Jason J. Amsden

Dissolvable films of silk fibroin for ultrathin conformal bio-integrated electronics

Electronics that are capable of intimate, non-invasive integration with the soft, curvilinear surfaces of biological tissues offer important opportunities for diagnosing and treating disease and for improving brain/machine interfaces. This article describes a material strategy for a type of bio-interfaced system that relies on ultrathin electronics supported by bioresorbable substrates of silk fibroin. Mounting such devices on tissue and then allowing the silk to dissolve and resorb initiates a spontaneous, conformal wrapping process driven by capillary forces at the biotic/abiotic interface. Specialized mesh designs and ultrathin forms for the electronics ensure minimal stresses on the tissue and highly conformal coverage, even for complex curvilinear surfaces, as confirmed by experimental and theoretical studies. In vivo, neural mapping experiments on feline animal models illustrate one mode of use for this class of technology. These concepts provide new capabilities for implantable and surgical devices.

Ultra-sensitive vibrational spectroscopy of protein monolayers with plasmonic nanoantenna arrays

Infrared absorption spectroscopy enabling direct access to vibrational fingerprints of the molecular structure is a powerful method for functional studies of bio-molecules. Although the intrinsic absorption cross-sections of IR active modes of proteins are nearly 10 orders of magnitude larger than the corresponding Raman cross-sections, they are still small compared to that of fluorescence-label based methods. Here, we developed a new tool based on collective excitation of plasmonic nanoantenna arrays and demonstrated direct detection of vibrational signatures of single protein monolayers. We first tailored the geometry of individual nanoantennas to form resonant structures that match the molecular vibrational modes. The tailored nanoantennas are then arranged in such a way that their in-phase dipolar coupling leads to a collective excitation of the ensemble with strongly enhanced near fields. The combined collective and individual plasmonic responses of the antenna array play a critical role in attaining signal enhancement factors of 104–105. We achieved measurement of the vibrational spectra of proteins at zeptomole levels for the entire array, corresponding to only 145 molecules per antenna. The near-field nature of the plasmonic enhancement of the absorption signals is demonstrated with progressive loading of the nanoantennas with varying protein film thicknesses. Finally, an advanced model based on nonequilibrium Greens function formalism is introduced, which explains the observed Fano-type absorption line-shapes and tuning of the absorption strengths with the antenna resonance.

Silicon electronics on silk as a path to bioresorbable, implantable devices

Many existing and envisioned classes of implantable biomedical devices require high performance electronicssensors. An approach that avoids some of the longer term challenges in biocompatibility involves a construction in which some parts or all of the system resorbs in the body over time. This paper describes strategies for integrating single crystalline silicon electronics, where the silicon is in the form of nanomembranes, onto water soluble and biocompatible silk substrates. Electrical, bending, water dissolution, and animal toxicity studies suggest that this approach might provide many opportunities for future biomedical devices and clinical applications.

Rapid Nanoimprinting of Silk Fibroin Films for Biophotonic Applications

With soft microand nanopatterned materials becoming increasingly useful for various optical, mechanical, electronic, microfluidic, and optofluidic devices, the extension of this paradigm to a pure protein-based material substrate would provide entirely new options for such devices. Silk fibroin is an appealing biopolymer for forming such devices because of its optical properties, mechanical properties, all aqueous processing, relatively easy chemical and biological functionalization, and biocompatibility. Biologically functionalized silk fibroin films can be patterned on the micro and nanoscale using a soft lithography casting technique while maintaining the biological activity of the embedded proteins. The combination of these properties could enable a new class of active optofluidic devices that merge high-quality photonic structures whose very material constituent responds, through the embedded proteins, to analytes infused through integrated microfluidics. However, the silk fibroin casting process takes 12–36 h, hindering the ability to rapidly produce multiple devices and the resulting silk structures contain artifacts due to drying and liftoff. In this communication, we will show that silk has the properties of an ideal nanoimprint resist enabling rapid device fabrication, which in combination with its optical properties and biocompatibility make it a new technology platform that seamlessly combines nanophotonics, biopolymeric and biocompatible materials. Optofluidics, though a relatively new field, is already undergoing evolution, finding applications to an ever-increasing range of problems, including varieties of biological sensing and detection. Initially optofluidics was developed as a fusion of microfluidics and photonics to enable compact, novel optical modulation technologies. The union of optical and fluidic confining structures, however, led optofluidic devices to be applied to sensing problems especially looking toward highly parallel, sensitive and low analyte volume applications. A further development of the optofluidic paradigm, introduced here through the use of silk, is to ‘‘activate’’ the constituent material of the device to make it chemically sensitive to species flowed past it. Typically, optofluidic devices are fabricated from materials usually found in photonics or microfluidics such as silica, silicon, polydimethylsiloxane or polymethacrylmethacrylate and other polymers. These materials, while possessing suitable and well-characterized optical and material properties are not inherently chemically sensitive or specific. It is possible to functionalize the surfaces of these materials with chemical reagents, however, a much broader range of sensitivities and specificities can be achieved if proteins or enzymes are used as the sensitizing agents. The use of proteins presents an issue in itself. Binding proteins (or chemicals receptive to them) to inorganic or synthetic polymer surfaces is complex. Ideally, a material such as silk fibroin that posseses excellent optical and mechanical qualities can be formed into a variety of optofluidic geometries and maintains the activity of embedded proteins is needed for realizing active optofluidic devices. A proof of concept presented here is to build a self-sensing nanoscale imprinted optofluidic device based on imprinted silk doped with lysed red blood cells. The device can be thought of as ‘‘self-analyzing’’ in that the single optofluidic component provides both chemical and spectral analysis due to the activation of the constituent imprinted silk. Nanoimprinting is a high-throughput lithography technique in which a mold is pressed onto a thermoplastic material heated above its glass-transition temperature. The softened material conforms to the mold due to applied pressure. Sub-100 nm structures by nanoimprint lithography were first demonstrated in polymethylmethacrylate (PMMA) and now structures as small as 10 nm are routinely achieved in PMMA. An ideal nanoimprint resist combines rapid imprinting times with low temperature and pressure as well as low surface energy to aid in mold removal. As such, the mold is often coated with a low surface energy surfactant. Nanoimprinting of biopolymers presents additional challenges because of a restricted parameter space that limits the ranges of temperature and pressures usable. However, in this communication, we demonstrate that silk fibroin films exhibit many characteristics of an ideal nanoimprint resist, which in combination with its optical properties and biocompatibility make it a new technology platform that seamlessly combines

Metamaterial Silk Composites at Terahertz Frequencies

www.MaterialsViews.com C O M Metamaterial Silk Composites at Terahertz Frequencies M U N IC By Hu Tao , Jason J. Amsden , Andrew C. Strikwerda , Kebin Fan , David L. Kaplan , Xin Zhang , * Richard D. Averitt , * and Fiorenzo G. Omenetto * A IO N Silk has been a highly desired and widely used textile since its fi rst appearance in ancient China. [ 1 , 2 ] Glossy and smooth, silk is favored not only by fashion designers, but also tissue engineers because it is mechanically tough but degrades harmlessly inside the body, offering new opportunities as a highly robust and biocompatible material substrate. [ 3 , 4 ] Since silk fi lms are optically transparent, it is possible to create a new collection of optical elements such as lenses and diffractive gratings, by 2D and/or 3D patterning of the silk fi lms. [ 5 , 6 ] Furthermore, silk fi broin has been proven to be a biologically favorable carrier that enables bio-dopants such as enzymes and proteins to maintain their functionality. [ 7 , 8 ] This opens the door to a new class of biophotonic devices that could potentially be implanted into the human body to monitor interactions between specifi c targets and embedded dopants. In addition to manipulating the silk fi lms and embedding appropriate dopants, it is desirable to incorporate resonant electromagnetic structures with the silk fi lms. This would enable hybrid silk-based sensors that couple bio-functionality with an easily measured electromagnetic response that changes in response to the local environment. Metamaterials are resonant sub-wavelength electromagnetic composites typically consisting of highly conducting metals. Importantly, metamaterials provide the means to design and control both the effective electric permittivity ( ε ) and magnetic permeability ( μ ). The power of metamaterials lies in the fact that it is possible to construct materials with a user designed electromagnetic response (often not available with naturally occurring materials) at a precisely controlled target frequency [ 9 ]

Implantable, multifunctional, bioresorbable optics

Advances in personalized medicine are symbiotic with the development of novel technologies for biomedical devices. We present an approach that combines enhanced imaging of malignancies, therapeutics, and feedback about therapeutics in a single implantable, biocompatible, and resorbable device. This confluence of form and function is accomplished by capitalizing on the unique properties of silk proteins as a mechanically robust, biocompatible, optically clear biomaterial matrix that can house, stabilize, and retain the function of therapeutic components. By developing a form of high-quality microstructured optical elements, improved imaging of malignancies and of treatment monitoring can be achieved. The results demonstrate a unique family of devices for in vitro and in vivo use that provide functional biomaterials with built-in optical signal and contrast enhancement, demonstrated here with simultaneous drug delivery and feedback about drug delivery with no adverse biological effects, all while slowly degrading to regenerate native tissue.

Low-threshold blue lasing from silk fibroin thin films

Silk is a natural biocompatible material that can be integrated in a variety of photonic systems and optoelectronic devices. The silk replication of patterned substrates with features down to tens of nanometers is exploited to realize highly transparent, mechanically stable, and free-standing structures with optical wavelength size. We demonstrate organic lasing from a blue-emitting stilbene-doped silk film spin-coated onto a one-dimensional distributed feedback grating (DFB). The lasing threshold is lower than that of organic DFB lasers based on the same active dye. These findings pave the way to the development of an optically active biocompatible technological platform based on silk.

Spectral analysis of induced color change on periodically nanopatterned silk films

We demonstrate controllable structural color based on periodic nanopatterned 2D lattices in pure protein films of silk fibroin. We show here periodic lattices in silk fibroin films with feature sizes of hundreds of nanometers that exhibit different colors as a function of varying lattice spacing. Further, when varying the index of refraction contrast between the nanopatterned lattice and its surrounding environment by applying liquids on top of the lattices, colorimetric shifts are observed. The effect is characterized experimentally and theoretically and a simple example of glucose concentration sensing is presented. This is the first example of a functional sensor based on silk fibroin optics.

Spatial and spectral detection of protein monolayers with deterministic aperiodic arrays of metal nanoparticles

Light scattering phenomena in periodic systems have been investigated for decades in optics and photonics. Their classical description relies on Bragg scattering, which gives rise to constructive interference at specific wavelengths along well defined propagation directions, depending on illumination conditions, structural periodicity, and the refractive index of the surrounding medium. In this paper, by engineering multifrequency colorimetric responses in deterministic aperiodic arrays of nanoparticles, we demonstrate significantly enhanced sensitivity to the presence of a single protein monolayer. These structures, which can be readily fabricated by conventional Electron Beam Lithography, sustain highly complex structural resonances that enable a unique optical sensing approach beyond the traditional Bragg scattering with periodic structures. By combining conventional dark-field scattering micro-spectroscopy and simple image correlation analysis, we experimentally demonstrate that deterministic aperiodic surfaces with engineered structural color are capable of detecting, in the visible spectral range, protein layers with thickness of a few tens of Angstroms.

Conformational changes in the photocycle of Anabaena sensory rhodopsin : Absence of the schiff base counterion protonation signal

Anabaena sensory rhodopsin (ASR) is a novel microbial rhodopsin recently discovered in the freshwater cyanobacterium Anabaena sp. PCC7120. This protein most likely functions as a photosensory receptor as do the related haloarchaeal sensory rhodopsins. However, unlike the archaeal pigments, which are tightly bound to their cognate membrane-embedded transducers, ASR interacts with a soluble cytoplasmic protein analogous to transducers of animal vertebrate rhodopsins. In this study, infrared spectroscopy was used to examine the molecular mechanism of photoactivation in ASR. Light adaptation of the pigment leads to a phototransformation of an all-trans/15-anti to 13-cis/15-syn retinylidene-containing species very similar in chromophore structural changes to those caused by dark adaptation in bacteriorhodopsin. Following 532 nm laser-pulsed excitation, the protein exhibits predominantly an all-trans retinylidene photocycle containing a deprotonated Schiff base species similar to those of other microbial rhodopsins such as bacteriorhodopsin, sensory rhodopsin II, and Neurospora rhodopsin. However, no changes are observed in the Schiff base counterion Asp-75, which remains unprotonated throughout the photocycle. This result along with other evidence indicates that the Schiff base proton release mechanism differs significantly from that of other known microbial rhodopsins, possibly because of the absence of a second carboxylate group at the ASR photoactive site. Several conformational changes are detected during the ASR photocycle including in the transmembrane helices E and G as indicated by hydrogen-bonding alterations of their native cysteine residues. In addition, similarly to animal vertebrate rhodopsin, perturbations of the polar head groups of lipid molecules are detected.