Paper-based microfluidic biofuel cell operating under glucose concentrations within physiological range

Highlights

• A compact paper-based biofuel cell that can be operated using a very limited sample volume is presented.

• The system explores the energy generated by glucose at concentrations within physiological range.

• The paper-based fuel cell has the potential of generating energy to power small biodevices.

Abstract

This work addresses the development of a compact paper-based enzymatic microfluidic glucose/O₂ fuel cell that can operate using a very limited sample volume (≈35 µl) and explores the energy generated by glucose at concentrations typically found in blood samples at physiological conditions (pH 7.4). Carbon paper electrodes combined with a paper sample absorption substrate all contained within a plastic microfluidic casing are used to construct the paper-based fuel cell. The anode catalysts consist of glucose dehydrogenase and [Os(4,4′-dimethoxy-2,2′-bipyridine)₂(poly-vinylimidazole)₁₀Cl]⁺ as mediator, while the cathode catalysts were bilirubin oxidase and [Os(2,2′-bipyridine)₂(poly-vinylimidazole)₁₀Cl]⁺ as mediator.

The fuel cell delivered a linear power output response to glucose over the range of 2.5–30 mM, with power densities ranging from 20 to 90 µW cm⁻². The quantification of the available electrical power as well as the energy density extracted from small synthetic samples allows planning potential uses of this energy to power different sensors and analysis devices in a wide variety of in-vitro applications.

Introduction

The need for alternative energy sources, able to produce clean electricity, has stimulated the investigation of new portable sources of sustainable and renewable energy production during the last couple of decades. In this respect, research in enzymatic fuel cells (EFC) has been encouraged due to their unique advantages over metal-based conventional fuel cells; namely, their environmental friendly nature related to their biological origin and their capability of working with very mild chemical reactants. However, despite the notable improvements performed in the increase of enzymatic catalytic activity and the electronic transfer to the fuel cell electrodes, enzyme fuel cells still lack of the required stability and lifetime to be considered a solid candidate for clean energy production.

Another major motivation for the development of enzymatic fuel cells (Luckarift et al., 2014) concerns to the production of electricity from glucose available in human physiological fluids, e.g. blood, plasma, saliva, tears and urine (Falk et al., 2014, Milton et al., 2015, Bandodkar and Wang, 2016, Conghaile et al., 2016). The possibility of envisaging the use of the enzymatic fuel cells as power sources for implantable devices has generated a lot of research efforts in the last decade. Despite the progress in this direction (Katz and MacVittie, 2013), to the best of our knowledge, implantable fuel cells have shown to be unable to work for more than a few days.

Lately, biofuel cells have found a very interesting applications niche in in-vitro applications. In this scenario, they would generate energy to power small biodevices using the electrical power extracted from the same sample to be analyzed (Bullen et al., 2006; Pinyou et al., 2015; Zhou, 2015). This turns to be particularly interesting for paper-based analytical devices (Yetisen et al., 2013), in which an integrated power source would enable to obtain unambiguous results without external instrumentation. In fact, paper has been applied as a substrate to develop power sources and batteries before (Sharifi et al., 2015). Among them, enzymatic fuel cells appear to be one of the most suitable power sources for paper-based devices in terms of environmental impact (Nguyen et al., 2014). Paper allows the confinement of liquids to specified regions and can wick fluids via capillary action allowing passive liquid transport (Osborn et al., 2010; Thom et al., 2013; Cate et al., 2014). Moreover, enzymatic paper-based devices have been identified as especially suitable for point-of-care purposes in the field of home health-care settings and for implementation in developing countries (Dungchai et al., 2009; Martinez et al., 2009; Nie et al., 2010).

Since their early appearance, the development of these microfluidic platforms was centered on the enhancement of analytical performance attained by system miniaturization (Meredith and Minteer, 2012). One way of increasing the integration level of these enzymatic paper-based systems is through immobilization of catalysts at the electrode surfaces. Catalysis in an EFC is achieved usually by a specific enzyme, with electron transfer between enzyme and electrode mediated by a redox species, so if these are immobilized on the electrode surface the cost involved in the power generation is minimized. Furthermore, enzymatic lifetime improved using immobilized catalysts.

In general, the power output generated by an EFC can be raised if porous electrode structures, that support fuel transport to the reaction sites, are used (Kim et al., 2006; Wen and Eychmüller, 2016). Therefore, the strategies followed by researchers have been focused on increasing the electrode reactive surface area and on optimizing an efficient charge transfer mechanism between the liquid phase, the catalytic solution and the electrode surface inside the system (MacAodha et al., 2012).

The enzymes are chosen depending on the characteristics required. Among the enzymes performing oxidation of glucose at the anode, FAD-dependent glucose dehydrogenase (GDH) is widely employed because it is unaffected by molecular oxygen present in solution (Tsujimura et al., 2006; Moehlenbrock et al., 2011). Regarding the electro-reduction of oxygen to water at the cathode the most reported enzyme is bilirubin oxidase (BOx), very suitable for applications requiring pH close to neutrality (Ivanov et al., 2010). Selection of redox mediator or appropriate redox potential and structure can enhance electron transfer between enzyme and electrode. Since the initial report on the use of ferrocene and its derivatives (Cass et al., 1984), osmium-based redox polymers have been widely utilized in enzymatic devices (Aquino Neto and De Andrade, 2013). Osmium redox polymer mediators have the flexibility that their redox potential can be tailored conveniently by altering the ligands attached to the osmium central metal atom (Forster and Vos, 1990, Gallaway and Calabrese Barton, 2008, Sakai et al., 2009) giving a broad voltage range that makes them suitable for both anode and cathode processes in glucose/ O₂ enzymatic fuel cells (Rengaraj et al., 2011; Osadebe et al., 2015).

With the aim of approaching enzymatic fuel cells to a practical application, we report the development of a compact paper-based enzymatic microfluidic glucose/O₂ fuel cell that is operated with a small sample volume at blood glucose concentration levels. In view of its applicability to real samples, the volume required to feed the device has been restricted to only 35 µl, a volume easily obtained from a finger prick. This was carried out using a flexible and lightweight device fabricated using laminated plastic materials with a paper-based core.