Diffusion in face-centered cubic (fcc) opals synthesized from 250 nm-diameter silica spheres was investigated by electrochemical methods and finite-element simulations. Opal modified electrodes (OME) ((111) opal surface orientation) were prepared by thermal evaporation of Au onto ∼1 mm-thick opals. Linear sweep voltammetry of Au OMEs in aqueous solutions containing an electroactive molecule and a supporting electrolyte (0.1 M Na 2 SO 4 ) was used to determine molecular diffusion coefficients, D fcc , within the opal. D fcc is related to the diffusion coefficient of the molecule in free solution, D sol , by the relationship D fcc , ) (E/τ)D sol , where E is the interstitial volume fraction of a fcc opal (E ) 0.260 for an infinitely thick opal) and τ is the tortuosity; the tortuosity reflects the increased distance traversed by molecules as they diffuse through the curved interstitial spaces of the opal lattice, and is a function of both the direction of transport relative to the lattice and the number of layers of spheres in the opal lattice. Finite-element simulations are used to compute τ for transport orthogonal to the (111), (110), and (100) surface orientations for 1−7 layers of spheres. Values of τ ) 1.9 ± 0.7 and 3.1 ± 1.2 were obtained from experiment for transport of Ru(NH) 6 3+ and Fe(CN) 6 4-normal to the (111) surface, respectively, in reasonable agreement with a value of ∼3.0 obtained from the simulation.Introduction. The recent interest in opals comprising a closepacked face center cubic (fcc) lattice of spheres (typically SiO 2 or polystyrene of submicrometer radius) is due, in part, to their application in the synthesis of photonic crystals, 1-10 energy storage media, 11-14 novel magnetic materials, 15-17 and sensors. 18 These materials are typically prepared by infusion or diffusional transport of precursor species through the opal lattice, followed by removal of the spheres to create an inverted opal structure. 1,[18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37] Molecular transport occurs within the tortuous interstitial spaces of the opal, Figure 1. Based on geometric factors alone, the effective diffusivity of molecules within the fcc lattice of spheres, D fcc , can be related to the diffusivitiy of molecules in free space, D sol , by eq 1:

This paper characterizes the Utah Slant Optrode Array (USOA) as a means to deliver infrared light deep into tissue. An undoped crystalline silicon (100) substrate was used to fabricate 10 × 10 arrays of optrodes with rows of varying lengths from 0.5 mm to 1.5 mm on a 400-μm pitch. Light delivery from optical fibers and loss mechanisms through these Si optrodes were characterized, with the primary loss mechanisms being Fresnel reflection, coupling, radiation losses from the tapered shank and total internal reflection in the tips. Transmission at the optrode tips with different optical fiber core diameters and light in-coupling interfaces was investigated. At λ = 1.55μm, the highest optrode transmittance of 34.7%, relative to the optical fiber output power, was obtained with a 50-μm multi-mode fiber butt-coupled to the optrode through an intervening medium of index n = 1.66. Maximum power is directed into the optrodes when using fibers with core diameters of 200 μm or less. In addition, the output power varied with the optrode length/taper such that longer and less tapered optrodes exhibited higher light transmission efficiency. Output beam profiles and potential impacts on physiological tests were also examined. Future work is expected to improve USOA efficiency to greater than 64%.

We studied the optical transmission and magneto-optical effect through a subwavelength hole array fabricated on a ferromagnetic cobalt (Co) thin film in comparison to a control unperforated Co film having the same thickness. We found that the perforated film sustains extraordinary transmission bands through the hole array, which can be well explained as due to light coupling to surface plasmons on the two film interfaces. We also found that due to resonant coupling to the surface plasmons, the magneto-optical Kerr effect in the spectral range of the anomalous transmission bands of the perforated Co film is much smaller than that in the control Co film.

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We present an encapsulation scheme that combines atomic layer deposited (ALD) Al₂O₃ and Parylene C for the encapsulation of implantable devices. The encapsulation performances of combining alumina and Parylene C was compared to individual layers of Parylene C or alumina and the bilayer coating had superior encapsulation properties. The alumina-Parylene coated interdigitated electrodes (IDEs) soaked in PBS for up to nine months at temperatures from 37 to 80 °C for accelerated lifetime testing. For 52-nm alumina and 6-μm Parylene C, leakage current was ∼20 pA at 5 VDC, and the impedance was about 3.5 MΩ at 1 kHz with a phase near -87° from electrochemical impedance spectroscopy for samples soaked at 67 °C for equivalent lifetime of 72 months at 37 °C. The change of impedance during the whole soaking period (up to 70 months of equivalent soaking time at 37 °C) over 1 to 10⁶ Hz was within 5%. The stability of impedance indicated almost no degradation of the encapsulation. Bias voltage effect was studied by continuously applying 5 VDC, and it reduced the lifetime of Parylene coating by ∼75% while it showed no measurable effect on the bilayer coating. Lifetime of encapsulation of IDEs with topography generated by attaching a coil and surface mount device (SMD) capacitor was about half of that of planer IDEs. The stable long-term insulation impedance, low leakage current, and better lifetime under bias voltage and topography made this double-layer encapsulation very promising for chronic implantable devices.