Evaluation of the Spectral Response of Functionalized Silk Inverse Opals as Colorimetric Immunosensors

Regenerated silk fibroin is a high molecular weight protein obtained by purifying the cocoons of the domesticated silkworm, Bombyx mori. This report exploits the aqueous processing and tunable β sheet secondary structure of regenerated silk to produce nanostructures (i.e., inverse opals) that can be used as colorimetric immunosensors. Such sensors would enable direct detection of antigens by changes in reflectance spectra induced by binding events within the nanostructure. Silk inverse opals were prepared by solution casting and annealing in a humidified atmosphere to render the silk insoluble. Next, antigen sensing capabilities were imparted to silk through a three step synthesis: coupling of avidin to silk surfaces, coupling of biotin to antibodies, and lastly antibody attachment to silk through avidin-biotin interactions. Varying the antibody enables detection of different antigens, as demonstrated using different protein antigens: antibodies, red fluorescent protein, and the beta subunit of cholera toxin. Antigen binding to sensors induces a red shift in the opal reflectance spectra, while sensors not exposed to antigen showed either no shift or a slight blue shift. This work constitutes a first step for the design of biopolymer-based optical systems that could directly detect antigens using commercially available reagents and environmentally friendly chemistries.

Silk Fibroin as Edible Coating for Perishable Food Preservation

The regeneration of structural biopolymers into micelles or nanoparticles suspended in water has enabled the design of new materials with unique and compelling properties that can serve at the interface between the biotic and the abiotic worlds. In this study, we leveraged silk fibroin quintessential properties (i.e. polymorphism, conformability and hydrophobicity) to design a water-based protein suspension that self-assembles on the surface of food upon dip coating. The water-based post-processing control of the protein polymorphism enables the modulation of the diffusion of gases through the silk fibroin thin membranes (e.g. O₂ and CO₂ diffusion, water vapour permeability), which is a key parameter to manage food freshness. In particular, an increased beta-sheet content corresponds to a reduction in oxygen diffusion through silk fibroin thin films. By using the dip coating of strawberries and bananas as proof of principle, we have shown that the formation of micrometre-thin silk fibroin membranes around the fruits helps the management of postharvest physiology of the fruits. Thus, silk fibroin coatings enhance fruits’ shelf life at room conditions by reducing cell respiration rate and water evaporation. The water-based processing and edible nature of silk fibroin makes this approach a promising alternative for food preservation with a naturally derived material.

Modulated Degradation of Transient Electronic Devices through Multilayer Silk Fibroin Pockets

The recent introduction of transient, bioresorbable electronics into the field of electronic device design offers promise for the areas of medical implants and environmental monitors, where programmed loss of function and environmental resorption are advantageous characteristics. Materials challenges remain, however, in protecting the labile device components from degradation at faster than desirable rates. Here we introduce an indirect passivation strategy for transient electronic devices that consists of encapsulation in multiple air pockets fabricated from silk fibroin. This approach is investigated through the properties of silk as a diffusional barrier to water penetration, coupled with the degradation of magnesium-based devices in humid air. Finally, silk pockets are demonstrated to be useful for controlled modulation of device lifetime. This approach may provide additional future opportunities for silk utility due to the low immunogenicity of the material and its ability to stabilize labile biotherapeutic dopants.

Materials for programmed, functional transformation in transient electronic systems

Materials and device designs are presented for electronic systems that undergo functional transformation by a controlled time sequence in the dissolution of active materials and/or encapsulation layers. Demonstration examples include various biocompatible, multifunctional systems with autonomous behavior defined by materials selection and layout.

All-water-based electron-beam lithography using silk as a resist

Traditional nanofabrication techniques often require complex lithographic steps and the use of toxic chemicals. To move from the laboratory scale to large scales, nanofabrication should be carried out using alternative procedures that are simple, inexpensive and use non-toxic solvents. Recent efforts have focused on nanoimprinting and the use of organic resists (such as quantum dot-polymer hybrids, DNA and poly(ethylene glycol)), which still require, for the most part, noxious chemicals for processing. Significant advances have been achieved using 'green' resists that can be developed with water, but so far these approaches have suffered from low electron sensitivity, line edge roughness and scalability constraints. Here, we present the use of silk as a natural and biofunctional resist for electron-beam lithography. The process is entirely water-based, starting with the silk aqueous solution and ending with simple development of the exposed silk film in water. Because of its polymorphic crystalline structure, silk can be used either as a positive or negative resist through interactions with an electron beam. Moreover, silk can be easily modified, thereby enabling a variety of 'functional resists', including biologically active versions. As a proof of principle of the viability of all-water-based silk electron-beam lithography (EBL), we fabricate nanoscale photonic lattices using both neat silk and silk doped with quantum dots, green fluorescent proteins (GFPs) or horseradish peroxidase (HRP).

A biomimetic composite from solution self-assembly of chitin nanofibers in a silk fibroin matrix

A chitin nanofiber-silk biomimetic nanocomposite with enhanced mechanical properties is self-assembled from solution to yield ultrafine chitin nanofibers embedded in a silk matrix.

Protein-protein nanoimprinting of silk fibroin films

Protein-protein imprinting of silk fibroin is introduced as a rapid, high-throughput method for the fabrication of nanoscale structures in silk films, through the application of heat and pressure. Imprinting on conformal surfaces is demonstrated with minor adjustments to the system, at resolutions comparable to other currently available nonplanar nanoimprint lithography techniques.

A physically transient form of silicon electronics

A remarkable feature of modern silicon electronics is its ability to remain physically invariant, almost indefinitely for practical purposes. Although this characteristic is a hallmark of applications of integrated circuits that exist today, there might be opportunities for systems that offer the opposite behavior, such as implantable devices that function for medically useful time frames but then completely disappear via resorption by the body. We report a set of materials, manufacturing schemes, device components, and theoretical design tools for a silicon-based complementary metal oxide semiconductor (CMOS) technology that has this type of transient behavior, together with integrated sensors, actuators, power supply systems, and wireless control strategies. An implantable transient device that acts as a programmable nonantibiotic bacteriocide provides a system-level example.

Focal Infection Treatment using Laser-Mediated Heating of Injectable Silk Hydrogels with Gold Nanoparticles

Medical treatment of subcutaneous bacterial abscesses usually involves systemic high-dose antibiotics and incision-drainage of the wound. Such an approach suffers from two main deficiencies: bacterial resistance to antibiotics and pain associated with multiple incision-drainage-wound packing procedures. Furthermore, the efficacy of high-dose systemic antibiotics is limited because of the inability to penetrate into the abscess. To address these obstacles, we present a treatment relying on laser-induced heating of gold nanoparticles embedded in an injectable silk-protein hydrogel. Although bactericidal nanoparticle systems have been previously employed based on silver and nitric oxide, they have limitations regarding customization and safety. The method we propose is safe and uses biocompatible, highly tunable materials: an injectable silk hydrogel and Au nanoparticles, which are effective absorbers at low laser powers such as those provided by hand held devices. We demonstrate that a single 10-minute laser treatment of a subcutaneous infection in mice preserves the general tissue architecture, while achieving a bactericidal effect - even resulting in complete eradication in some cases. The unique materials platform presented here can provide the basis for an alternative treatment of focal infections.

Silk-based conformal, adhesive, edible food sensors

An array of passive metamaterial antennas fabricated on all protein-based silk substrates were conformally transferred and adhered to the surface of an apple. This process allows the opportunity for intimate contact of micro- and nanostructures that can probe, and accordingly monitor changes in, their surrounding environment. This provides in situ monitoring of food quality. It is to be noted that this type of sensor consists of all edible and biodegradable components, holding utility and potential relevance for healthcare and food/consumer products and markets.