Silk Fibroin Protein
There has been much recent interest in silk fibroin in the biomedical disciplines in recent years, stemming from its biocompatibility, resorbability, and excellent mechanical properties. Although initially its biomedical use was in scaffolds for tissue engineering and regenerative medicine applications, silk has also shown recent promise in areas as diverse as drug delivery and optics. This is due to its wide range of available mechanical properties, leading to variable flexibility, hydration swelling, and in vivo degradation rate. These parameters can be controlled simply though process control of silk water content and protein secondary structure conformation. Combined with fabrication techniques designed for fabrication of traditional electronic components on silk substrates, silk films, in particular, show promise for the expanding role of silk at the interface between biology and high technology.
Eliciting no immune response, encapsulation of an implantable device in silk would help to make the contained components “invisible” to the in vivo environment. Reducing the interaction between contained electronic and optical components and the surrounding tissue not only prevents harm to the tissue by the components, but also prevents harm to the electronics by the tissue, and premature degradation of the device. Silk device encapsulation has recently been studied, and has furthermore been shown to help create transient degradation in biodegradable devices.
For multilayer devices, the functions of each layer determine the optimal properties of the substrate (i.e. flexible yet water stable for electronics fabrication), and a strong interface between these layers will be essential to successful all-in one fabrication. In such a model for silk devices rapid multilayer fabrication could allow for complex devices to be constructed in individual layers, and then assembled into a completed implantable construct. We are working to establish techniques to generate laminates with bond strengths on the order of 3MPa, and allow for combination of layers of varying individual mechanical and physical properties Thus, by leveraging the complex mechanical phase space, silk fibroin fabrication would be an effective means of combining the various layers into a singular device.
The controllable degradation properties further make silk an excellent matrix for stabilization of active biological and chemical components. Several studies have been conducted utilizing silk as a vehicle for achieving controllable drug release profiles, and it has just been discovered that silk can provide excellent stability to active small molecule and viral pharmaceuticals. This could extend the shelf life of the viral component of an all in one device, and allowing for the possibility of an off-the-shelf implant, as well as allowing for release profile control to ensure efficacy of the treatment as well as safety in the patient or study model.
Additionally, nanoimprinting techniques could be used to spatially modulate light, and provide options for nanoscale devices, without the use of any additional materials. Recent research in this area has established a high throughput technique for silk-silk imprinting, allowing for 100nm resolution imprinting across several generations, with sub one-minute timescales.
Combining all of these available properties, with silk fibroin acting as a bridge, a multilayer biocompatible devices can be constructed with modular assembly, bringing controllable drug release and spatial light modulation to a defined in vivo location, in a robust, flexible, all-in-one package with extended shelf life.
Implantable, Transient Devices
In collaboration with researchers in John Rogers’ Group at the University of Illinois Urbana Champaign (UIUC), we have been investigating implantable silk devices that last for non-infinite timescales. Apart from the silk related transience achieved through the methods discussed above, the devices include bioresorbable semiconductor and metal components for completely dissolvable passive and active devices. The electronics are made of magnesium conductors and interconnects with silicon nanomembrane based active components. These dissolve in seconds upon immersion in water, and can have their life extended through deposition of additional oxide protection layers. By combining the power of these dissolvable electronics with our suite of silk fabrication tools, we seek to build implantable devices with triggerable degradation profiles for both short and long term medical implants with no need for device recovery.