Enhanced Radio Frequency Biosensor for Food Quality Detection Using Functionalized Carbon Nanofillers.

This paper outlines an improved design of inexpensive, wireless and battery free biosensors for in situ monitoring of food quality. This type of device has an additional advantage of being operated remotely. To make the device, a portion of an antenna of a passive 13.56 MHz radio frequency identification (RFID) tag was altered with a sensing element composed of conductive nanofillers/particles, a binding agent, and a polymer matrix. These novel RFID tags were exposed to biogenic amine putrescine, commonly used as a marker for food spoilage, and their response was monitored over time using a general-purpose network analyzer. The effect of conductive filler properties, including conductivity and morphology, and filler functionalization was investigated by preparing sensing composites containing carbon particles (CPs), multiwall carbon nanotubes (MWCNTs), and binding agent grafted-multiwall carbon nanotubes (g-MWCNTs), respectively. During exposure to putrescine, the amount of reflected waves, frequency at resonance, and quality factor of the novel RFID tags decreased in response. The use of MWCNTs reduced tag cutoff time (i.e., faster response time) as compared with the use of CPs, which highlighted the effectiveness of the conductive nanofiller morphology, while the addition of g-MWCNTs further accelerated the sensor response time as a result of localized binding on the conductive nanofiller surface. Microstructural investigation of the film morphology indicated a better dispersion of g-MWCNTs in the sensing composite as compared to MWCNTs and CPs, as well as a smoother texture of the surface of the resulting coating. These results demonstrated that grafting of the binding agent onto the conductive particles in the sensing composite is an effective way to further enhance the detection sensitivity of the RFID tag based sensor.

Hydrogel discs for digital microfluidics.

Hydrogels are networks of hydrophilic polymer chains that are swollen with water, and they are useful for a wide range of applications because they provide stable niches for immobilizing proteins and cells. We report here the marriage of hydrogels with digital microfluidic devices. Until recently, digital microfluidics, a fluid handling technique in which discrete droplets are manipulated electromechanically on the surface of an array of electrodes, has been used only for homogeneous systems involving liquid reagents. Here, we demonstrate for the first time that the cylindrical hydrogel discs can be incorporated into digital microfluidic systems and that these discs can be systematically addressed by droplets of reagents. Droplet movement is observed to be unimpeded by interaction with the gel discs, and gel discs remain stationary when droplets pass through them. Analyte transport into gel discs is observed to be identical to diffusion in cases in which droplets are incubated with gels passively, but transport is enhanced when droplets are continually actuated through the gels. The system is useful for generating integrated enzymatic microreactors and for three-dimensional cell culture. This paper demonstrates a new combination of techniques for lab-on-a-chip systems which we propose will be useful for a wide range of applications.

Digital microfluidic hydrogel microreactors for proteomics.

Proteolytic digestion is an essential step in proteomic sample processing. While this step has traditionally been implemented in homogeneous (solution) format, there is a growing trend to use heterogeneous systems in which the enzyme is immobilized on hydrogels or other solid supports. Here, we introduce the use of immobilized enzymes in hydrogels for proteomic sample processing in digital microfluidic (DMF) systems. In this technique, preformed cylindrical agarose discs bearing immobilized trypsin or pepsin were integrated into DMF devices. A fluorogenic assay was used to optimize the covalent modification procedure for enzymatic digestion efficiency, with maximum efficiency observed at 31 µg trypsin in 2-mm diameter agarose gel discs. Gel discs prepared in this manner were used in an integrated method in which proteomic samples were sequentially reduced, alkylated, and digested, with all sample and reagent handling controlled by DMF droplet operation. Mass spectrometry analysis of the products revealed that digestion using the trypsin gel discs resulted in higher sequence coverage in model analytes relative to conventional homogenous processing. Proof-of-principle was demonstrated for a parallel digestion system in which a single sample was simultaneously digested on multiple gel discs bearing different enzymes. We propose that these methods represent a useful new tool for the growing trend toward miniaturization and automation in proteomic sample processing.

A circular cross-section PDMS microfluidics system for replication of cardiovascular flow conditions.

Since the inception of soft lithography, microfluidic devices for cardiovascular research have been fabricated easily and cost-effectively using the soft lithography method. The drawback of this method was the fabrication of microchannels with rectangular cross-sections, which did not replicate the circular cross-sections of blood vessels. This article presents a novel, straightforward approach for the fabrication of microchannels with circular cross-sections in poly(dimethylsiloxane) (PDMS), using soft lithography. The method exploits the polymerization of the liquid silicone oligomer around a gas stream when both of them are coaxially introduced in the microchannel with a rectangular cross-section. We demonstrate (i) the ability to control the diameter of circular cross-sections of microchannels from ca. 40-100 mum; (ii) the fabrication of microchannels with constrictions, and (iii) the capability to grow endothelial cells on the inner surface of the microchannels.

Durable, region-specific protein patterning in microfluidic channels.

We present a straightforward, accessible method to covalently pattern proteins in poly(dimethyl siloxane) (PDMS) microchannels. Our approach includes (i) region-specific photografting of a layer of poly(acrylamide) (PAAm) and (ii) bioconjugation of PAAm with a desired protein. The method produces symmetric protein patterns on all channel walls, which have high specificity and pattern fidelity, are compatible with a variety of geometries and exhibit excellent longevity under shear stresses of up to 1 dyn/cm. We demonstrate the generality of the method by creating multi-protein gradients within microfluidic microchannels and by in-situ patterning of islands of multiple proteins. Protein activity was observed by the digestion of BODIPY-casein using channels patterned with trypsin.

Augmenting microgel flow via receptor-ligand binding in the constrained geometries of microchannels.

We investigated the flow dynamics of biotin-conjugated microgel capsules in avidin-conjugated microchannel constrictions. Microgels were prepared using a microfluidic assembly approach. Biotinylated microgels passing through avidin-modified constrictions slowed relative to several control systems. This effect was observed below a critical velocity of the microgels in the channel-at-large. The reduction in microgel velocity in the constriction occurred for several different sizes of microgels and orifices. Soft compliant microgels showed a lower velocity in the constriction relative to rigid microgels with the same concentration of biotin on the surface, due to the ability of the softer microgels to deform in the orifice and maximize their surface area when in contact with the orifice wall.

Flow of microgel capsules through topographically patterned microchannels.

We investigated the flow dynamics of microgel capsules in topographically patterned microfluidic devices. For microgels flowing through channel constrictions, or orifices, we observed three phenomena: (i) the effect of confinement, (ii) the role of interactions between the microgels and the channel surface, and (iii) the effect of the velocities of microgels prior to their passage through an orifice. We studied negatively charged alginate microgels and positively charged alginate microgels coated with N-(2-hydroxy)propyl-3-trimethylammonium chitosan chloride (HTCC). Aqueous dispersions of microgels were driven through poly(dimethyl siloxane) microchannels carrying a weak negative surface charge. The velocity of the continuous phase, and hence, the velocity of the microgels increased as they passed through topographically patterned orifices. Alginate microgels were observed to have a larger increase in velocity relative to HTCC-coated alginate microgels. This effect, which was attributed to electrostatic attraction or repulsion, was found to be strongest for orifices with dimensions close to the microgel diameter. For example, when 75 microm-diameter microgels flowed through a 76 microm orifice, alginate gels (negatively charged) experienced a 2x greater increase in velocity than HTCC-coated (positively charged) microgels. This effect was exaggerated at lower initial flow rates. For example, when 75 microm-diameter microgels flowed through an 80 microm orifice, a two-fold difference in the velocity changes of the two microgel types was observed when the initial flow rate was 275 microm s(-1), while a three-fold difference in velocity changes was observed when the initial flow rate was 130 microm s(-1). We speculate that these studies will be useful for modeling the flow of suspensions of cells or other biologically relevant particles for a wide range of applications.