Constrained shrinking of nanoimprinted pre-stressed polymer films to achieve programmable, high-resolution, miniaturized nanopatterns.

Nanoimprint lithography is an emerging technology to form patterns and features in the nanoscale. Production of nanoscale patterns is challenging particularly in the sub-50 nm range. Pre-stressed polymer films with embedded microscale pattern can be miniaturized by shrinking induced due to thermal stress release. However, when pre-stressed films are thermally nanoimprinted with sub-micron features and shruken, they lose all the topographical features due to material recovery. Here we report a new approach that prevents recovery and allows retention of shrunken patterns even at the scale of <50 nm. We have discovered that when the shrinking process is mechanically constrained in one direction, the thermal treatment only relieves the stress in the orthogonal direction leading to a uniaxial shrinkage in that direction while preserving the topographical features. A second step, with the constraint in the orthogonal direction leads to biaxial shrinkage and preservation of all of the topographical features. This new technique can produce well defined and high resolution nanostructures at dimensions below 50 nm. The process is programmable and the thermal treatment can be tuned to shrink features to various dimension below the original imprint which we use to produce tunable and gradient plasmonic structures.

Tissue-in-a-Tube: three-dimensional in vitro tissue constructs with integrated multimodal environmental stimulation.

Three-dimensional (3D) in vitro tissue models are superior to two-dimensional (2D) cell cultures in replicating natural physiological/pathological conditions by recreating the cellular and cell-matrix interactions more faithfully. Nevertheless, current 3D models lack either the rich multicellular environment or fail to provide appropriate biophysical stimuli both of which are required to properly recapitulate the dynamic in vivo microenvironment of tissues and organs. Here, we describe the rapid construction of multicellular, tubular tissue constructs termed Tissue-in-a-Tube using self-assembly process in tubular molds with the ability to incorporate a variety of biophysical stimuli such as electrical field, mechanical deformation, and shear force of the fluid flow. Unlike other approaches, this method is simple, requires only oxygen permeable silicone tubing that molds the tissue construct and thin stainless-steel pins inserted in it to anchor the construct and could be used to provide electrical and mechanical stimuli, simultaneously. The annular region between the tissue construct and the tubing is used for perfusion. Highly stable, macroscale, and robust constructs anchored to the pins form as a result of self-assembly of the extracellular matrix (ECM) and cells in the bioink that is filled into the tubing. We demonstrate patterning of grafts containing cell types in the constructs in axial and radial modes with clear interface and continuity between the layers. Different environmental factors affecting cell behavior such as compactness of the structure and size of the constructs can be controlled through parameters such as initial cell density, ECM content, tubing size, as well as the distance between anchor pins. Using connectors, network of tubing can be assembled to create complex macrostructured tissues (centimeters length) such as fibers that are bifurcated or columns with different axial thicknesses which can then be used as building blocks for biomimetic constructs or tissue regeneration. The method is versatile and compatible with various cell types including endothelial, epithelial, skeletal muscle cells, osteoblast cells, and neuronal cells. As an example, long mature skeletal muscle and neuronal fibers as well as bone constructs were fabricated with cellular alignment dictated by the applied electrical field. The versatility, speed, and low cost of this method is suited for widespread application in tissue engineering and regenerative medicine.

Electrokinetic transport and distribution of antibacterial nanoparticles for endodontic disinfection.

AIM: To assess a novel, noninvasive intervention capable of mobilizing charged antibacterial nanoparticles to the apical portions of the root canal system, utilizing the principles of electrokinetics. METHODS: Experiments were conducted in three stages. Stage-1: A computer model was generated to predict and visualize the electric field and current density distribution generated by the proposed intervention. Stage-2: Transport of chitosan nanoparticles (CSnp) was evaluated qualitatively using a transparent microfluidic model with fluorescent-labelled CSnp. Stage-3: An ex vivo model was utilized to study the antimicrobial efficacy of the proposed treatment against 3-week-old monospecies E. faecalis biofilms. Scanning electron microscopy (SEM) was also utilized in this stage to confirm the deposition of CSnp. RESULTS: The results of the computer simulations predicted an electric field and current density that reach their maxima at the apical constriction of the root canal. Correspondingly, the microfluidic experiments demonstrated rapid, controlled CSnp transport throughout the simulated root canal anatomy with subsequent distribution and deposition in the apical constriction as well as periapical regions. Infected root canals when subjected to the novel treatment method resulted in a mean bacterial reduction of 2.1 log CFU. SEM analysis revealed electrophoretic deposition of chitosan nanoparticles onto the root canal dentine walls in the apical region. CONCLUSION: The findings from this study demonstrate that the combination of cationic antibacterial nanoparticles with a low-intensity electric field results in particle transportation (electrophoresis) and deposition within the root canal. This results in a synergistic antibiofilm efficacy and has the potential to enhance root canal disinfection.

Continuous flow microreactor for protein PEGylation.

PEGylation is increasingly being utilized to enhance the therapeutic efficacy of biopharmaceuticals. Various chemistries and reaction conditions have been established to synthesize PEGylated proteins and more are being developed. Both the extent of conversion and selectivity of protein PEGylation are highly sensitive to process variables and parameters. Therefore, microfluidic-based high-throughput screening platforms would be highly suitable for optimization of protein PEGylation. As part of this study, a poly-dimethylsiloxane-based continuous flow microreactor system was designed and its performance was compared head-to-head with a batch reactor. The reactants within the microreactor were contacted by passive micromixing based on chaotic advection generated by staggered herringbone grooves embedded in serpentine microchannels. The microreactor system was provided with means for on-chip reaction quenching. Lysozyme was used as the model protein while methoxy-polyethylene glycol-(CH2)5COO-NHS was used as the PEGylation reagent. Full mixing was achieved close to the microreactor inlet, making the device suitable for protein PEGylation. The effect of mixing type, i.e., simple stirring versus chaotic laminar mixing on PEGylation, was investigated. Higher selectivity (as high as 100% selectivity) was obtained with the microreactor while the conversion was marginally lower.

Design of a flat field concave-grating-based micro-Raman spectrometer for environmental applications.

In order to simplify the design process of microfabricated concave gratings, simplified algorithms for fast characterization of the concave grating were developed. These algorithms can be used to assist system designers using ray-tracing software in the determination of optimum design parameters considering the requirements and restrictions for specific applications. According to the algorithms, it is feasible to design a flat field microconcave grating with a 4 mm grating radius as a key component in a micro-Raman spectrometer system for inline environmental monitoring applications. This microspectrometer operates over the spectral wavelength band from 785 nm to 1000 nm and has a spectral resolution of 2 nm at 900 nm. The total size of the system is 1 mm×4 mm×3.7 mm, making it one of the smallest for this wavelength range and spectrum resolution.

Electrochemical growth of high-aspect ratio nanostructured silver chloride on silver and its application to miniaturized reference electrodes.

The sensitivity of many biological and chemical sensors is critically dependent on the stability of the potential of the reference electrode being used. The stability of a reference electrode's potential is highly influenced by the properties of its surface. In this paper, for the first time, the formation of nanosheets of silver chloride on silver wire is observed and controlled using high anodic constant potential (>0.5 V) and pulsed electrodeposition. The resulting nanostructured morphology substantially improves the electrode's potential stability in comparison with the conventional globular surface structure. The increased stability is attributed to the increase in the surface area of the silver chloride produced by the nanosheet formation.

Finite-Element Modelling of Biotransistors.

Current research efforts in biosensor design attempt to integrate biochemical assays with semiconductor substrates and microfluidic assemblies to realize fully integrated lab-on-chip devices. The DNA biotransistor (BioFET) is an example of such a device. The process of chemical modification of the FET and attachment of linker and probe molecules is a statistical process that can result in variations in the sensed signal between different BioFET cells in an array. In order to quantify these and other variations and assess their importance in the design, complete physical simulation of the device is necessary. Here, we perform a mean-field finite-element modelling of a short channel, two-dimensional BioFET device. We compare the results of this model with one-dimensional calculation results to show important differences, illustrating the importance of the molecular structure, placement and conformation of DNA in determining the output signal.

Microfabricated reference electrodes and their biosensing applications.

Over the past two decades, there has been an increasing trend towards miniaturization of both biological and chemical sensors and their integration with miniaturized sample pre-processing and analysis systems. These miniaturized lab-on-chip devices have several functional advantages including low cost, their ability to analyze smaller samples, faster analysis time, suitability for automation, and increased reliability and repeatability. Electrical based sensing methods that transduce biological or chemical signals into the electrical domain are a dominant part of the lab-on-chip devices. A vital part of any electrochemical sensing system is the reference electrode, which is a probe that is capable of measuring the potential on the solution side of an electrochemical interface. Research on miniaturization of this crucial component and analysis of the parameters that affect its performance, stability and lifetime, is sparse. In this paper, we present the basic electrochemistry and thermodynamics of these reference electrodes and illustrate the uses of reference electrodes in electrochemical and biological measurements. Different electrochemical systems that are used as reference electrodes will be presented, and an overview of some contemporary advances in electrode miniaturization and their performance will be provided.