Optimizing Molecular Weight of Lyophilized Silk As a Shelf-Stable Source Material

Storage of silk proteins in liquid form can lead to excessive waste from premature gelation, thus an alternative storage strategy is proposed using lyophilization to generate soluble and shelf-stable powder formats for on-demand use. Initial solution stability studies highlighted instabilities of higher-molecular-weight silks that could not be resolved by solution modifications such as autoclaving, pH increases, dilution, or combinations thereof. Conversely, shelf-stable lyophilized stock powders of silk fibroin of moderate to low molecular weights were developed that could be fully constituted even after 1 year of storage at elevated temperatures. Increasing dried silk powder loading in aqueous solution facilitated increased silk solution concentrations-here up to 80 mg/mL solubility was demonstrated across a range of formulations. Powders generated from silk solutions with higher-molecular-weight distributions were less soluble than moderate or lower-molecular-weight versions, despite no differences in their solution glass-transition temperatures. Instead, the aggregation and β-sheet content of lyophilized higher molecular weight stock solutions were identified as the cause of the reduced powder solubility by circular dichroism and dynamic light scattering analyses. The solubility and molecular weight profiles of all formulations investigated were preserved after storing the lyophilized materials over 1 year, even at 37 °C. No long-term powder stability behaviors were influenced by the addition of a secondary drying step in the lyophilization procedure, suggesting that this protocol could be scaled without the burden of lengthy process times. Taken together, these findings provide a very flexible and potentially cost-saving approach to producing shelf-stable silk fibroin stock materials based on the use of moderate to lower-molecular-weight lyophilized preparations. This utility is demonstrated with the formation of silk material formats from the stored powders, including films, gels, and salt-leached porous scaffolds. In turn, a more efficient system allowing full resolubilization will enable stockpiling powder for on-demand usage and for deployment of dried silks for application demands in field settings.