Rapid transfer-based micropatterning and dry etching of silk microstructures

Generation of Complex Tunable Multispectral Signatures with Reconfigurable Protein-Based, Plasmonic-Photonic Crystal Hybrid Nanostructures

Structurally colored materials, which rely on the interaction between visible light and nanostructures, produce brilliant color displays through fine control of light interference, diffraction, scattering, or absorption. Rationally combining different color-selective functions into a single form offers a powerful strategy to create programmable optical functions which are otherwise difficult, if not impossible to obtain. By leveraging structural protein templates, specifically silk fibroin, nanostructured materials that combine plasmonic and photonic crystal paradigms are shown here. This confluence of function enables directional, tunable, and multiple co-located optical responses derived from the interplay between surface plasmon resonance and photonic bandgap effects. Several demonstrations are shown with programmable coloration at varying viewing sides, angle, and by solvent infiltration, opening avenues for smart displays and multi-mode information encoding applications.

Development of robust, ultra-smooth, flexible and transparent regenerated silk composite films for bio-integrated electronic device applications

Regenerated Silk Fibroin (RSF) films are considered promising substrate candidates primarily in the field of bio-integrated electronic device applications. The key issues that ought to be addressed to exploit the inherent advantages of silk thin films include enhancing their flexibility and chemical durability. Such films find a plethora of applications, the significant one being conformal, transparent microelectrode arrays. Elevated temperatures that are regularly used in lithographic processes tend to dehydrate RSF films, making them brittle. Furthermore, the solvents/etchants used in typical device fabrication results in the formation of micro-cracks. This paper addressed both these issues by developing composite films and studying the effect of biodegradable additives in enhancing flexibility and chemical durability without compromising on optical transparency and surface smoothness. Through our rigorous experimentation, regenerated silk blended with Polyvinyl Alcohol (Silk/PVA) is identified as the composite for achieving the objectives. Furthermore, the Cyto-compatibility studies suggest that Silk/PVA, along with all other silk composites, have shown above 80% cell viability, as verified using L929 fibroblast cell lines. Going a step further, we demonstrated the successful patterning of 32 channel optically transparent microelectrode array (MEA) pattern, with a minimum feature size of 5 μm above the free-standing and optically transparent Silk/PVA composite film.

Dry Surface Treatments of Silk Biomaterials and Their Utility in Biomedical Applications

Silk-based materials are widely used in biomaterial and tissue engineering applications due to their cytocompatibility and tunable mechanical and biodegradation properties. Aqueous-based processing techniques have enabled the fabrication of silk into a broad range of material formats, making it a highly versatile material platform across multiple industries. Utilizing the full potential of silk in biomedical applications frequently requires modification of silk's surface properties. Dry surface modification techniques, including irradiation and plasma treatment, offer an alternative to the conventional wet chemistry strategies to modify the physical and chemical properties of silk materials without compromising their bulk properties. While dry surface modification techniques are more prevalent in textiles and sterilization applications, the range of modifications available and resultant changes to silk materials all point to the utility of dry surface modification for the development of new, functional silk biomaterials. Dry surface treatment affects the surface chemistry, secondary structure, molecular weight, topography, surface energy, and mechanical properties of silk materials. This Review describes and critically evaluates the effect of physical dry surface modification techniques, including irradiation and plasma processes, on silk materials and discusses their utility in biomedical applications, including recent examples of modulation of cell/protein interactions on silk biomaterials, in vivo performance of implanted biomaterials, and applications in material biofunctionalization and lithographic surface patterning approaches.

A method for large-scale implantation of 3D microdevice ensembles into brain and soft tissue

Wireless networks of implantable electronic sensors and actuators at the microscale (sub-mm) level are being explored for monitoring and modulation of physiological activity for medical diagnostics and therapeutic purposes. Beyond the requirement of integrating multiple electronic or chemical functions within small device volumes, a key challenge is the development of high-throughput methods for the implantation of large numbers of microdevices into soft tissues with minimal damage. To that end, we have developed a method for high-throughput implantation of ~100-200 µm size devices, which are here simulated by proxy microparticle ensembles. While generally applicable to subdermal tissue, our main focus and experimental testbed is the implantation of microparticles into the brain. The method deploys a scalable delivery tool composed of a 2-dimensional array of polyethylene glycol-tipped microneedles that confine the microparticle payloads. Upon dissolution of the bioresorbable polyethylene glycol, the supporting array structure is retrieved, and the microparticles remain embedded in the tissue, distributed spatially and geometrically according to the design of the microfabricated delivery tool. We first evaluated the method in an agarose testbed in terms of spatial precision and throughput for up to 1000 passive spherical and planar microparticles acting as proxy devices. We then performed the same evaluations by implanting particles into the rat cortex under acute conditions and assessed the tissue injury produced by our method of implantation under chronic conditions.

Controlling silk fibroin conformation for dynamic, responsive, multifunctional, micropatterned surfaces

Protein micro/nanopatterning has long provided sophisticated strategies for a wide range of applications including biointerfaces, tissue engineering, optics/photonics, and bioelectronics. We present here the use of regenerated silk fibroin to explore wrinkle formation by exploiting the structure-function relation of silk. This yields a biopolymer-based reversible, multiresponsive, dynamic wrinkling system based on the protein's responsiveness to external stimuli that allows on-demand tuning of surface morphologies and properties. The polymorphic transitions of silk fibroin enable modulation of the wrinkle patterns and, consequently, the material's physical properties. The interplay between silk protein chains and external stimuli enables control over the protein film's wrinkling dynamics. Thanks to the versatility of regenerated silk fibroin as a technological substrate, a number of demonstrator devices of varying utility are shown ranging from information encoding to modulation of optical transparency and thermal regulation.

Photolithographic Masking Method to Chemically Pattern Silk Film Surfaces

A method has been developed for selectively patterning silk surfaces using a photolithographic process to mask off sections of silk films, which allows selective and precise patterning of features down to 40 μm. This process is highly versatile, utilizes only low-cost equipment and can be used to rapidly prototype flat silk substrates with spatially controlled chemical patterns. Here we demonstrate the usefulness of this technique to deposit fluorescent dyes, labeled proteins and conducting polymers or to modify the surface charge of the silk protein in desired regions on a silk film surface.

Programmable Vanishing Multifunctional Optics

Physically transient optics, a form of optics that can physically disappear with precisely controlled degradation behaviors, has widespread applications including information security, drug release, and degradable implants. Here, a set of silk-based programmable vanishing, biologically functional, multichromatic diffractive optical elements (MC-DOEs) is reported. Silk proteins produced by silkworms and spiders are mechanically robust, biocompatible, biodegradable, and importantly, optically transparent, which open up new opportunities for a set of fully degradable transient optical devices with no need of metallic or semiconductor components. Compared with monochromatic DOEs, MC-DOEs carry out richer information for more practical applications such as encryption and decryption of multilevel information, quantitative sensing/monitoring of chemical/biological cascade reactions, and effective treatment of infections caused by multiple pathogens.

Wafer-Scale Multilayer Fabrication for Silk Fibroin-Based Microelectronics

Silk fibroin is an excellent candidate for biomedical implantable devices because of its biocompatibility, controllable biodegradability, solution processability, flexibility, and transparency. Thus, fibroin has been widely explored in biomedical applications as biodegradable films as well as functional microstructures. Although there exists a large number of patterning methods for fibroin thin films, multilayer micropatterning of fibroin films interleaved with metal layers still remains a challenge. Herein, we report a new wafer-scale multilayer microfabrication process named aluminum hard mask on silk fibroin (AMoS), which is capable of micropatterning multiple layers composed of both fibroin and inorganic materials (e.g., metal and dielectrics) with high-precision microscale alignment. To the best of our knowledge, our AMoS process is the first demonstration of wafer-scale multilayer processing of both silk fibroin and metal micropatterns. In the AMoS process, aluminum deposited on fibroin is first micropatterned using conventional ultraviolet (UV) photolithography, and the patterned aluminum layer is then used as a mask to pattern fibroin underneath. We demonstrate the versatility of our fabrication process by fabricating fibroin microstructures with different dimensions, passive electronic components composed of both fibroin and metal layers, and functional fibroin microstructures for drug delivery. Furthermore, because one of the crucial advantages of fibroin is biocompatibility, we assess the biocompatibility of our fabrication process through the culture of highly susceptible primary neurons. Because the AMoS process utilizes conventional UV photolithography, the principal advantages of our process are multilayer fabrication with high-precision alignment, high resolution, wafer-scale large area processing, no requirement for chemical modification of the protein, and high throughput and thus low cost, all of which have not been feasible with silk fibroin. Therefore, the proposed fabrication method is a promising candidate for batch fabrication of functional fibroin microelectronics (e.g., memristors and organic thin film transistors) for next-generation implantable biomedical applications.

Precise Protein Photolithography (P3): High Performance Biopatterning Using Silk Fibroin Light Chain as the Resist

Precise patterning of biomaterials has widespread applications, including drug release, degradable implants, tissue engineering, and regenerative medicine. Patterning of protein-based microstructures using UV-photolithography has been demonstrated using protein as the resist material. The Achilles heel of existing protein-based biophotoresists is the inevitable wide molecular weight distribution during the protein extraction/regeneration process, hindering their practical uses in the semiconductor industry where reliability and repeatability are paramount. A wafer-scale high resolution patterning of bio-microstructures using well-defined silk fibroin light chain as the resist material is presented showing unprecedent performances. The lithographic and etching performance of silk fibroin light chain resists are evaluated systematically and the underlying mechanisms are thoroughly discussed. The micropatterned silk structures are tested as cellular substrates for the successful spatial guidance of fetal neural stems cells seeded on the patterned substrates. The enhanced patterning resolution, the improved etch resistance, and the inherent biocompatibility of such protein-based photoresist provide new opportunities in fabricating large scale biocompatible functional microstructures.

Physically Transient Resistive Switching Memory Based on Silk Protein

Physically transient resistive switching devices based on silk protein are successfully demonstrated. The devices can be absolutely dissolved in deionized water or in phosphate-buffered saline in 2 h. At the same time, a reasonable resistance OFF/ON ratio of larger than 10(2) and a retention time of more than 10(4) s are achieved for nonvolatile memory applications.

The Fabrication of Ordered Bulk Heterojunction Solar Cell by Nanoimprinting Lithography Method Using Patterned Silk Fibroin Mold at Room Temperature

The performance of organic solar cell is greatly determined by the nanoscale heterojunction morphology, and finding a practical method to achieve advantageous nanostructure remains a challenge. We demonstrate here that ordered bulk heterojunction (OBHJ) solar cell can be fabricated assisted by a simple, cost-effective nanoimprinting lithography method using patterned silk fibroin film mold at room temperature. The P3HT nanogratings were achieved by nanoimprinting lithography (NIL) process, and phenyl-C61-butyric acid methyl ester (PCBM) was spin-coated on the top of P3HT nanogratings. The conducting capacity of P3HT nanograting film has little difference compared with the unimprinted film in the vertical direction, due to the same edge-on chain alignment. However, it can be found that the fabrication of OBHJ nanostructure using room temperature NIL technique with patterned silk fibroin mold is able to promote optical absorption, interfacial area, and bicontinuous pathway. Therefore, the ordered heterojunction morphology plays an important part in improving device performance due to efficient exciton diffusion, dissociation, and reducing charge recombination rate.

Aqueous multiphoton lithography with multifunctional silk-centred bio-resists

Silk and silk fibroin, the biomaterial from nature, nowadays are being widely utilized in many cutting-edge micro/nanodevices/systems via advanced micro/nanofabrication techniques. Herein, for the first time to our knowledge, we report aqueous multiphoton lithography of diversiform-regenerated-silk-fibroin-centric inks using noncontact and maskless femtosecond laser direct writing (FsLDW). Initially, silk fibroin was FsLDW-crosslinked into arbitrary two/three-dimensional micro/nanostructures with good elastic properties merely using proper photosensitizers. More interestingly, silk/metal composite micro/nanodevices with multidimension-controllable metal content can be FsLDW-customized through laser-induced simultaneous fibroin oxidation/crosslinking and metal photoreduction using the simplest silk/Ag⁺ or silk/[AuCl₄]⁻ aqueous resists. Noticeably, during FsLDW, fibroin functions as biological reductant and matrix, while metal ions act as the oxidant. A FsLDW-fabricated prototyping silk/Ag microelectrode exhibited 10⁴-Ω⁻¹m⁻¹-scale adjustable electric conductivity. This work not only provides a powerful development to silk micro/nanoprocessing techniques but also creates a novel way to fabricate multifunctional metal/biomacromolecule complex micro/nanodevices for applications such as micro/nanoscale mechanical and electrical bioengineering and biosystems.

Scientists are increasingly realising the potential for natural materials in micro- and nanofabrication. Here, the authors employ silk-based resists for aqueous multiphoton lithography towards generating intricate structures by femtosecond direct writing.

Regenerative Engineering and Bionic Limbs

Amputations of the upper extremity are severely debilitating, current treatments support very basic limb movement, and patients undergo extensive physiotherapy and psychological counselling. There is no prosthesis that allows the amputees near-normal function. With increasing number of amputees due to injuries sustained in accidents, natural calamities and international conflicts, there is a growing requirement for novel strategies and new discoveries. Advances have been made in technological, material and in prosthesis integration where researchers are now exploring artificial prosthesis that integrate with the residual tissues and function based on signal impulses received from the residual nerves. Efforts are focused on challenging experts in different disciplines to integrate ideas and technologies to allow for the regeneration of injured tissues, recording on tissue signals and feed-back to facilitate responsive movements and gradations of muscle force. A fully functional replacement and regenerative or integrated prosthesis will rely on interface of biological process with robotic systems to allow individual control of movement such as at the elbow, forearm, digits and thumb in the upper extremity. Regenerative engineering focused on the regeneration of complex tissue and organ systems will be realized by the cross-fertilization of advances over the past thirty years in the fields of tissue engineering, nanotechnology, stem cell science, and developmental biology. The convergence of toolboxes crated within each discipline will allow interdisciplinary teams from engineering, science, and medicine to realize new strategies, mergers of disparate technologies, such as biophysics, smart bionics, and the healing power of the mind. Tackling the clinical challenges, interfacing the biological process with bionic technologies, engineering biological control of the electronic systems, and feed-back will be the important goals in regenerative engineering over the next two decades.

Coatings and films made of silk proteins

Silks are a class of proteinaceous materials produced by arthropods for various purposes. Spider dragline silk is known for its outstanding mechanical properties, and it shows high biocompatibility, good biodegradability, and a lack of immunogenicity and allergenicity. The silk produced by the mulberry silkworm B. mori has been used as a textile fiber and in medical devices for a long time. Here, recent progress in the processing of different silk materials into highly tailored isotropic and anisotropic coatings for biomedical applications such as tissue engineering, cell adhesion, and implant coatings as well as for optics and biosensors is reviewed.

Integration of a silk fibroin based film as a luminescent down-shifting layer in ITO-free organic solar cells

A bio-derived silk-fibroin film doped with a luminescent dye and its application as luminescent down-shifting layer in organic solar cells.

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.

Direct transfer of subwavelength plasmonic nanostructures on bioactive silk films

By a reusable transfer fabrication technique, we demonstrate high-fidelity fabrication of metal nanoparticles, optical nanoantennas, and nanohole arrays directly on a functional silk biopolymer. The ability to reproducibly pattern silk biopolymers with arbitrarily complex plasmonic arrays is of importance for a variety of applications in optical biosensing, tissue engineering, cell biology, and the development of novel bio-optoelectronic medical devices.

Silk materials--a road to sustainable high technology

This review addresses the use of silk protein as a sustainable material in optics and photonics, electronics and optoelectronic applications. These options represent additional developments for this technology platform that compound the broad utility and impact of this material for medical needs that have been recently described in the literature. The favorable properties of the material certainly make a favorable case for the use of silk, yet serve as a broad inspiration to further develop biological foundries for both the synthesis and processing of Nature's materials for technological applications.

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.