Synthesis and characterization of biocompatible nanodiamond-silk hybrid material.

A new hybrid material consisting of nanodiamonds (NDs) and silk has been synthesized and investigated. NDs can contain bright fluorescence centers, important for bioprobes to image biological structures at the nanoscale and silk provides a transparent, robust matrix for these nanoparticles in-vivo or in-vitro. The ND-silk hybrid films were determined to be highly transparent in the visible to near infrared wavelength range. The NDs embedded in silk exhibited significant enhancement of emission relative to air, correlating with theoretical predictions. Furthermore, animal toxicity tests confirmed ND-silk films to be non-toxic in an in-vivo mice model.

Measuring the dispersive properties of liquids using a microinterferometer.

Using a single-beam, compact interferometer, we measure the refractive index of liquids in the near IR. This highly compact device relies on a silica capillary with a 50 µm inner diameter: it uses a minimal volume of test liquid, isolates the liquid from the humid atmosphere, has broadband operation, and is inherently mechanically stable. These characteristics, in combination with straightforward data acquisition, make it particularly well-suited for measuring the optical properties in the near IR of a wide range of liquids. Using this refractometer, we measure the refractive index of high-index liquids that are expected to be hydroscopic. The accuracy of the refractometer (±0.1%) is demonstrated through measuring the indices of air and pure water. We show that the hydroscopic behavior of the probed liquids has little influence on their optical properties in the near IR.

Bio-microfluidics: biomaterials and biomimetic designs.

Bio-microfluidics applies biomaterials and biologically inspired structural designs (biomimetics) to microfluidic devices. Microfluidics, the techniques for constraining fluids on the micrometer and sub-micrometer scale, offer applications ranging from lab-on-a-chip to optofluidics. Despite this wealth of applications, the design of typical microfluidic devices imparts relatively simple, laminar behavior on fluids and is realized using materials and techniques from silicon planar fabrication. On the other hand, highly complex microfluidic behavior is commonplace in nature, where fluids with nonlinear rheology flow through chaotic vasculature composed from a range of biopolymers. In this Review, the current state of bio-microfluidic materials, designs and applications are examined. Biopolymers enable bio-microfluidic devices with versatile functionalization chemistries, flexibility in fabrication, and biocompatibility in vitro and in vivo. Polymeric materials such as alginate, collagen, chitosan, and silk are being explored as bulk and film materials for bio-microfluidics. Hydrogels offer options for mechanically functional devices for microfluidic systems such as self-regulating valves, microlens arrays and drug release systems, vital for integrated bio-microfluidic devices. These devices including growth factor gradients to study cell responses, blood analysis, biomimetic capillary designs, and blood vessel tissue culture systems, as some recent examples of inroads in the field that should lead the way in a new generation of microfluidic devices for bio-related needs and applications. Perhaps one of the most intriguing directions for the future will be fully implantable microfluidic devices that will also integrate with existing vasculature and slowly degrade to fully recapitulate native tissue structure and function, yet serve critical interim functions, such as tissue maintenance, drug release, mechanical support, and cell delivery.

Facile fabrication of gelatin-based biopolymeric optical waveguides.

The rapid development in optical detection techniques for sensing applications has led to an increased need for biocompatible, biodegradable, and disposable optical components. We present a controllable fabrication technique for an entirely biopolymeric planar optical waveguide via simple spin-coating. The refractive index difference, thermal responsive properties, and inherent biocompatibility of gelatin and agarose were exploited in the fabrication of thin, stacked films that efficiently guide light in a core layer with higher index of refraction. These planar waveguides were fabricated using a simple spin-coating technique, which resulted in controllable layer thicknesses and smooth layer interfaces. This technique, therefore, offers a path for routine engineering of biopolymer structures with contrasting refractive indices. The thermal stability of the gelatin core layer was improved using two crosslinkers; glutaraldehyde or microbial Transglutaminase. Light guiding in the core layer of the waveguide was demonstrated using a simple He-Ne laser setup. Guiding efficiency was further illustrated by directly embedding fluorescent markers within the core layer and detecting their spectral signature. Combined with the biopolymers' inherent biocompatibility and biodegradability, our simple strategy to fabricate disposable optical components holds the potential for the development of applications in biological sensing and implantable biomedical devices.

Bioactive "self-sensing" optical systems.

Free-standing silk films are useful materials to manufacture nanopatterned optical elements and to immobilize bio-dopants such as enzymes while maintaining their biological activity. These traits were combined by incorporating hemoglobin into free-standing silk diffraction gratings to fabricate chemically responsive optofluidic devices responsive to ambient gas conditions, constituting a simple oxygen sensor. This type of self-analyzing optical system is enabled by the unique ability to reproduce high-fidelity optical structures in silk while maintaining the activity of entrapped proteins such as hemoglobin. These bioactive optical devices offer a direct readout capability, adding utility into the bioresponsive material arena.

Planar microfluidic chamber for generation of stable and steep chemoattractant gradients.

The extracellular availability of growth factors, hormones, chemokines, and neurotransmitters under gradient conditions is required for directional cellular responses such as migration, axonal pathfinding, and tissue patterning. These responses are, in turn, important in disease and developmental processes. This article addresses critical barriers toward devising a chemotaxis assay that is broadly applicable for different kinds of cancer cells through the design of a microfluidic chamber that produces a steep gradient of chemoattractant. Photolithography was used to create microchannels for chemoattractant delivery, flow diversion barriers/conduits, and small outlets in the form of apertures. The 1-microm apertures were made at the active surface by uncapping a thin (1.5 microm) layer of AZ1518. This process also created a vertical conduit that diverted the flow such that it occurred perpendicularly to the active, experimental surface where the gradients were measured. The other side of the vertical conduit opened to underlying 20-microm deep channels that carried microfluidic flows of tracer dyes/growth factors. Modeled data using computational fluid dynamics produced gradients that were steep along the horizontal, active surface. This simulation mirrors empirically derived gradients obtained from the flow analyses of fluorescent compounds. The open chamber contains a large buffer volume, which prevents chemoattractant saturation and permits easy cell and compound manipulation. The technique obviates the use of membranes or laminar flow that may hinder imaging, rinsing steps, cell seeding, and treatment. The utility of the chamber in the study of cell protrusion, an early step during chemotaxis, was demonstrated by growing cancer cells in the chamber, inducing a chemoattractant gradient using compressed air at 0.7 bar, and performing time-lapse microscopy. Breast cancer cells responded to the rapidly developed and stable gradient of epidermal growth factor by directing centroid positions toward the gradient and by forming a leading edge at a speed of 0.45 microm/min.

Transverse characterization of high air-fill fraction tapered photonic crystal fiber.

We demonstrate tapering of a high air-fill fraction photonic crystal fiber by using the flame-brushing technique. Transverse probing along the taper allows us to ascertain how the microstructure is preserved during tapering. Experimental results are compared with numerical simulations performed with the finite-difference time-domain and plane-wave expansion methods. Through this investigation we find that the fiber geometry is well preserved throughout the tapering process and we resolve the apparent discrepancies between simulation and experiment that arise through the finite extent of the fiber microstructure.

A new slant on photonic crystal fibers.

We present the novel use of microstructured optical fibers not as "light-pipes", but in a transverse geometry to manipulate the light propagating across the fiber. Fundamental and higher-order bandgaps were observed experimentally in this geometry using a number of techniques. The comparison of the measured spectra with photonic band structure and Finite-Difference Time-Domain simulations provide strong evidence that the spectral features are a result of the periodic nature of the fiber microstructure in the transverse direction.