Perspective use of direct human blood as an energy source in air-breathing hybrid microfluidic fuel cells
Introduction
The recently developed glucose enzymatic biofuel cells (BFC's) could become an important alternative for producing energy to power either biomedical devices or active sensors [1], [2]. Nevertheless, the currently developed glucose biofuel cells still yield low current and power densities and poor stability with a short lifetime mainly due to inefficient immobilization [3], [4]. Hybrid fuel cell (HFC) systems are an alternative studied based on an enzymatic catalyst and an abiotic material, combining the best properties of both types of catalysts and assuming that abiotic material works properly under physiological conditions (pH 7, 37 °C) [5]. Furthermore, to become implantable, the glucose bio or hybrid fuel cells must solve others issues: biomaterial use, on-chip fabrication and the ability to operate at the glucose and oxygen concentrations available in blood or other bodily fluids. In this context, glucose oxidase (GOx) has been used to fabricate the bioanode due to its high catalytic activity and selectivity for β-d-glucose under physiological conditions [6]. Pt has been selected as an abiotic catalyst for the cathode due to its high oxygen reduction activity. Pt is also biocompatible and operates efficiently in the human body [7], [8]. Furthermore, using an abiotic catalyst reduces the catalytic inactivity to ions or organic molecules relative to enzymes such as bilirubin oxidase or laccase that are usually used in the cathode [9].
Some relevant works evaluating glucose enzymatic biofuel cells under physiological conditions are listed in Table 1. The maximum performance was reported by Pan Caofeng et al. [10] at 0.03 mW cm−2 using phosphate-buffered saline (PBS) as the electrolyte; nevertheless, the same work found a critical power decrease when the BFC was tested using human blood (0.0056 mW cm−2). Similar behaviour had already been presented by Xiaoju Wang et al., [11] who performed an additional test using plasma. An improved performance was achieved [12], [13], [14] and primarily corresponded to the enrichment of glucose to 10 mM in the solutions, which is not a natural concentration for blood or human serum (3–5 mM). However, substituting human serum with bovine serum [5], [6], [12] lowered the performance. The work presented in Refs. [13], [15] evaluated BFCs under quiet conditions in non-human media, which neglects the fluid dyamics of the human body, an important challenge to overcome. Additionally, using mediators [16] is typical when evaluating BFCs which is an important limitation for implantation.
The use of microfluidic platforms to build electrochemical microdevices for energy conversion transfers the operating principles of HFCs to a novel membraneless fuel cell technology, sometimes called a microfluidic fuel cell, for building implantable on-chip power sources. The following work reports a hybrid air-breathing microfluidic fuel cell (HμFC) composed by microfabricated components that incorporate a bioanode and an abiotic cathode as electrodes. The performance of the hybrid device is evaluated in the presence of glucose as fuel, which is obtained from three sources: phosphate buffer, human serum and human blood; and oxygen taken directly from air as oxidant which is a great advantage to increase the limiting reagent normally found in these micro fuel cells. The encouraging results obtained make this easily constructed microdevice a promising candidate for use as a power supply for in vivo applications due to its high performance.
Section snippets
Electrode fabrication
The bioanode was constructed from two important elements: a biocompatible substrate made of ARcare®8890 (Adhesives Research Inc) covered by 0.66 cm2 of adhesive graphite paper (Fig. 1a–a′) and a double deposit of GOx/VC-MWCNT-GA.
The enzyme catalytic ink contained glucose oxidase (GOx) (5 mg mL−1) dissolved in 0.1 M phosphate buffer (PB) at pH 7 and immobilised with 1% glutaraldehyde (GA) via cross-linking. In a parallel process, multi-walled carbon nanotubes (MWCNTs) were cleaned by refluxing
Electrochemical measurements
The air-breathing HμFC performance exhibited the following behaviours for the four conditions described in Section 2.4 (Fig. 3): for case (I), the open circuit potential (OCP) was approximately 0.86 V, which provides acceptable thermodynamic stability for a hybrid fuel cell [19]. However, the OCP and power density decreased considerably for cases II and III relative to the first case (from 0.86 to 0.66 to 0.52 V and 0.62 to 0.33 to 0.22 mW cm−2). This behaviour might be attributable to the high
Conclusions
In summary a small air-breathing hybrid microfluidic fuel cell powered using glucose from human serum and blood as a fuel with O2 supplied from air was fabricated. This study may offer a new type of hybrid microfluidic fuel cell with excellent performance (0.2 mW cm−2) and good stability as a possible power source for in vivo use. The enzymatic activity determined for the GOx/VC-MWCNT-GA bioanode exhibited a good operational stability during the HμFC time tests under different conditions.
The
Acknowledgements
The authors wish to thank the Mexican Council for Science and Technology CONACYT for financial support through the ANR-CONACYT project (Grant 163114).
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