Seokheun Choi

Paper-based batteries: a review.

There is an extensively growing interest in using paper or paper-like substrates for batteries and other energy storage devices. Due to their intrinsic characteristics, paper (or paper-like) batteries show outstanding performance while retaining low cost, multifunctionality, versatility, flexibility and disposability. In this overview, we review recent achievements in paper (or paper-like) batteries as well as their applications. Various types of paper power devices are discussed including electrochemical batteries, biofuel cells, lithium-ion batteries, supercapacitors, and nanogenerators. Further scientific and technological challenges in this field are also discussed.

A μL-scale micromachined microbial fuel cell having high power density

We report a MEMS (Micro-Electro-Mechanical Systems)-based microbial fuel cell (MFC) that produces a high power density. The MFC features 4.5-μL anode/cathode chambers defined by 20-μm-thick photo-definable polydimethylsiloxane (PDMS) films. The MFC uses a Geobacter-enriched mixed bacterial culture, anode-respiring bacteria (ARB) that produces a conductive biofilm matrix. The MEMS MFC generated a maximum current density of 16,000 μA cm(-3) (33 μA cm(-2)) and power density of 2300 μW cm(-3) (4.7 μW cm(-2)), both of which are substantially greater than achieved by previous MEMS MFCs. The coulombic efficiency of the MEMS MFC was at least 31%, by far the highest value among reported MEMS MFCs. The performance improvements came from using highly efficient ARB, minimizing the impact of oxygen intrusion to the anode chamber, having a large specific surface area that led to low internal resistance.

A paper-based microbial fuel cell: instant battery for disposable diagnostic devices.

We present a microfabricated paper-based microbial fuel cell (MFC) generating a maximum power of 5.5 μW/cm(2). The MFC features (1) a paper-based proton exchange membrane by infiltrating sulfonated sodium polystyrene sulfonate and (2) micro-fabricated paper chambers by patterning hydrophobic barriers of photoresist. Once inoculum and catholyte were added to the MFC, a current of 74 μA was generated immediately. This paper-based MFC has the advantages of ease of use, low production cost, and high portability. The voltage produced was increased by 1.9 × when two MFC devices were stacked in series, while operating lifetime was significantly enhanced in parallel.

Methods of reducing non-specific adsorption in microfluidic biosensors

Non-specific adsorption (NSA) of biomolecules is a persistent challenge in microfluidic biosensors. Microfluidic biosensors often have immobilized bioreceptors such as antibodies, enzymes, DNAs, etc, via linker molecules such as SAMs (self-assembled monolayers) to enhance immobilization. However, the linker molecules are very susceptible to NSA, causing false responses and decreasing sensitivity. In this paper, we present design methods to reduce the NSA of alkanethiol SAMs, which are popular linker molecules on microfluidic biosensors. Three design parameters were studied for two different chain-length SAMs (n = 2 and 10): (i) SAM incubation time, (ii) surface roughness [0.8 nm and 4.4 nm RMS (root mean square)] and (iii) gold crystal re-growth along (1?1?1) the target orientation. NSA was monitored by surface plasmon resonance (SPR). The results suggest that increased SAM incubation time reduces NSA, and that short-chain SAMs respond more favorably than the long-chain SAMs. Both SAMs were shown to be sensitive to surface roughness, and long-chain SAMs reduced NSA by 75%. Gold crystal re-growth along (1?1?1) the target orientation profoundly reduced NSA on the short-chain SAM. On a gold surface where surface roughness was 0.8 nm and there was strong directional alignment along the (1?1?1) gold crystal, final concentrations of nonspecifically bound proteins were 0.05 ng mm?2 (fibrinogen) and 0.075 ng mm?2 (lysozyme)?significantly lower than other known methods. The results show that optimizing three parameters (SAM incubation time, gold surface roughness and gold crystal orientation) improved SAM sensitivity for fibrinogen?anti-fibrinogen conjugates by a factor of 5 in 2.94 pM, suggesting that the methods are effective for reducing NSA in microfluidic biosensors.

Surface plasmon resonance protein sensor using Vroman effect

We report a new surface plasmon resonance (SPR) protein sensor using the Vroman effect for real-time, sensitive and selective detection of protein. The sensor relies on the competitive nature of protein adsorption onto the surface, directly depending upon proteins molecular weight. The sensor uses SPR for highly sensitive biomolecular interactions detection and the Vroman effect for highly selective detection. By using the Vroman effect we bypass having to rely on bio-receptors and their attachment to transducers, a process known to be complex and time-consuming. The protein sensor is microfabricated to perform real-time protein detection using four different proteins including aprotinin (0.65kDa), lysozyme (14.7kDa), streptavidine (53kDa), and isolectin (114kDa) on three different surfaces, namely a bare-gold surface and two others modified by OH- and COOH-terminated self-assembled monolayer (SAM). The real-time adsorption and displacement of the proteins are observed by SPR and evaluated using an atomic force microscope (AFM). The sensor can distinguish proteins of at least 14.05kDa in molecular weight and demonstrate a very low false positive rate. The protein detector can be integrated with microfluidic systems to provide extremely sensitive and selective analytical capability.

A contour-mode film bulk acoustic resonator of high quality factor in a liquid environment for biosensing applications

This letter reports an acoustic resonator of high quality factors (Qs) operating in liquid media. The film bulk acoustic resonator (FBAR) is made of a ring-shaped piezoelectric aluminum nitride thin film, and is excited in a contour mode. By having a low motional resistance upon coupling with liquids, the contour mode FBAR achieved Qs up to 189, more than 12× over the state-of-the-art FBARs in liquids. The resonator was characterized by an aptamer—thrombin binding pair for a biosensor and showed a mass resolution of 1.78 ng/cm2.

A High-Quality-Factor Film Bulk Acoustic Resonator in Liquid for Biosensing Applications

We report a high-quality-factor (Q) film bulk acoustic resonator (FBAR) operating in liquid environments. By integrating a microfluidic channel to a longitudinal-mode FBAR, a Q of up to 150 is achieved with direct liquid contacting. A transmission line model is used to theoretically predict the Q behavior of the FBAR. The model suggests an oscillatory pattern of Q as a function of the channel thickness and the acoustic wavelength in the liquid, which is experimentally verified by precisely controlling the channel thickness. This FBAR biosensor is characterized in liquids for the real-time in situ monitoring of the competitive adsorption/exchange of proteins, the Vroman effect. The FBAR offers a minimum detectable mass of 1.35 ng/cm2 and is successfully implemented in a Pierce oscillator as a portable sensing module.

A microfluidic biosensor based on competitive protein adsorption for thyroglobulin detection

We report a microfluidic sensing platform for the detection of thyroglobulin (Tg) using competitive protein adsorption. Serum Tg is a highly specific biomarker for residual thyroid tissue, recurrence and metastases after treatment for differentiated thyroid cancer (DTC). Conventional Tg detection techniques require complicated immobilization of antibodies and need to form a sandwich assay using additional secondary antibodies to enhance the sensitivity. We present a fundamentally different sensing technique without using antibody immobilization on a microfluidic platform. We engineer two surfaces covered by two known proteins, immunoglobulin G (IgG) and fibrinogen, with different affinities onto the surfaces. The microfluidic device offers a selective protein sensing by being displaced by a target protein, Tg, on only one of the surfaces. By utilizing the competitive protein adsorption, Tg displaces a weakly bound protein, IgG; however, a strongly bound protein, fibrinogen, is not displaced by Tg. The surface plasmon resonance (SPR) sensorgrams show that five human serum proteins, albumin, haptoglobin, IgG, fibrinogen and Tg, have different adsorption strengths to the surface and the competitive adsorption of individuals controls the exchange sequence. The adsorption and exchange are evaluated by fluorescent labeling of these proteins. Tg in a protein mixture of albumin, haptoglobin, and Tg is selectively detected based on the exchange reaction. By using the technique, we obviate the need to rely on antibodies as a capture probe and their attachment to transducers.

Powering point-of-care diagnostic devices

Effective and rapid point-of-care (POC) diagnostics have the capability to revolutionize public healthcare both in developed and developing countries. One of the key challenges that is critical to address in developing POC devices is to effectively and sufficiently power them. In developing countries, where the electricity grid is not well established and the use of batteries is not cost-effective, power supplies are the most problematic issue for stand-alone and self-sustained POC devices. In this review, we provide an overview of techniques for powering POC diagnostic devices for use in both developed and developing countries, as well as detailed discussions of recent advancements in POC devices. Then, we discuss next-generation POC diagnostics and their power source strategies.

A 3D paper-based enzymatic fuel cell for self-powered, low-cost glucose monitoring.

In this work, we demonstrate a novel low-cost, self-powered paper-based biosensor for glucose monitoring. The device operating mechanism is based on a glucose/oxygen enzymatic fuel cell using an electrochemical energy conversion as a transducing element for glucose monitoring. The self-powered glucose biosensor features (i) a 3D origami paper-based structure for easy system integration onto paper, (ii) an air-cathode on paper for low-cost production and easy operation, and (iii) a screen printed chitosan/glucose oxidase anode for stable current generation as an analytical signal for glucose monitoring. The sensor showed a linear range of output current at 1-5mM glucose (R(2)=0.996) with a sensitivity of 0.02 µA mM(-1). The advantages offered by such a device, including a low cost, lack of external power sources/sophisticated external transducers, and the capacity to rapidly generate reliable results, are well suited for the clinical and social settings of the developing world.