Miniature Biofuel Cell as a Potential Power Source for Glucose

Article pubs.acs.org/ac

Miniature Biofuel Cell as a Potential Power Source for GlucoseSensing Contact Lenses Magnus Falk,†,¶ Viktor Andoralov,†,¶ Maria Silow,‡ Miguel D. Toscano,‡ and Sergey Shleev*,†,# †

Biomedical Sciences, Health & Society, Malmö University, 205 06 Malmö, Sweden Novozymes A/S, 2880 Bagsværd, Denmark # The Department of Chemical Enzymology, A.N. Bach Institute of Biochemistry, 119 071 Moscow, Russia ‡

S Supporting Information *

ABSTRACT: A microscale membrane-less biofuel cell, capable of generating electrical energy from human lachrymal liquid, was developed by utilizing the ascorbate and oxygen naturally present in tears as fuel and oxidant. The biodevice is based on three-dimensional nanostructured gold electrodes covered with abiotic (conductive organic complex) and biological (redox enzyme) materials functioning as efficient anodic and cathodic catalysts, respectively. Three-dimensional nanostructured electrodes were fabricated by modifying 100 μm gold wires with 17 nm gold nanoparticles, which were further modified with tetrathiafulvalene-tetracyanoquinodimethane conducting complex to create the anode and with Myrothecium verrucaria bilirubin oxidase to create the biocathode. When operated in human tears, the biodevice exhibited the following characteristics: an open circuit voltage of 0.54 V, a maximal power density of 3.1 μW cm−2 at 0.25 V and 0.72 μW cm−2 at 0.4 V, with a stable current density output of over 0.55 μA cm−2 at 0.4 V for 6 h of continuous operation. These findings support our proposition that an ascorbate/ oxygen biofuel cell could be a suitable power source for glucose-sensing contact lenses to be used for continuous health monitoring by diabetes patients.

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general directions in the determination technique: spectral determination, utilizing glucose sensitive hydrogels embedded with photonic crystals9−12 or fluorescence-based techniques,13−15 and electrochemical determination.7,16 One of the first reports of a flexible electrochemical biosensor used for glucose monitoring in human tears, saliva, and sweat was published in 1995 by Mitsubayashi et al.,17 where the biodevice was based on immobilized glucose oxidase. This progress was followed by a number of publications dedicated to the development of sensors suitable for measurements in situ while being selective and sensitive toward glucose.7,16,18−21 Improvements have also been made in microelectronics fabrication.22,23 Furthermore, integration of glucose biosensors into contact lenses has recently been realized by several research groups, who have been able to fabricate high performance bionic contact lenses for glucose monitoring in the eye.7,16,24 With different types of contact lenses, including integrated glucose biosensors already developed, functional and practical bionic contact lenses could potentially be realized in the near future. In fact, not only have glucose sensors been incorporated into contact lenses but also simple single-pixel displays,25 representing the first step in the development of contact lenses with functional displays. However, one persistent problem for

he occurrence of diabetes, a chronic systemic disease characterized by raised blood glucose concentration, is steadily increasing, especially in developed countries. Moreover, the treatment of the disease is a major cost for modern healthcare systems,1 with costs expected to rapidly increase even further. 2 Diabetes can be controlled by proper administration of insulin, where correct timing and dosage of the injections are important. Self-monitoring is widely used for controlling the level of glucose in the blood and usually performed by an invasive test, drawing a small amount of blood from the patient and measuring the amount of glucose using a portable glucometer. Nonetheless, this method is not ideal, and development of devices for continuous noninvasive monitoring of blood glucose could increase safety and convenience of testing, leading to improved public health and reduced medical costs. Noninvasive tests have received significant research interest, with typical noninvasive sensors being based on optical techniques for transcutaneous determination of the glucose in the blood.3−6 As an alternative, recent research efforts have been dedicated toward the determination of blood glucose indirectly using human lachrymal liquids (tear fluid), where a glucose-sensing element can be implemented directly into a contact lens, thus allowing for continuous noninvasive monitoring. Most importantly for the viability of such devices, recent studies have shown a direct correlation between tear and blood glucose concentration.7,8 The development of tear glucose sensors integrated into contact lenses includes two © 2013 American Chemical Society

Received: March 5, 2013 Accepted: June 5, 2013 Published: June 5, 2013 6342

dx.doi.org/10.1021/ac4006793 | Anal. Chem. 2013, 85, 6342−6348

Analytical Chemistry

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these types of devices is the implementation of a suitable power source, where integration with inductive links or RF circuits has been suggested. Such solutions are not ideal and can be rather complicated requiring inconvenient equipment. As an alternative power source, integration of a biofuel cell (BFC) into the bionic contact lens has recently been suggested, where enzyme catalysts were employed to convert the chemical energy from biofuel (glucose) and biooxidant (oxygen) available in tear fluid into electrical energy.26,27 BFCs present to be a very attractive alternative as a power supply for bionic contact lenses, since miniature biodevices can potentially be produced at a low cost and without complicated designs, especially when utilizing enzymes immobilized directly on the electrode surfaces without the usage of any mediators.27 For the specific application as a power source for a glucosesensing device, utilization of glucose as a fuel would be very problematic. In this case, a more suitable fuel source present in tears should be employed. Thus, in order to avoid the oxidization of the glucose present in human lachrymal liquid, we have designed a miniature membrane-less BFC utilizing ascorbate instead of glucose to generate electrical power. The power production should thereby not affect the concentration of glucose and influence the sensor performance. To create a stable anode, which efficiently electrooxidizes ascorbate, nanostructured microelectrodes were modified with tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ), known as one of the best catalysts for ascorbate oxidation.28 The complex is a well-known mediator, has been utilized to create enzyme-based biosensors, and was also recently used in a direct glucose fuel cell.28,29 The well-known enzyme, bilirubin oxidase (BOx), which efficiently reduces oxygen under physiological conditions, was utilized as a catalyst to create the direct electron transfer (DET) based biocathode.30−34 When employing efficient and specific catalysts immobilized on the electrode surfaces, like TTF-TCNQ and BOx, cross-reactions are minimized and a miniature, membrane- and compartment-less design can be used. The ascorbate/O2 BFC, which powers a contact lens-based glucose sensor with wireless electronics, could serve as a self-contained biodevice for continuous noninvasive monitoring of the level of glucose in the blood. A conceptual scheme of such a selfpowered bionic contact lens is shown in Figure 1, where a BFC together with a glucose sensor, interface chip, simple display, and antenna have virtually been integrated on a lens. A contact lens placed in the eye is exposed to an aqueous layer of basal tears,35 which has a very different composition compared to reflex tears, produced by emotional or mechanical stimulations.36,37 Collection of basal tears is, therefore, necessary to keep the content equivalent to the normal state in the eye,38 in order to properly investigate the performance of a BFC intended to operate as part of a bionic lens. Below, experimental proof that a TTF-TCNQ/BOx based miniature BFC can generate sufficient power from ascorbate and oxygen available in basal tears, without influencing the glucose concentration, to power modern ultralow power electronics is provided.39

Figure 1. Conceptual scheme of a bionic contact lens consisting of (1) a biocathode, modified with AuNPs and BOx, (2) an anode, modified with AuNPs and TTF-TCNQ complex, (3) a glucose biosensor, as described in the work by Chu et al.,7 (4) an interface chip, (5) a simple display, and (6) an antenna. The BFC supplies power to the other electronic components by converting the chemical energy existing in tears into electrical energy, where ascorbate is oxidized to dehydroascorbic acid and oxygen is reduced to water, thereby creating a glucose-sensing electronic contact lens.

TCNQ, and TTF were all purchased from Sigma-Aldrich (St. Louis, MO, U.S.A.). All chemicals used were of analytical grade. All solutions were prepared with ultrapure water (18 MOhm cm) using a PURELAB UHQ II system from ELGA Labwater (High Wycombe, U.K.). A 50 mM phosphate buffer, pH 7.4, containing 150 mM NaCl (i.e., phosphate buffer saline, PBS) was used as a physiological buffer. Au wires with a diameter of 0.1 mm were purchased from Goodfellow Cambridge Ltd. (Huntingdon, England). Enzyme. The MvBOx was produced recombinantly in Aspergillus oryzae, as previously reported.40 The purification was performed via a two-step chromatographic procedure using an Ä kta Explorer FPLC system from GE Healthcare (Little Chalfont, Buckinghamshire, U.K.) as follows: First, the fermentation broth was filtered through a Whatman Glass Microfilter sandwich and then through a Nalgene 0.2 μm Bottle Top filter. Then, (NH4)2SO4 was added to 1.5 M, and the pH was adjusted to 8.0, after which the broth was filtered again through a 0.2 μm filter to remove any possible precipitate. Then, the broth was loaded, according to the manufacturers recommendations, onto a ∼60 mL Phenyl sepharose (GE Healthcare) column pre-equilibrated with 5 column volumes of 20 mM Tris/Acetate, 1.5 M (NH4)2SO4, pH 8. After washing with 5 column volumes of the equilibration buffer, the enzyme was eluted by 2.5 column volumes of the elution buffer and 20 mM Tris/Acetate, pH 8. Throughout the washing and elution steps, 10 mL fractions were collected. All fractions were screened for activity on syringaldazine. The active fractions

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EXPERIMENTAL SECTION Chemicals. Sulphuric acid was purchased from Fluka (Buchs, Switzerland), sodium chloride from Merck (Darmstadt, Germany), ascorbic acid and D-glucose from BDH chemicals (Poole, England), and cellulose acetate, disodium phosphate, monosodium phosphate, gold chloride (AuCl), dopamine, 6343



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U.S.A.) in a similar way as the microelectrodes and used for investigations under model conditions. Experimental Details. Measurements under model conditions were performed by recording chronopotentiometry using a three-electrode rotating disk electrode system, RDE−2 from BASi (West Lafayette, U.S.A.), at a rotation rate of 1000 rpm. A Ag|AgCl|3 M NaCl electrode RE−5B (207 mV vs NHE) from BASi (West Lafayette, U.S.A.) and a platinum wire mesh from Sigma-Aldrich were used as reference and counter electrodes, respectively. Electrochemical characterization of the microelectrodes was performed using a μAutolab Type III/FRA2 potentiostat/ galvanostat from Metrohm (Riverview, FL, U.S.A.) in a macrocell with a total volume of 30 mL. Due to the limited amount of basal tears that could be collected, detailed characterization of the BFC was carried out in a microcell, consisting of a glass capillary with a volume of roughly 10 μL and an inside diameter of 1.5 mm. Linear sweep voltammetry was performed by connecting the anode as a working electrode and the biocathode as a combined counter and reference electrode, sweeping the potential with a scan speed of 0.1 mV s−1. Measurements with the microcell were kept at roughly 100% humidity by placing the cell in a humidified box, thereby preventing the evaporation of electrolytes from the glass capillary. Further characterization of the system was performed using the BFC setup in chronoamperometric mode and recording the current output over time at an applied voltage of 0.4 V, in a glass capillary with a volume of 25 μL. Electrochemical impedance was measured with a voltage amplitude perturbation of 5 mV using a 50 kHz to 10 mHz frequency range. Fitting of the impedance data was done using ZView program 2.0 from ZView 2.80 from Scribner Associates Inc. (Southern Pines, NC, U.S.A.) in the frequency region from 1 kHz to 10 mHz.

were then pooled and buffer-shifted to 20 mM Tris/Acetate, pH 8, before the next chromatographic step. In the second step, the pooled fractions were applied, according to the manufacturers recommendations, onto a ∼20 mL Source 15Q anion exchange (GE Healthcare) column pre-equilibrated with 5 column volumes of 20 mM Tris/Acetate, pH 8. After loading the enzyme, the column was washed with 5 column volumes of the equilibration buffer and eluted by a linear gradient from 0 to 100% of 20 mM Tris/acetate, 0.5 M Na2SO4, pH 8. All fractions were screened for activity on syringaldazine and for sample purity by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE; Novex NuPAGE 4−12% Bis-Tris gradient gels, Life Technologies Corp.). The active fractions of sufficient purity were then pooled and concentrated into a final concentration of 10 mg mL−1 using Amicon Ultra spin filters from Millipore Corporation (Billerica, MA, U.S.A.), with a MW cutoff of 10 kDa. Protein concentration was determined by measuring the absorbance at 280 nm and using the theoretical extinction coefficient derived from the amino acid sequence. The highly purified preparation of the enzyme, as confirmed by SDS-PAGE, was stored in a 50 mM Tris/acetate buffer, pH 8 at −18 °C. Human Lachrymal Liquid. Basal tears from apparently healthy male volunteers were collected by gently moving a glass capillary to the tear meniscus and waiting for the capillary to fill up, which is the typical method for collection of human lachrymal liquid. This method causes very little irritation to the eye, resulting in the collected tear fluid containing minimal contamination from mucus and serum components.41 This is a very time-consuming process and collection of only 15 μL can take close to 1 h. Electrode Design. Electrodes were fabricated based on Au microwires. The wires were insulated using cellulose acetate dissolved in acetone (15 mg mL−1). A length of roughly 3 mm was left uninsulated for use as working electrode surface. The electrodes were gently polished with 0.05 μm aluminum oxide powder from Struers (Westlake, OH, U.S.A.); thereafter, they were cleaned electrochemically by cycling the potential between −0.2 and 1.7 V (vs Ag|AgClKClsat) at a scan rate of 100 mV s−1 for 20 cycles in 0.5 M H2SO4. Au nanoparticles (AuNPs) with an average diameter of 17 nm were synthesized using the citrate reduction method as previously described42 and concentrated by centrifugation as previously outlined.26 The microbiocathode was prepared according to a previously developed method.26 Briefly, Au microwire electrodes were modified with AuNPs by repeatedly applying drops of concentrated solution to cleaned electrodes and allowing the solution to dry. The procedure increased the real surface area by approximately 100 times, creating three-dimensional (3D) nanostructured electrodes. Thereafter, the electrodes were electrochemically cleaned once more and incubated in a BOx solution (4 mg mL−1) for 2 h; subsequently, they were washed with buffer and ready for use. The microanode was prepared with AuNPs in the same way as the microbiocathode, thereafter dipped in a saturated TCNQ solution followed by a saturated TFF solution, both dissolved in acetone. The potential of the electrode was registered in PBS containing 0.5 mM ascorbic acid, and the TFF-TCNQ complex formation was repeated until it was lower than 110 mV vs normal hydrogen electrode (NHE). The macrosized anode and biocathode were prepared using rotating disk electrodes (RDEs) with a diameter of 3 mm from BASi (West Lafayette,

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RESULTS AND DISCUSSION Characteristics under Model Conditions. Both electrodes, i.e., the biocathode and the anode, were investigated under model conditions using a macro-scaled setup and compared with electrodes without a catalyst. The results are shown in Figure 2, where the potential of the electrodes was monitored at different ascorbate concentrations, naturally present in human basal tears at concentrations up to 0.665 mM.43,44 When the biocathode was modified with the enzyme, a significant shift was observed in the potentials to the cathodic region (cf. curves 1 and 2 in Figure 2), indicating efficient bioelectroreduction of oxygen. As the concentration of ascorbate was increased, the onset voltage of the biocathode was decreased, leading to a decrease in the open circuit voltage (OCV) of the BFC. Even though the enzymatic process is highly specific, some interfering reactions can affect the cathode due to the catalytic activity of the supporting material and, consequently, decrease its efficiency.45,46 It is evident from Figure 2 that ascorbate is such an interfering reagent, which depolarizes the biocathode. Electrooxidation of ascorbate is known to occur on many electrode materials at quite low overpotentials.47−49 However, this problem is difficult to mitigate, since separation of the fuel by employing membranes would be very difficult to implement in a contact lens and it might also cause some diffusion limitations. A possible solution would be to block the supporting material with a dense monolayer of properly oriented enzyme, using a similar strategy, as already reported for laccase-based biocathodes.50,51 6344



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the microanode in the presence of different oxidants is presented in Figure 3.

Figure 2. Open circuit potentials of typical (1) AuNPs with adsorbed BOx (biocathode), (2) bare AuNPs, and (3) AuNPs and TTF-TCNQ complex (anode) vs NHE (noted on the y-axis). The corresponding cell potential differences are also noted in the Figure, where (1) is used as cathode and either (2) or (3) as anode (Ecathode − Eanode, numbers given in the figure). Measurements were performed in air-saturated PBS at different concentrations of ascorbate (marked in the figure) using RDEs with a rotation rate of 1000 rpm. The average differences in the registered potential between different biocathodes were measured to be ±7% in the presence of ascorbate and ±3% without the biofuel. The average differences in the registered potential between different anodes and AuNPs modified electrodes without catalysts were estimated to be ±4% and ±5%, respectively.

Figure 3. Linear sweep voltammograms of microanode in a macrocell with air-saturated PBS containing: (1) 5 mM glucose; (2) 5 mM glucose and 0.2 mM ascorbate; (3) 5 mM glucose, 0.2 mM ascorbate, and 0.2 mM dopamine, with an average difference in the registered current output between different electrodes of ±7%. Scan rate: 5 mV s−1.

No activity toward glucose oxidation was observed in a wide potential range (Figure 3, curve 1), even though the biofuel was present at roughly one hundred times higher concentration than what recent studies show basal tears to contain.26,57 In the same potential range, the electrode was electrochemically very active toward ascorbate and dopamine (Figure 3, curves 2 and 3). The onset of dopamine oxidation started at a higher potential than oxidation of ascorbate, and a clear separation of the anodic waves could be observed (Figure 3, curve 3). These results demonstrate that the TTF-TCNQ modified anode is a suitable choice for the anodic element of a BFC, intended to operate in a contact lens as a power supply for a glucose sensor. No electrooxidation of glucose occurred; hence, the glucose level in the tear fluid can be assumed to be unaffected by the operation of the BFC. This is very important so as not to interfere with the supposed operation of a glucose sensor, which would reduce the accuracy of the blood-glucose measurement. Oxidation of dopamine occurs due to the supporting layer of AuNPs, where large oxidation currents are observed at concentrations of dopamine above 200 μM on the bare AuNPs electrode, as well as on electrodes modified with TTF-TCNQ (Supporting Information, Supporting Figures S-1 and S-2). The performance of the microbiocathode in air-saturated buffers is shown in Figure 4. Since human tears are air-saturated during waking hours,60 an air-saturated solution contains a similar amount of oxygen as is present in the normal state of the human eye. The current output of the cathode at large overpotential is similar in buffer containing only glucose, as well as glucose and ascorbate, albeit the onset potential is roughly 50 mV lower in the presence of 0.2 mM ascorbate (cf. curves 1 and 2 in Figure 4). This is in agreement with the observations under model conditions (Figure 2). Under these conditions, the current output of the biocathode was significantly higher compared to the current output of the anode, and the power output of the BFC in the high-voltage region would clearly be limited by the anode. Detailed electrochemical investigations of the biocathode have also been previously performed,26 where diffusion limited currents were obtained in simple buffers and a clear

Alternatively, a so-called air-breathing biocathode could be employed.52,53 These different strategies could be used to increase the efficiency of the biocathode, by increasing both the onset voltage and the output current. By modifying the AuNP modified anode with TTF-TCNQ complex, we observed a shift in the potential toward the anodic region (cf. curves 2 and 3 in Figure 2). In the presence of ascorbate, the potential of the TTF-TCNQ modified anode is close to the thermodynamic redox potential of the dehydroascorbic acid/ascorbate couple (0.077 V54). The low overpotential needed for ascorbate oxidation indicates that efficient electrooxidation of the biofuel occurs at the anode with an immobilized catalyst. From the investigations under model conditions, one can conclude that the OCV of the BFC (Ecathode − Eanode in Figure 2) is higher than 0.5 V for the typical concentration range of ascorbate in human lachrymal liquid, i.e., from 20 up to 665 μM.43,44,55 Reported sensors for noninvasive glucose monitoring were embedded into contact lenses operated at a voltage of 0.4 V,7,16 and the development of modern ultralow-power electronics has led to electronic systems that run on 0.5 V already being produced.56 This indicates that powering of a contact lens-based glucose-sensing biodevice with an ascorbate/ oxygen BFC is realistic and can be practically exploited in the near future. Performance of Anode and Biocathode. Human lachrymal liquid contains a multitude of organic low molecular weight compounds, which can be electrooxidized.44 Thus, aside from ascorbate and glucose, the performance of the anode and biocathode was therefore also investigated in the presence of dopamine. Studies have shown that tears contain glucose at concentrations of roughly 50 μM26,57 up to 600 μM35 and dopamine at concentrations of roughly 60 nM58 up to 2 mM.59 The large variation reported in the literature can be attributed to different collection methods, which can significantly influence liquid composition,36,37 as well as the small collected volumes with low amounts of bioanalytes. The performance of 6345



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and biocathode obtained under model conditions (cf. curves 1 and 3 in Figure 2). Two regions can be distinguished on the power density plot (Figure 5, curve 1): (i) one region at a voltage above roughly 0.4 V, in all likelihood, corresponds to electrooxidation of ascorbate, and (ii) another one below 0.4 V, most probably, corresponds to electrooxidation of dopamine in combination with ascorbate and other antioxidants present in the solution. These results are in agreement with the data obtained from the microanode operating in buffer solutions (Figure 3). A maximal power density of 3.1 μW cm−2 at a cell voltage of 0.25 V was obtained in tears, reduced to 0.72 μW cm−2 at a cell voltage of 0.4 V with roughly a similar result at 0.5 V. As discussed above, a voltage of 0.4 is important for practical applications because of the requirements for modern electronics and developed glucose sensors embedded into contact lenses.7,16 When the BFC was operated in buffer solution containing only ascorbate as a biofuel (Figure 5, curve 2), one distinct peak at the high voltage region of the power density plot was observed, which was attributed to electrooxidation of ascorbate. The fact that the OCV of the BFC is lower in buffer than in basal tears is in good correlation with the results obtained with the model system (cf. Figures 2 and 5), where a higher ascorbate concentration yield lowered the OCV due to interfering reactions occurring at the biocathode. The peak seen when no fuel is present can be attributed to oxidation of the TTF-TCNQ complex. All results confirmed that the described BFC works by utilizing ascorbate as a fuel in the voltage region of interest. Notably, the maximum power output was over three times as high in ascorbate containing buffer as in tears in the high voltage region (cf. curves 1 and 2 above 0.4 V in Figure 5), in all likelihood, due to a lower concentration than 0.5 mM of ascorbate actually present in human basal tears. As mentioned above, a wide range of concentrations of different analytes present in human lachrymal liquid have been reported in the literature.26,35−37,43,44,55,57−59 However, a precise estimation of the ascorbate level in basal tears is difficult from the data obtained in the present work, since the diffusion parameters of the different media must also be taken into account. The current output of the BFC at a fixed potential of 0.4 V is shown in Figure 6. A large drop in output current was observed within the first hour of operation, from 3.5 down to 0.8 μA cm−2 in human lachrymal tears, and from 18 down to 2 μA cm−2 in ascorbate containing PBS. Thereafter, the current output was rather stabilized (Figure 6). One can suggest that the initial drop in current is not due to actual destabilization of electrodes, but rather the specific potential distribution and organization of diffusion properties of the microcell, as was confirmed by impedance studies of microelectrodes in the microcell (see additional details in Supporting Information). These conditions should be compared with what would occur to a bionic contact lens in the human eye, where similar microscale conditions could possibly apply. Despite this, the BFC was able to provide a constant current output of over 0.55 μA cm−2 for more than 6 h, when operated in basal tears. In addition, theoretical calculations based on the varying tear flow and ascorbate concentration in human lachrymal liquid, as reported in the literature, were performed, showing that an electric power output between 0.03 and 22.1 μW can be obtained (see details of calculations, as well as threedimensional figure in the Supporting Information, Supporting

Figure 4. Linear sweep voltammograms of a typical microbiocathode in macrocell with air-saturated PBS containing: (1) 5 mM glucose; (2) 5 mM glucose and 0.2 mM ascorbate; or (3) 5 mM glucose, 0.2 mM ascorbate, and 0.2 mM dopamine, with an average difference in the registered current output between different electrodes of ±12%. Scan rate: 5 mV s−1.

drop in onset voltage for the reductive current was observed in the presence of ascorbate. With the addition of dopamine to the buffer solution, the performance of the biocathode was significantly reduced (Figure 4, curve 3). To characterize the influence of dopamine on the performance further, experiments were carried out under model conditions with a range of dopamine concentration from 0 up to 2000 μM. A slight reduction in the OCV was observed at concentrations below 20 μM dopamine, up to roughly 50 mV, whereas at higher concentrations a large drop was observed, with a reduction of over 350 mV in the potential at 2000 μM dopamine (Supporting Information, Supporting Figure S-3). Furthermore, the catalytic current from the reduction of oxygen was significantly suppressed at such a high dopamine concentration (Supporting Information, Supporting Figure S-4), reducing the efficiency of the biocathode due to concomitant oxidation of dopamine occurring at the supporting AuNPs layer. With a high concentration of the biofuel, i.e., > 0.2 mM, this effect is very pronounced. Biofuel Cell Performance. The typical power output of the BFC, when operated in basal tears as well as in a simple buffer, is displayed in Figure 5. When the BFC was operated in human lachrymal liquid, an OCV of around 0.54 V was registered, close to the potential difference between the anode

Figure 5. Typical power density plots for miniature BFC operating in a microcell containing (1) human basal tears, (2) PBS with 0.5 mM ascorbate (AH−), and (3) PBS, with an average difference in the registered power output between different BFCs of ±15%. Scan rate: 0.1 mV s−1. 6346



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incorporated into suitable flexible biocompatible polymeric materials are currently an ongoing investigation in our laboratories.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information about the electrochemical oxidation of dopamine at the biofuel cell electrodes, impedance studies of microelectrodes in the microcell, and theoretical power output calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 6. Typical operational stability of the BFC tested in a microcell containing (1) basal tears and (2) PBS with 0.5 mM ascorbate as a drop in actual current output and percentage (inset), with an average difference in the registered current output between different BFCs of ±13%. Applied potential: 0.4 V.

Author Contributions ¶

M. Falk and V. Andoralov contributed equally to the present work. All authors contributed to the writing of this manuscript and have read and approved the final version.

Notes

Figure S-7). Both practically obtained and theoretically calculated powers might at first seem to be very low. However, the development of modern ultralow power electronics has already enabled devices with very low current requirement that run on 0.5 V to be created.56 It is difficult to significantly scale down the voltage even further, while still preserving the reliable performance of the electronic circuits, whereas the current can be scaled down from mA to pA in modern transistors. Considering the available space on a contact lens, a size of 1 cm2 for the BFC is fully possible. If a breathing cathode was to be designed, the size could effectively be doubled by utilizing both sides of the contact lens. The stability of the BFC is sufficient for the intended application, given that typical contact lenses are often created for several or even one-day use. The biocompatibility of the BFC would need to be thoroughly assessed before any devices could be ready for human trials. There are, however, no negative reasons why this should be a major issue considering the fact that the BFC is not actually implanted but rather the biodevice operates ex vivo.27 Although, the utilization of TTF-TCNQ complex could raise concerns about the toxicity of the BFC; the complex has been investigated in animal models and found to be of low toxicity.61 When TTF-TCNQ was applied directly to the eye, no common or localized reactions were observed.61 There is also no evidence that the other components of the BFC would be harmful; moreover, by embedding the BFC into the polymer material of a contact lens, we do not expect there to be any deleterious effects to the human eye.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the European Commission (FP7 project NMP4-SL-2009-229255) and by the Russian Foundation for Basic Research (12-04-33102 and 13-04-12083).

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REFERENCES

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CONCLUSIONS By utilizing a miniature membrane-less ascorbate/oxygen BFC, we have shown that sufficient electrical power can be generated from human basal tears, without influencing the glucose concentration of the liquid. Considering the stability and power output of the BFC combined with recent advances in modern ultralow power electronics and contact lens based glucose sensors, the BFC could be utilized as part of the design of a bionic contact lens and allow self-powered, noninvasive, continuous glucose monitoring to be realized. This development holds great promise in the future as an aid for diabetes patients and could help improve public health as well as reduce medical costs. Fabrication and characterization of BFCs 6347



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Miniature Biofuel Cell as a Potential Power Source for Glucose

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