Society, adapted with permission from [117]. Article and further information regarding permissions can be accessed at this direct link: https://doi.org/10.1021/acsmacrolett.7b00006.

Figure 10

Synthetic strategy using polyfunctional trithiocarbonates as RAFT agents to synthesis silk-mimetic multiblock copolymers by sequential addition of methyl acrylate and N-acryloyl-L-phenylalanine monomers.

Figure 11

Multi-step strategy for the synthesis of silk-mimetic copolymers using naturally derived silk fibroin [128]. Here, (a) enzymatically fragmented silk fibroin (silk fibroin peptide, SFP) is (b) modified with vinyl acyl chloride and (c) then copolymerized with acrylonitrile.