Salt Ions-Directed Hierarchical Organization of Silk Fibroin for Novel Polymeric Materials

Polymeric materials, ubiquitous in daily life and substantially underlying technology and economy, stem from the organization of polymeric chains across multiple length scales, which determines the properties and functions via a well-established processing-property relationship. However, most polymeric materials are characterized by non-renewable feedstocks, bulky and energy-intensive processes, such as thermoforming and compression molding, and environmentally persistent waste. The bulky formation may be questionable because of the less precision in molecule manipulation, leading to compromised functionality and various wastes. Other characteristics of polymer processing raise concerns over energy and environmental sustainability, reflected by the depletion of fossil resources, considerable greenhouse emissions, and the accumulation of microplastics. These issues pose critical barriers to promoting the sustainable growth and prosperity of our societies on the global level and, therefore, highlight an urgent requirement for innovative molecular mechanisms in the organization of polymer chains toward novel polymeric materials.

Here, I propose a mechanism to organize polymeric chains into materials based on specific ion effects on silk fibroin, a structural protein macromolecule derived from silkworm cocoons (heavy chain MW: 390 kDa) and an analog to block copolymer. Salt ions have long been known for their capability, e.g., ranked in the Hofmeister series, to precipitate proteins from aqueous solutions, which now could be explained by the competition between salt ions and polypeptide chains for water molecules. Such sophisticated molecular-ion interactions are usually through relatively weak forces, such as hydrogen bonding and hydrophobic forces; it is also associated with the properties of ions, such as volume, hydration capacity, polarizability, and surface tension, as well as the local surface chemistry on the backbone and the amino acid residues of polypeptide chains. Notably, the rich information in such interactions may be vital in eliminating the need for intensive energy input.

We have demonstrated that salt ions in aqueous solutions are capable of organizing silk fibroin molecules into water-insoluble materials, unlike protein precipitates, most likely via a sequence of assembling events at hierarchical length scales, including secondary structure formation, nano-fibrillogenesis, and macroscopic phase separation. Furthermore, this salt ion-based organization is compatible with multiple fabrication approaches, including extrusion-based 3D printing, spin coating, and casting, all of which are at ambient and aqueous conditions and exhibit appropriate manufacturing performance in shape control and mechanical strength. Finally, it implies a strategy to use salt ions to manipulate molecular organization and, consequently, the properties of silk fibroin material, which often involves secondary structures and structural morphology. For example, NaCl leads to an opaque appearance along with nanoporous morphology and a higher ratio of β-sheets, while K₂HPO₄ results in visual transparency along with dense morphology and fewer β-sheets.

In conclusion, the salt ion-directed hierarchical assembly of silk fibroin is a promising mechanism for devising novel polymeric materials. It is primarily due to a collection of traits, including manipulation at the molecular level, renewable and degradable feedstocks from sericulture, ambient and aqueous processing conditions, and compatibility with fabrication methods to produce shape-controlled and structurally integral materials. Notably, a further understanding of specific ion effects on silk fibroin, via advanced characterizations and computational tools, could lead to unprecedented pathways of polymer organization, thus leading to new properties. It also could constitute essential principles for organizing other existing and de novo macromolecules into novel materials.