Silk powder, left, is made into plastic medical devices (Chunmei Li and David Kaplan, Tufts University)
17 Dec. 2019. Engineers developed a more efficient process using heat to make items from raw silk into manufactured goods, including bio-compatible medical devices. A team from Tufts University in Medford, Massachusetts describes the process in yesterday’s issue of the journal Nature Materials (paid subscription required).
Researchers from the lab of biomedical engineering professor David Kaplan are seeking better ways to convert silk, a naturally occurring, strong, and durable material into medical supplies and devices. Most current methods, say the authors, involve chemical solvents that reduce the stability of fibroin proteins in silk that provide the material its superior properties. As a result, fabricating goods from silk often requires making trade-offs that sacrifice strength or durability for efficiency.
Kaplan’s lab studies silk’s polymer chemistry, aiming for new methods for capturing silk’s natural stability in manufactured goods. In their paper, the team uses heat to regulate the crystalline content of silk, allowing the material to be transformed into medical and other products with simple and direct methods much like other plastics. In addition, their process also allows for adjusting silk’s polymer chemistry to add other desired properties, such as therapeutic compounds, to the material.
The new process, borrowed in part from plastics manufacturing, converts freeze-dried silk fibroin proteins into a powder of nanoscale pellets in some cases as small as 30 nanometers, where 1 nanometer equals 1 billionth of a meter. These tiny silk protein pellets become the raw material for fabricating goods, and are stable enough to be stored and shipped to manufacturing sites in bulk like other raw materials.
Under pressure and temperatures ranging from 97 to 145 degrees Celsius, the crystalline content of the nanoscale pellets breaks down, then reassembles their molecules, allowing the material to be formed into intermediate materials or final products. In addition, this molecular rearrangement allows for adjusting the physical properties of the manufactured silk as needed for producing specific items, as well as adding therapeutic molecules, such as antibiotics or enzyme treatments to the material.
The researchers produced plastic bone screws from their silk fabrication process that in lab animals allows for formation of new bone tissue and are resorbed as new bone grows around them. In addition, the team discovered they could adjust the resorbtion rate of the silk-based plastic by changing the heat level during the manufacturing stage, which changes the crystalline structure and its water-absorbing ability. Further tests with plastic ear canals added an enzyme to the material’s chemistry that allows the piece to degrade faster than normal.
The Tufts team calls their process solid-state manufacturing. Kaplan notes in a university statement released through EurekAlert that “this new solid-state manufacturing approach can significantly cut the time and cost of producing many [medical devices] and offer even greater flexibility in their form and properties.” Kaplan adds that “this new approach avoids the complications with solution-based supply chains for the silk protein, which should facilitate scale up in manufacturing.”
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