James N. McMullin

Integrated wavelength-selective optical waveguides for microfluidic-based laser-induced fluorescence detection.

We demonstrate the fabrication and characterization of a novel, inexpensive microchip capable of laser induced fluorescence (LIF) detection using integrated waveguides with built-in optical filters. Integrated wavelength-selective optical waveguides are fabricated by doping poly(dimethysiloxane) (PDMS) with dye molecules. Liquid-core waveguides are created within dye-doped PDMS microfluidic chips by filling channels with high refractive index liquids. Dye molecules are allowed to diffuse into the liquid core from the surrounding dye-doped PDMS. The amount of diffusion is controlled by choosing either polar (low diffusion) or apolar (high diffusion) liquid waveguide cores. The doping dye is chosen to absorb excitation light and to transmit fluorescence emitted by the sample under test. After 24 h, apolar waveguides demonstrate propagation losses of 120 dB cm(-1) (532 nm) and 4.4 dB cm(-1) (633 nm) while polar waveguides experience losses of 8.2 dB cm(-1) (532 nm) and 1.1 dB cm(-1) (633 nm) where 532 and 633 nm light represent the excitation and fluorescence wavelengths, respectively. We demonstrate the separation and detection of end-labelled DNA fragments using polar waveguides for excitation light delivery and apolar waveguides for fluorescence collection. We demonstrate that the dye-doped waveguides can provide performance comparable to a commercial dielectric filter; however, for the present choice of dye, their ultimate performance is limited by autofluorescence from the dye. Through the detection of a BK virus polymerase chain reaction (PCR) product, we demonstrate that the dye-doped PDMS system is an order of magnitude more sensitive than a similar undoped system (SNR: 138 vs. 9) without the use of any external optical filters at the detector.

Single-step fabrication of refractive microlens arrays

Arrays of submillimeter microlenses are made from droplets of UV-curable optical adhesive dispensed from a pressurized syringe under computer control. Measurements of the focal length uniformity, the minimum focused spot size, and the spherical aberration are presented. An excellent lens diameter and focal length uniformity are achieved over 100 element arrays.

Strong Bragg gratings photoinduced by 633-nm illumination in evaporated As2Se3 thin films.

Bragg gratings are used in several photonic devices to reflect, and thus to isolate, specific wavelengths of light. Gratings can be photoinduced in chalcogenide glasses by illumination of bandgap light in an interference pattern. We used holographic interferometry to create Bragg gratings in amorphous As2Se3 thin films with a period of 0.56 microm by illumination with 633-nm light. The quality of the gratings was tested in real time, and refractive-index modulations as high as 0.037 were measured. These gratings were found to be stable over a period of several months if they were kept in the dark.

The ABCD matrix in arbitrarily tapered quadratic-index waveguides

The general form for the ABCD matrix in an arbitrarily tapered quadratic-index waveguide is given in terms of the two independent solutions of the second-order differential ray-tracing equation. A procedure for finding taper functions which admit analytical solutions in terms of known functions is presented with several examples.

Laser fabrication of integrated microfluidic/micro-optic systems

A system for fabricating integrating microfluidic and micro-optic systems is described. Microchannel walls and multimode waveguides are formed in UV-curable optical adhesive by moving the substrate under computer control in the focus of a HeCd laser beam. Optical properties of the system were measured and two system examples are presented.

PDMS biochips with integrated waveguides

A general method is described for the fabrication of polydimethylsiloxane (PDMS) lab-on-a-chip (LOC) devices with integrated optic and fluidic elements. The PDMS core layer containing the optic and fluidic components is cast and cured under pressure on a silicon master. Subsequently, outer layers of lower-index PDMS are bonded to the core layer to provide optical and fluidic confinement. The functionality of the waveguides and microchannels is demonstrated by the detection and identification of two different types of fluorescent polystyrene beads in a pressure-driven flow inside a microfluidic channel in a device fabricated by this process.

Integrated optical measurement of microfluid velocity

The integration of multimode ion-exchange waveguides and etched microchannels in glass biochips is described. The waveguides were used to carry laser light to specific points in the channels for the detection of fluorescent microparticles. Anomalous etching was observed at the intersections of the waveguides and the microchannels resulting in the partial etching of the waveguides to form side-channels to the main channels. When filled with fluid, these side-channels have a lower index of refraction than the surrounding waveguides causing the laser light to illuminate the microchannel in two places. It is demonstrated how the double-peaked signal from a passing fluorescent microparticle can be used to directly determine the velocity of the liquid in microchannels.

A multi-layer biochip with integrated hollow waveguides

The fabrication of multi-layer microfluidic biochips with integrated optical waveguides is described. The optical waveguides are metallized silicon v-grooves formed by anisotropic etching and capped by metal strips formed on pyrex glass. Microchannels are fabricated on the other side of the pyrex layer by isotropic wet etching and capped by a layer of PDMS. Optical coupling between the waveguides and microchannels is achieved by reflection from the end-facets of the waveguides. Numerical simulations and measurements of the waveguide properties are described and the excitation and detection of laser-induced fluorescence from 15 µm microspheres is demonstrated.

Integrated diffraction grating for lab-on-a-chip microspectrometers

Optical microspectrometers are potentially important components for improving the functionality of lab-on-a-chip systems used for detection and identification of cells and other biological material. For disposable and portable applications, monolithic integration of the microfluidic channels, optical waveguides and diffraction grating is desirable. This paper presents the design and simulation of a transmission grating that can be integrated with microfluidic channels for on-chip fluorescence detection. The grating design features two stigmatic points and large facet sizes that can be easily fabricated in low-cost polymers. Scalar simulations predict grating efficiencies greater than 74% for wavelengths from 500 nm to 700 nm in the -2 diffraction order.

Rapid and cheap prototyping of a microfluidic cell sorter

Development of a microfluidic device is generally based on fabrication‐design‐fabrication loop, as, unlike the microelectronics design, there is no rigorous simulation‐based verification of the chip before fabrication. This usually results in extremely long, and hence expensive, product development cycle if micro/nano fabrication facilities are used from the beginning of the cycle. Here, we illustrate a novel approach of device prototyping that is fast, cheap, reliable, and most importantly, this technique can be adopted even if no state‐of‐the‐art microfabrication facility is available. A water‐jet machine is used to cut the desired microfluidic channels into a thin steel plate which is then used as a template to cut the channels into a thin sheet of a transparent and cheap polymer material named Surlyn® by using a Hot Knife™. The feature‐inscribed Surlyn sheet is bonded in between two microscope glass slides by utilizing the techniques which has been being used in curing polymer film between dual layer automotive glasses for years. Optical fibers are inserted from the sides of chip and are bonded by UV epoxy. To study the applicability of this prototyping approach, we made a basic microfluidic sorter and tested its functionalities. Sample containing microparticles is injected into the chip. Light from a 532‐nm diode laser is coupled into the optical fiber that delivers light to the interrogation region in the channel. The emitted light from the particle is collected by a photodiode (PD) placed over the detection window. The device sorts the particles into the sorted or waste outlets depending on the level of the PD signal. We used fluorescent latex beads to test the detection and sorting functionalities of the device. We found that the system could detect all the beads that passed through its geometric observation region and could sort almost all the beads it detected.