The use of silk in medical devices

Recent innovations of the use of Silk in medical devices.

Sutures made of silk have been used by humans for centuries. This article looks at some recent innovations that show the many diverse applications of silk in a medical context.

There are many factors to consider when designing a medical device, and in the case of invasive devices, one of the more significant factors is the choice of materials from which the device is made. The properties of the material must match those required for a particular use. For example, it may be advantageous for heart defect closure devices to be bioresorbable, but this would be a definite disadvantage to a pacemaker. Invasive medical devices must above all else be biocompatible, which is defined as “the capability of a prosthesis implanted in the body to exist in harmony with tissue without causing deleterious changes”. Whilst it may take many years to perfect the materials used for such medical devices, one material has been millions of years in the making: silk.

Silks are produced by members of the class Arachnida (e.g. orb-weaving spiders such as Nephila clavipes), and by several species of worm in the order Lepidoptera (silkworms such as Bombyx mori). Silk is mainly made from the protein fibre fibroin, and in nature provides a structural role in cocoon formation, nest building, web formation, and egg protection. The particular properties that evolution has bestowed upon silk (impressive mechanical properties, including high ultimate tensile strength and Young’s modulus, not to mention biocompatibility) has resulted in the extensive study of silk as a biomaterial. To date, spider silk has yet to be produced on a commercial scale, but silk produced from silkworms has been farmed and exploited for millennia, and is readily obtainable.

One novel use of silk in a medical device was explored in Perrone et al (Nature Communications/DOI: 10.1038/ncomms4385) in their article “The Use of Silk-based Devices for Fracture Fixation”. The article describes screws made of silk that could replace the current gold-standard metallic fixation systems. Metal fracture fixation devices can cause a reduction in bone density due to stress shielding, and furthermore must subsequently be removed. Attempts at producing bioresorbable devices that do not need removal have been made, but these have produced an unacceptable inflammatory response, with negative impacts on bone remodelling. The silk-based devices that were studied however are seemingly advantageous in many respects, including being self-tapping, bioresorbable, and also biocompatible by exhibiting a minimal inflammatory response.

The notion of using silk-based devices, such as screws, to promote bone growth and speed the healing process was also considered by James et al in their article “Silk Biomaterials-Mediated miRNA Functionalised Orthopaedic Devices (Tissue Engineering DOI: 10.1089/ten.TEA.2017.0455). This paper particularly aims to promote osteconductivity i.e. where the device acts as a scaffold for new bone growth. The paper concerned the microRNA molecule miR-214, which is very important is normal skeletal development. It was found that knockdown of miR-214 lead to the increase of bone formation. Silk screws were subsequently developed with a film of anti-sense miR-214, which blocks miR-214 locally and thus promotes bone growth at the site of the screw. Although early in development, the results of this paper are very encouraging.

Diverse medical uses of silk are also found in the patent literature.

Silk has been used in the laboratory as a scaffold for cell culture. One such patent application, WO2016/154428 in the name of The Board of Regents of the University of Texas System, describes a cell culture system including a silk fibroid scaffold, culture media, and pancreatic cells. The pancreatic cells grown in the tissue culture system have physiological and morphological features like those of in vivo pancreatic cells. The inventors found that using this scaffold, a greater number of high quality cells were produced.

Surgical meshes, in particular those used for tissue repair (especially hernias) have been used for over a century. Silk is one such material used in the creation of these meshes. US9308079 in the name of Allergen Inc., describes a recent development where the silk mesh has a particular knit pattern that substantially prevents unraveling and preserves the stability of the mesh when cut. The knitted mesh includes at least two yarns laid in a knit direction that are engaged with each other to define a plurality of nodes.

WO2017015387, filed in the name of several applicants including Tufts University, is directed to a silk-borne medical device for treating otitis. The application describes methods for making resorbable ear tubes, formed from silk fibroin solution.

WO2014176451, in the name of the Trustees of Tufts College, describes a bioresorbable biopolymer stent made from silk fibroin, which can be deployed within a blood vessel and resorbed by the body over a predetermined time period after the blood vessel has been remodeled. Furthermore, the stent can include a ratcheting mechanism to aid deployment. The stent can be expanded when in place, and the ratcheting mechanism locks the stent in the expanded configuration.

Making alternative use of silk’s properties, WO2014152097 in the name of Banner Pharmacaps Inc. describes a hard or soft capsule made from an interpenetrating network of a silk polymer and a film-forming natural polymer. The silk-based capsules have been found to be more robust, exhibit better seams (so do not leak), and possess improved stability under storage, as compared to analogous capsules prepared without the solubilized silk polymer. In addition, the interpenetrating network can function to cross-link the capsule shell. This can be advantageous in that it slows down the release of the drug within the capsule, which can be highly desirable for extended release formulations (e.g., for the delivery of active ingredients in the colon).

A further application of silk meshes is described in WO2014036210, in the name of Allergan Inc.  This application is directed to a silk mesh used for treating plantar fasciitis. The silk mesh optionally comprises a low dose corticosteroid.

The unique properties of silk mean that it has a diverse range of potential applications and it will be interesting to see the use it is put to in the future.

Should you wish to discuss this topic, please contact the author, Chris Burnett, or your usual AA Thornton contact.

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