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Waste-to-energy conversion from a microfluidic device
Journal of Power Sources 360 (2017) 80e86
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Waste-to-energy conversion from a microfluidic device pez-Gonza lez a, R.J. Jime nez-Valde s b, A. Moreno-Zuria a, F.M. Cuevas-Mun ~ iz a, B. Lo c b a, * J. Ledesma-García , J.L. García-Cordero , L.G. Arriaga n y Desarrollo Tecnolo gico en Electroquímica, Quer Centro de Investigacio etaro, 76703, Mexico n y de Estudios Avanzados del Instituto Polit Unidad Monterrey, Centro de Investigacio ecnico Nacional, Vía del Conocimiento 201, Parque PIIT, Apodaca, n CP 66628, Mexico Nuevo Leo c noma de Quer Facultad de Ingeniería, Universidad Auto etaro, Quer etaro, 76010, Mexico a
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A cell culture media is evaluated as fuel in a microfluidic fuel cell. The cathode compartment is open in an air-breathing concept. Three types of devices are evaluated: An inorganic, a hybrid and biofuel cell. The inorganic fuel cell exhibited the highest performance with cell culture media. The work demonstrates that the waste could be used to produce electrical energy.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2016 Received in revised form 19 May 2017 Accepted 31 May 2017
This work reports the successful harvesting of energy from waste produced in a microfluidic device using a fuel cell. A miniaturized glucose air-breathing microfluidic fuel cell (ABmFFC) was designed, fabricated and tested with three different configurations according to their electrode nature: inorganic, hybrid and biofuel cell. Each ABmFFC was characterized using an ideal medium, with sterile cell culture medium, and with waste produced on a microfluidic device. The inorganic-ABmFFC exhibited the highest performance compared to the rest of the configurations. As a proof-of-concept, cancer cells were cultured on a microfluidic device and the consumed cell culture media (glucose concentration <11 mM) was used as an energy source without further treatment, into the inorganic-ABmFFC. The fuel cell generated a maximum total power of 5.2 mW, which is enough energy to power low-consumption microelectronic chips. This application demonstrates that the waste produced by microfluidic applications could be potentially scavenged to produce electrical energy. It also opens the possibility to develop truly energy self-sufficient portable devices. © 2017 Elsevier B.V. All rights reserved.
Keywords: Glucose oxidation Microfluidic fuel cell Lab-on-a-chip Coupling devices Air-breathing
1. Introduction In 2012, 81.7% of the global total primary energy supply was met
* Corresponding author. E-mail address: [email protected] (L.G. Arriaga). http://dx.doi.org/10.1016/j.jpowsour.2017.05.118 0378-7753/© 2017 Elsevier B.V. All rights reserved.
by fossil fuels. However, fossil fuels had the highest CO2 and greenhouse gases emissions (99.5%), which has detrimental effects on the environment and in our health [1]. Renewable energy sources are expected to cope with the ever increase amount of energy consumption and mitigate climate change in an efficient and more sustainable way. Most known renewable energies include
lez et al. / Journal of Power Sources 360 (2017) 80e86 pez-Gonza B. Lo
wind, solar, geothermal, biomass, ocean, and hydropower energies. Classified as a type of biomass energy, waste-to-energy (WTE) taps into the solid waste produced by agricultural, industrial or domestic activities to produce energy in the form of heat and electricity [2,3]. Although the main purpose is to minimise landfilling, the waste can be incinerated to produce heat, which is later transformed into other forms of energy. However, the organic portion of the solid waste can be used to generate bio-hydrogen, bio-oil, bio-gas, and bio-alcohols, through anaerobic digestion and gasification [2,4]. The Unites States Environmental Protection Agency has called WTE one of the cleanest sources of energy [3]. Although WTE technologies utilize large volumes of waste to generate energy, even small volumes could be useful to produce it. These small amounts of energy could be sufficient to power wireless biosensors, sensor networks, and portable electronics [5] but most importantly could have a significant environmental impact by utilizing waste that otherwise would have been disposed. Alternative energies are important to achieve better sustainability of our world; no matter how little energy they produce. One of the most prominent characteristics of microfluidics is its low volume consumption of samples and reagents. As such, it may not be thought that any by-product or waste of an assay performed in a microfluidic platform could be utilized to generate energy. Indeed, such an assay must deliver enough organic material (biofuel) in a continuous flow for long periods of time. However, some microfluidics applications conform to these conditions. One of them is microfluidic cell culture, in which the culture media is rich in glucose and can act as a biofuel. In this microfluidic application, cells are captured on a microchamber and the culture media is perfused over the cells for long periods of time, often longer than two weeks [6]. The cells absorb only a small percentage of the nutrients in the media while the rest is wasted. Other potential microfluidic applications where by-products could be reutilized include microreactors for chemical and material synthesis, with typical operating working volumes in the mL min1 range [7e9]. Chemical microreactors can produce millilitres of by-products that could be further exploited to produce bio-alcohols (e.g. ethanol and methanol). It is therefore plausible to exploit the waste produced from certain microfluidic platforms to generate energy, albeit of the low power produced. In this paper we demonstrate that liquid waste generated from a microfluidic device can be successfully harvested to produce electric power. We cultured cancer cells in a microfluidic device and coupled its output to a microfluidic fuel-cell (mFFC) “dongle”. Our mFFC dongle is a miniaturized version of an air-breathing fuel cell that was fabricated using micromachining techniques [10,11]. Glucose contained in the culture media is used in a chemical reaction to convert it directly into electrical energy. This proof-ofconcept opens up the possibility to connect our mFFC dongle to other microfluidic applications that produce waste rich in biofuels [12,13]. 1.1. Microfluidic fuel-cell dongle Microfluidic fuel cells (mFFC) convert the energy of a chemical reaction into electrical energy. mFFC's have the advantage that they do not require a membrane to separate the electrolyte from each side because they operate under a laminar flow regime. The two streams flow in a microchannel and a natural interface is formed: anolyte and catholyte are separated minimizing the crossover effect [14]. This allows the miniaturization of the fuel cells, and reduce costs associated to the membrane [15]. In a mFFC, the oxidation of a fuel (eg. glucose [16], ethanol [17] and methanol [18]) occurs in the anode while the reduction of oxygen in water (in most of the cases) occurs in the cathode.
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Different types of fuel cells exist, each adapted for a specific type of fuel. Prominent among them is the air-breathing fuel cell (ABFFC), which has a window on the cathode side that allows the diffusion of oxygen from air to the cathode, increasing the concentration of oxygen, and thus improving the global performance of the fuel cell in terms of current and power density. In this work, we evaluated three different types of microfluidic ABFFCs (ABmFFC): inorganic (I-ABmFFC, both electrodes, anode and cathode, are inorganic), hybrid (H-ABmFFC, one of the electrode is enzymatic and the other one is inorganic), and biofuel cell (BABmFFC, in which both electrodes are enzymatic). Each of the fuel cells was evaluated under three different conditions: an ideal medium (i.e. an alkaline buffer in the presence of glucose), a sterile real medium (cell culture media RPMI), and used cell culture media (i.e. cell culture media consumed by cells). 2. Experimental 2.1. Microfluidic fuel cell construction Utilizing micromachining techniques, an air-breathing microfluidic fuel cell (ABmFFC) was constructed using a mini CNC (Deacitec, XR-1000 model). The first step was to fabricate 15.4 mm 14.1 mm x 1 mm polycarbonate walls. Next, a microfluidic channel composed of two Silastic® pieces was cut out using a Silhouette cutting plotter that was aligned to form a window of 10 mm 6 mm. This was then assembled into one piece using a double-sided adhesive. We used an aluminium adhesive to serve as the current collector. Once the electrodes were incorporated into the channel, the ABmFFC assembly was completed using clamping pieces made of acrylic (Fig. 1a and c). 2.1.1. Inorganic air-breathing microfluidic fuel cell (I-ABmFFC) electrodes The synthesis of Au/C was reported previously by our group [19] and Pt/C 20% w/w was purchased by E-tek. Prior to closing the microfluidic fuel cell, the anodic and cathodic catalysts, Au/C and Pt/C, respectively, were deposited on Toray carbon paper (EC-TP1060T thickness: 0.19 mm (7.5 mils), 25 wt% PTFE as a standard loading) electrode collectors that were 0.0084 cm2 in area via the spraying method (2 mg cm2 of catalyst load) (Fig. 1b). 2.1.2. Hybrid and biofuel air-breathing microfluidic fuel cell (HABmFFC, B- ABmFFC) electrodes For the H-ABmFFC and B-ABmFFC, a bioanode (GOx) was constructed submerging Toray carbon paper electrode for 15min on a catalytic ink with the following components: for each mg of glucose oxidase, 7 mL of Nafion, 13 mL of glutaraldehyde solution, 990 mL of 0.1 M PBS pH 7, 20 mg of Vulcan XC-72 carbon and 10 mg of Tetrabutylammonium bromide (TBAB). This ink was then sonicated for 20 min and mixed by a vortex for other 15 min. The bioanode was dried at room temperature. The procedure for the biocathode construction was previously described by our working group [20], the ink obtained was adsorbed on Toray carbon paper (Fig. 1b). 2.2. Microfluidic fuel cell evaluation The ABmFFCs were evaluated using polarization curves obtained at 20 mVs-1 with 0.3mLh1 of volumetric flow. All the electrochemical experiments were performed in a Potentiostat Bio-Logic VSP with a variable load. For the I-ABmFFC, the solutions were 5 mM glucose in 0.3 M KOH and 0.3 M KOH as the anolyte and catholyte, respectively (Ideal conditions). In the cell culture media, RPMI 1640 (Gibco®, pH 7.4) was used as the anolyte solution and 0.3 M KOH was used as the catholyte solution. The glucose
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Fig. 1. (a) Each microfluidic fuel cell consists of two electrodes embedded in a sealing layer and sandwiched between two pieces of acrylic (PMMA). Current collectors are connected to the electrodes. (b) Table shows the different material used for each fuel cell configuration. (c) A photograph of a microfluidic fuel cell.
concentration of the media was measured to be 11 mM (real conditions-RPMI). Another experiment with a waste cell culture medium as anolyte was performed (this solution was obtained from a static cell culture). For the H-ABmFFC: Ideal conditions (Anolyte: 5 mM Glucose in PBS pH 7.4; Catholyte: 0.3 M KOH); real conditions-RPMI (Anolyte: RPMI 1640 y RPMI waste; Catholyte: 0.3 M KOH). For the B-ABmFFC: Ideal conditions (Anolyte: 5 mM Glucose in PBS pH 7.4; Catholyte: PBS pH 5); real conditions-RPMI (Anolyte: RPMI 1640 y RPMI waste; Catholyte: PBS pH 5). 2.3. Microfluidic device construction The microfluidic device was made using soft lithography [21]. Master moulds were composed of 400 silicon wafers coated with SU8-2050 photoresist (Microchem, USA) to a thickness of 70 mm, according to the manufacturer's instructions. PDMS (Sylgard 184, Dow Corning, USA) was poured over the master mould and cured for 1 h at 80 C. The device was then peeled off and plasma bonded to a microscope glass slide (Fig. S2). 2.4. Cell culture experiment Microfluidic chambers were washed with PBS (Sigma®) for 10 min, incubated with fibronectin (Gibco®) at a concentration of 200 mg mL1 for one hour, and washed again with PBS. Next, TC1 cells (a tumour cell line derived from lung epithelial mice cells C57BL/6, a generous donation from Dr. Arturo Chavez) at a concentration of 3 106 cells/mL were loaded into the device. The chip was placed in a custom-made CO2 incubator with 5% CO2 at 37 C and 70% humidity. Cell viability was monitored with an inverted microscope (Axio Observer, Zeiss), and pictures of the chambers were taken every 12 h for three days. RPMI media was flowed through the chip at 0.3 mL h1 for the duration of the assay. 2.5. Coupled microfluidic fuel cell and LOC evaluation For the coupling experiment, the I-ABmFFC was used in order to observe its behaviour, the output of the microfluidic device was connected to the anode electrode inlet of the I-ABmFFC to use the RPMI-consumed media as fuel. In this fluid, the glucose concentration was lower than the initial glucose concentration (11 mM). This lower concentration is attributed to the consumption of nutrients by the cells. The coupled device (cell culture lab on a chip þ I-ABmFFC) was evaluated using polarization curves at a flow rate of 0.3 mL h1. The materials used as the anode and cathode were Au/C and Pt/C, respectively, and 0.3 M KOH was used as the catholyte solution. These experiments took place over two days. In the last day a chrono-amperometric evaluation at 200 mV was performed in order to observe the catalyst stability. All the electrochemical experiments were performed in a Potentiostat Bio-
Logic VSP with a variable load. 3. Results and discussion For the I-ABmFFC operating under ideal conditions, was employed 5 mM Glucose in 0.3 M KOH and 0.3 M KOH as the anolyte and catholyte, respectively. As expected, the performance of these solutions was superior to sterile cell culture medium, Fig. 2. Under ideal conditions, the OCV (open circuit voltage) was 0.82 V and 13.42 mA cm2 of maximum current density. In RPMI medium, the OCV was 0.68 V and the maximum current density was 9.29 mA cm2. The maximum power output in RPMI medium (2.00 mW cm2) was about half the power output under ideal conditions (4.40 mW cm2). This reduction in performance could be attributed to changes in pH (from 13.5 to 7.4), which seriously affected the oxidation reaction carried out in the anode electrode. In addition, others components present in RPMI, such as amino acids and vitamins, could be adsorbing onto the gold electrode surface, decreasing the active catalytic sites [22]. The result of the blank experiment is showed in Fig. S1. An H-ABmFFC (containing an enzymatic anode and an inorganic cathode [20,23]) and a B-ABmFFC (containing both enzymatic electrodes [24,25]) were also fabricated and tested. The operating pH of these ABmFFCs is 7.4, which is the optimum pH to oxidize glucose by the glucose oxidase enzyme. In addition, it has been reported that enzymatic electrodes are capable of working with biological media (environment where the inorganic electrodes are easily poisoned [26]), and they are more selective to their substrate than inorganic electrodes. The performance of the H-ABmFFC and BABmFFC for the real and ideal conditions are summarized in Table 1 and Fig. 2. For the H-ABmFFC, the OCV obtained under ideal conditions (5 mM glucose in PBS 7.4 as anolyte and 0.3 M KOH as catholyte) was 0.76 V, higher than the real condition medium (RPMI) at just 0.34 V. However, the maximum current density obtained in RPMI was 4.55 mA cm2, compared to 9.80 mA cm2 for the ideal conditions. These results demonstrate the benefits of using a hybrid microfluidic cells, because the enzyme is a better catalyst than the inorganic catalysts (Au, Pt or Cu) under biological conditions, whereas platinum is a very good catalyst for the oxygen reduction reaction in alkaline media [16]. For the waste RPMI the maximum current density was decreased by 35% (6.11 mA cm2); in this case the glucose concentration could have been decreased but also the waste cell products could have been adsorbed on the electrode, inhibiting the efficiency of the glucose oxidase enzyme [27]. The performance for the B-ABmFFC operating under ideal and real conditions is very similar: 0.44 V vs 0.43 V; 2.82 mA cm2 vs 3.96 mA cm2; and 0.43 mW cm2 for both conditions. There is no practical difference between the two because the pH for both anolyte solutions is 7.4. For the waste RPMI medium our results show a decrease in 50e80% of the parameters (voltage, current
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Fig. 2. Polarization curves of the ABmFFC with anolyte being either (a) RPMI sterile medium or (b) RPMI waste medium.
Table 1 Parameters obtained from I-ABmFFC, H-ABmFFC and B-ABmFFC at 0.3 mL h1 of flow rate.
InorganicABmFFC
Hybrid ABmFFC
Bio ABmFFC
Anode
Cathode
Fuel
Oxidant
OCV (V)
Current (mA)
Power (mW)
Au/C
Pt/C
112.90
37.04
Pt/C
0.68
78.03
16.80
Au/C
Pt/C
0.62
60.48
11.17
GOx
Pt/C
0.76
38.22
7.98
GOx
Pt/C
0.34
82.07
9.32
GOx
Pt/C
0.40
51.49
6.13
GOx
Lac-ABTS/C
0.44
23.69
3.61
GOx
Lac-ABTS/C
0.43
33.26
3.61
GOx
Lac-ABTS/C
Air 0.3 M KOH Air 0.3 M KOH Air 0.3 M KOH Air 0.3 M KOH Air 0.3 M KOH Air 0.3 M KOH Air PBS pH 5 Air PBS pH 5 Air PBS pH 5
0.82
Au/C
5 mM Glucose in 0.3 M KOH (Ideal conditions) Glucose (11 mM) e RPMI (Real conditions-RPMI) Glucose (<11 mM) e RPMI
0.23
13.02
0.84
5 mM Glucose in PBS pH 7.4 (Ideal conditions) Glucose (11 mM) e RPMI (Real conditions-RPMI) Glucose (<11 mM) e RPMI 5 mM Glucose in PBS pH 7.4 (Ideal conditions) Glucose (11 mM) e RPMI (Real conditions-RPMI) Glucose (<11 mM) e RPMI
density and power density). These results are similar to those previously reported for biofuel cells. The low power density obtained could be related to a poor electron transfer between the enzyme and the electrode surface [28]. The result of the blank experiment for the B-ABmFFC is shown in Fig. S1. The results of the three types of ABmFFC evaluated are summarized in Table 1. It is obvious the better performance of I-ABmFFC, which we selected to operate in tandem with the cell-culture microfluidic system. 3.1. WTE from a microfluidic cell-culture system The waste-to-energy setup consists of an air-breathing microfluidic fuel cell (ABmFFC) connected in tandem to a cell-culture microfluidic chip, as shown in Fig. 3. The cancer cells are initially flowed into the microfluidic device and allowed to attach to the surface. The outlet of the cell-culture chip is connected to the inlet of the ABmFFC using short plastic tubing. Culture media previously perfused to the cells is fed into the ABmFFC without further treatment. Next, having selected the I-ABmFFC as the fuel cell of choice, its performance using waste generated in real time from a microfluidic
cell-culture system was evaluated. The microfluidic culture-system consists of eight parallel microfluidic chambers with a total volume of 36.65 mL (see design in Fig. S2). The output of this microfluidic system was connected to the anode electrode inlet of the I-ABmFFC. Lung cancer epithelial cells (C57BL/6) were seeded in the microfluidic device and allowed to attach. Cells morphology before attaching is initially rounded (Fig. S3a) but after the second day the cells became elongated, an indication that they have attached to the surface of the microfluidic chambers (Fig. S3b). During the first day of cell culturing the output power of the IABmFFC was measured to be 0.51 mW/cm2, but increased slightly to 0.63 mW cm2 on the second day, Fig. 4a. The low value measured during the first day could be attributed to an increase in glucose consumption by the cells (cells consume high quantities of glucose to grow and reproduce [29]). It is also possible that cell debris or unattached cells are dragged into the electrodes, blocking their surface and thus decreasing their performance. We observed that the amount of cells not adhered to the glass substrate on the second day was lower than the first day. In the half-cell study (no cells) we observed that the potential at which the oxidation of glucose in media RPMI sterile and RPMI waste shifted to more positive potentials (0.05 V), which could be attributed to a change in pH,
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Fig. 3. (a) Photograph of the cell culture microfluidic device connected to the air-breathing microfluidic fuel cell (ABmFFC) coupled with the lab-on-a-chip platform. (b) Bright-field and (c) fluorescent image of the cells cultured on the chip.
Fig. 4. (a) Polarization curve comparison for the microfluidic fuel cell: Real conditions-RPMI vs coupled conditions (day 1, red; day 2, green). (b) Chronoamperometric curve of the coupled fuel cell (10 min, 200 mV). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. S4. Indeed, the colour on the pH indicator of the medium changed from an alkaline to an acid state. A chrono-amperometric evaluation at 200 mV on the second day was performed Fig. 4b. Performance was steady, which
suggests that even when operating continuously the electrodic materials are not poisoned by organic molecules present in the culture media. After 10 min of testing, the power harvested was 2.72 mAh with a maximum power of 3.36 mW, enough energy to
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power a microsensor device [30]. On the third day we performed the same evaluation and measured a total energy extracted of 16 mJ. Also, the cell viability to test for any contamination resulting from back-pressure of the I-ABmFFC was evaluated. Cells were stained with fluorescein diacetate (FDA) to determine their viability. The results indicate that the cells remained viable after three days, confirming that even for sensitive assays (such as cell culturing), there was no contamination from the ABmFFC electrodes (Fig. S5). When compared to other miniaturized fuel cells with our IABmFFC has a reasonable performance (OCV 0.82 V), despite glucose (210 kJ mol1) is thermodynamically less energetic than methanol (702 kJ mol1) or formic acid (286 kJ mol1), see Table S1. Furthermore, under real conditions (RPMI media), the uncoupled configuration performed better than methanol/air fuel cells [30,31] even in the absence of strong oxidizers, such as H2O2 or KMnO4, which could be used to achieve similar power levels [32,33]. This could be attributed to the use of the air breathing technology in fuel cells, which allows the oxygen to be readily available to react at the cathode electrode. 4. Conclusions From the analysis of the results of each configuration of ABmFFC, it was observed that the I- ABmFFC got the better performance in the cell culture medium and waste medium reuse. However, the hybrid and biofuel cell have potential application as power source from waste medium reuse, because of the glucose selectivity of the glucose oxidase. This study could lead to the development of truly energy self-sufficient lab-on-a-chip devices. In summary, we have demonstrated that electric power can be indeed generated from a microfluidic device using the cell culture medium waste as fuel in a mFFC. The performance of the coupled device was comparable to other microfluidic fuel cells that, nevertheless, are not integrated to other microsystems. One of the advantages is the cell culture medium waste does not require a pretreatment and thus could be used directly in the mFFC. The energy harvested with our mFFC dongle can find many applications, such as powering small sensors that could in turn monitor reactions inside the microfluidic device, or the energy produced could simply be stored in batteries. Acknowledgements The authors thank the Mexican Council for Science and Technology (CONACyT) for financial support through the Fondo Sectorial AEM-CONACyT, Grants 248511 and 262771, “Fondo Sectorial SEP- CONACYT” Grant CB-256097, and the project “Fondo de Forn”, Grant FIN201618. Part of this work talecimiento a la Investigacio was performed in the Laboratorio Nacional de Micro y Nanofluídica, pez-Gonza lez, A. Moreno-Zuria and R.J. Grant 280485. B. Lo nez-Valde s acknowledge CONACYT for the doctoral Jime scholarships. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.05.118. References [1] C. Acar Dincer, A review on clean energy solutions for better sustainability, Int. J. Energ. Res. 39 (2015) 585e606. [2] R. Kothari, V.V. Tyagi, A. Pathak, Waste-to-energy: a way from renewable energy sources to sustainable development, Renew. Sust. Energ. Rev. 14 (2010) 3164e3170.
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Toner, Clinical microfluidics for neutrophil genomics and proteomics, Nat. Med. 16 (2010) 1042e1047. [22] A. Brouzgou, P. Tsiakaras, Electrocatalysts for glucose electrooxidation reaction: a review, Top. Catal. 58 (2015) 1311e1327. n[23] R.A. Escalona-Villalpando, A. Dector, D. Dector, A. Moreno-Zuria, S.M. Duro n-Valencia, L.G. Arriaga, J. Ledesma-García, Glucose microTorres, M. Galva fluidic fuel cell using air as oxidant, Int. J. Hydrogen Energ (2016), http:// dx.doi.org/10.1016/j.ijhydene.2016.04.238. [24] A. Dector, R.A. Escalona-Villalpando, D. Dector, V. Vallejo-Becerra, vez-Ramírez, L.G. Arriaga, J. Ledesma-García, Perspective use of direct A.U. Cha human blood as an energy source in air-breathing hybrid microfluidic fuel cells, J. Power Sources 288 (2015) 70e75. €le, T.T. Nguyen-Boisse, L. Renaud, S. Tingry, Integration of canti[25] D. Desmae levered porous electrodes into microfluidic co-laminar enzymatic biofuel cells: toward higher enzyme loadings for enhanced performance, Microelectron. Eng. 165 (2016) 23e26. [26] M. Tominaga, M. Nagashima, K. Nishiyama, I. Taniguchi, Surface poisoning during electrocatalytic monosaccharide oxidation reactions at gold electrodes dium, Electrochem. Commun. 9 (2007) 1892e1898. in alkaline me nchez, Microfluidic fuel cells for energy generation, Lab. [27] M. Safdar, J. J€ anis, S. Sa Chip 16 (2016) 2754e2758. [28] T. Ivanov, K. Sundmacher Vidakovi c-Koch, Recent advances in enzymatic fuel cells: experiments and modeling, Energies 3 (2010) 803e846. [29] L. Han, H. Zhang, W.Z. Guo, D.R. Wysham, A.K. Roque, X. Willson, C. Sheng, V.L. Zhou, Bae-Jump, Glucose promotes cell proliferation, glucose uptake and invasion in endometrial cancer cells via AMPK/mTOR/S6 and MAPK signaling, Gynecol. Oncol. 138 (2015) 668e675.
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[30] S. Tominaka, S. Ohta, H. Obata, T. Osaka, R. Alkire, Prospects of on-chip fuel cell performance: improvement based on numerical simulation, Energy Environ. Sci. 4 (2011) 162e171. [31] S. Tominaka, S. Ohta, H. Obata, T. Momma, T. Osaka, On-chip fuel cell: micro direct methanol fuel cell of an air-breathing, membraneless, and monolithic design, J. Am. Chem. Soc. 130 (2008) 10456e10457. [32] M. Shen, S. Walter, L. Dovat, M.A.M. Gijs, Planar micro-direct methanol fuel
cell prototyped by rapid powder blasting, Microelectron. Eng. 88 (2011) 1884e1886. pez-Montesinos, N. Yossakda, A. Schmidt, F.R. Brushett, W.E. Pelton, [33] P.O. Lo P.J.A. Kenis, Design, fabrication, and characterization of a planar, silicon-based, monolithically integrated micro laminar flow fuel cell with a bridge-shaped microchannel cross-section, J. Power Sources 196 (2011) 4638e4645.
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Waste-to-energy conversion from a microfluidic device pez-Gonza lez a, R.J. Jime nez-Valde s b, A. Moreno-Zuria a, F.M. Cuevas-Mun ~ iz a, B. Lo c b a, * J. Ledesma-García , J.L. García-Cordero , L.G. Arriaga n y Desarrollo Tecnolo gico en Electroquímica, Quer Centro de Investigacio etaro, 76703, Mexico n y de Estudios Avanzados del Instituto Polit Unidad Monterrey, Centro de Investigacio ecnico Nacional, Vía del Conocimiento 201, Parque PIIT, Apodaca, n CP 66628, Mexico Nuevo Leo c noma de Quer Facultad de Ingeniería, Universidad Auto etaro, Quer etaro, 76010, Mexico a
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
A cell culture media is evaluated as fuel in a microfluidic fuel cell. The cathode compartment is open in an air-breathing concept. Three types of devices are evaluated: An inorganic, a hybrid and biofuel cell. The inorganic fuel cell exhibited the highest performance with cell culture media. The work demonstrates that the waste could be used to produce electrical energy.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 December 2016 Received in revised form 19 May 2017 Accepted 31 May 2017
This work reports the successful harvesting of energy from waste produced in a microfluidic device using a fuel cell. A miniaturized glucose air-breathing microfluidic fuel cell (ABmFFC) was designed, fabricated and tested with three different configurations according to their electrode nature: inorganic, hybrid and biofuel cell. Each ABmFFC was characterized using an ideal medium, with sterile cell culture medium, and with waste produced on a microfluidic device. The inorganic-ABmFFC exhibited the highest performance compared to the rest of the configurations. As a proof-of-concept, cancer cells were cultured on a microfluidic device and the consumed cell culture media (glucose concentration <11 mM) was used as an energy source without further treatment, into the inorganic-ABmFFC. The fuel cell generated a maximum total power of 5.2 mW, which is enough energy to power low-consumption microelectronic chips. This application demonstrates that the waste produced by microfluidic applications could be potentially scavenged to produce electrical energy. It also opens the possibility to develop truly energy self-sufficient portable devices. © 2017 Elsevier B.V. All rights reserved.
Keywords: Glucose oxidation Microfluidic fuel cell Lab-on-a-chip Coupling devices Air-breathing
1. Introduction In 2012, 81.7% of the global total primary energy supply was met
* Corresponding author. E-mail address: [email protected] (L.G. Arriaga). http://dx.doi.org/10.1016/j.jpowsour.2017.05.118 0378-7753/© 2017 Elsevier B.V. All rights reserved.
by fossil fuels. However, fossil fuels had the highest CO2 and greenhouse gases emissions (99.5%), which has detrimental effects on the environment and in our health [1]. Renewable energy sources are expected to cope with the ever increase amount of energy consumption and mitigate climate change in an efficient and more sustainable way. Most known renewable energies include
lez et al. / Journal of Power Sources 360 (2017) 80e86 pez-Gonza B. Lo
wind, solar, geothermal, biomass, ocean, and hydropower energies. Classified as a type of biomass energy, waste-to-energy (WTE) taps into the solid waste produced by agricultural, industrial or domestic activities to produce energy in the form of heat and electricity [2,3]. Although the main purpose is to minimise landfilling, the waste can be incinerated to produce heat, which is later transformed into other forms of energy. However, the organic portion of the solid waste can be used to generate bio-hydrogen, bio-oil, bio-gas, and bio-alcohols, through anaerobic digestion and gasification [2,4]. The Unites States Environmental Protection Agency has called WTE one of the cleanest sources of energy [3]. Although WTE technologies utilize large volumes of waste to generate energy, even small volumes could be useful to produce it. These small amounts of energy could be sufficient to power wireless biosensors, sensor networks, and portable electronics [5] but most importantly could have a significant environmental impact by utilizing waste that otherwise would have been disposed. Alternative energies are important to achieve better sustainability of our world; no matter how little energy they produce. One of the most prominent characteristics of microfluidics is its low volume consumption of samples and reagents. As such, it may not be thought that any by-product or waste of an assay performed in a microfluidic platform could be utilized to generate energy. Indeed, such an assay must deliver enough organic material (biofuel) in a continuous flow for long periods of time. However, some microfluidics applications conform to these conditions. One of them is microfluidic cell culture, in which the culture media is rich in glucose and can act as a biofuel. In this microfluidic application, cells are captured on a microchamber and the culture media is perfused over the cells for long periods of time, often longer than two weeks [6]. The cells absorb only a small percentage of the nutrients in the media while the rest is wasted. Other potential microfluidic applications where by-products could be reutilized include microreactors for chemical and material synthesis, with typical operating working volumes in the mL min1 range [7e9]. Chemical microreactors can produce millilitres of by-products that could be further exploited to produce bio-alcohols (e.g. ethanol and methanol). It is therefore plausible to exploit the waste produced from certain microfluidic platforms to generate energy, albeit of the low power produced. In this paper we demonstrate that liquid waste generated from a microfluidic device can be successfully harvested to produce electric power. We cultured cancer cells in a microfluidic device and coupled its output to a microfluidic fuel-cell (mFFC) “dongle”. Our mFFC dongle is a miniaturized version of an air-breathing fuel cell that was fabricated using micromachining techniques [10,11]. Glucose contained in the culture media is used in a chemical reaction to convert it directly into electrical energy. This proof-ofconcept opens up the possibility to connect our mFFC dongle to other microfluidic applications that produce waste rich in biofuels [12,13]. 1.1. Microfluidic fuel-cell dongle Microfluidic fuel cells (mFFC) convert the energy of a chemical reaction into electrical energy. mFFC's have the advantage that they do not require a membrane to separate the electrolyte from each side because they operate under a laminar flow regime. The two streams flow in a microchannel and a natural interface is formed: anolyte and catholyte are separated minimizing the crossover effect [14]. This allows the miniaturization of the fuel cells, and reduce costs associated to the membrane [15]. In a mFFC, the oxidation of a fuel (eg. glucose [16], ethanol [17] and methanol [18]) occurs in the anode while the reduction of oxygen in water (in most of the cases) occurs in the cathode.
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Different types of fuel cells exist, each adapted for a specific type of fuel. Prominent among them is the air-breathing fuel cell (ABFFC), which has a window on the cathode side that allows the diffusion of oxygen from air to the cathode, increasing the concentration of oxygen, and thus improving the global performance of the fuel cell in terms of current and power density. In this work, we evaluated three different types of microfluidic ABFFCs (ABmFFC): inorganic (I-ABmFFC, both electrodes, anode and cathode, are inorganic), hybrid (H-ABmFFC, one of the electrode is enzymatic and the other one is inorganic), and biofuel cell (BABmFFC, in which both electrodes are enzymatic). Each of the fuel cells was evaluated under three different conditions: an ideal medium (i.e. an alkaline buffer in the presence of glucose), a sterile real medium (cell culture media RPMI), and used cell culture media (i.e. cell culture media consumed by cells). 2. Experimental 2.1. Microfluidic fuel cell construction Utilizing micromachining techniques, an air-breathing microfluidic fuel cell (ABmFFC) was constructed using a mini CNC (Deacitec, XR-1000 model). The first step was to fabricate 15.4 mm 14.1 mm x 1 mm polycarbonate walls. Next, a microfluidic channel composed of two Silastic® pieces was cut out using a Silhouette cutting plotter that was aligned to form a window of 10 mm 6 mm. This was then assembled into one piece using a double-sided adhesive. We used an aluminium adhesive to serve as the current collector. Once the electrodes were incorporated into the channel, the ABmFFC assembly was completed using clamping pieces made of acrylic (Fig. 1a and c). 2.1.1. Inorganic air-breathing microfluidic fuel cell (I-ABmFFC) electrodes The synthesis of Au/C was reported previously by our group [19] and Pt/C 20% w/w was purchased by E-tek. Prior to closing the microfluidic fuel cell, the anodic and cathodic catalysts, Au/C and Pt/C, respectively, were deposited on Toray carbon paper (EC-TP1060T thickness: 0.19 mm (7.5 mils), 25 wt% PTFE as a standard loading) electrode collectors that were 0.0084 cm2 in area via the spraying method (2 mg cm2 of catalyst load) (Fig. 1b). 2.1.2. Hybrid and biofuel air-breathing microfluidic fuel cell (HABmFFC, B- ABmFFC) electrodes For the H-ABmFFC and B-ABmFFC, a bioanode (GOx) was constructed submerging Toray carbon paper electrode for 15min on a catalytic ink with the following components: for each mg of glucose oxidase, 7 mL of Nafion, 13 mL of glutaraldehyde solution, 990 mL of 0.1 M PBS pH 7, 20 mg of Vulcan XC-72 carbon and 10 mg of Tetrabutylammonium bromide (TBAB). This ink was then sonicated for 20 min and mixed by a vortex for other 15 min. The bioanode was dried at room temperature. The procedure for the biocathode construction was previously described by our working group [20], the ink obtained was adsorbed on Toray carbon paper (Fig. 1b). 2.2. Microfluidic fuel cell evaluation The ABmFFCs were evaluated using polarization curves obtained at 20 mVs-1 with 0.3mLh1 of volumetric flow. All the electrochemical experiments were performed in a Potentiostat Bio-Logic VSP with a variable load. For the I-ABmFFC, the solutions were 5 mM glucose in 0.3 M KOH and 0.3 M KOH as the anolyte and catholyte, respectively (Ideal conditions). In the cell culture media, RPMI 1640 (Gibco®, pH 7.4) was used as the anolyte solution and 0.3 M KOH was used as the catholyte solution. The glucose
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Fig. 1. (a) Each microfluidic fuel cell consists of two electrodes embedded in a sealing layer and sandwiched between two pieces of acrylic (PMMA). Current collectors are connected to the electrodes. (b) Table shows the different material used for each fuel cell configuration. (c) A photograph of a microfluidic fuel cell.
concentration of the media was measured to be 11 mM (real conditions-RPMI). Another experiment with a waste cell culture medium as anolyte was performed (this solution was obtained from a static cell culture). For the H-ABmFFC: Ideal conditions (Anolyte: 5 mM Glucose in PBS pH 7.4; Catholyte: 0.3 M KOH); real conditions-RPMI (Anolyte: RPMI 1640 y RPMI waste; Catholyte: 0.3 M KOH). For the B-ABmFFC: Ideal conditions (Anolyte: 5 mM Glucose in PBS pH 7.4; Catholyte: PBS pH 5); real conditions-RPMI (Anolyte: RPMI 1640 y RPMI waste; Catholyte: PBS pH 5). 2.3. Microfluidic device construction The microfluidic device was made using soft lithography [21]. Master moulds were composed of 400 silicon wafers coated with SU8-2050 photoresist (Microchem, USA) to a thickness of 70 mm, according to the manufacturer's instructions. PDMS (Sylgard 184, Dow Corning, USA) was poured over the master mould and cured for 1 h at 80 C. The device was then peeled off and plasma bonded to a microscope glass slide (Fig. S2). 2.4. Cell culture experiment Microfluidic chambers were washed with PBS (Sigma®) for 10 min, incubated with fibronectin (Gibco®) at a concentration of 200 mg mL1 for one hour, and washed again with PBS. Next, TC1 cells (a tumour cell line derived from lung epithelial mice cells C57BL/6, a generous donation from Dr. Arturo Chavez) at a concentration of 3 106 cells/mL were loaded into the device. The chip was placed in a custom-made CO2 incubator with 5% CO2 at 37 C and 70% humidity. Cell viability was monitored with an inverted microscope (Axio Observer, Zeiss), and pictures of the chambers were taken every 12 h for three days. RPMI media was flowed through the chip at 0.3 mL h1 for the duration of the assay. 2.5. Coupled microfluidic fuel cell and LOC evaluation For the coupling experiment, the I-ABmFFC was used in order to observe its behaviour, the output of the microfluidic device was connected to the anode electrode inlet of the I-ABmFFC to use the RPMI-consumed media as fuel. In this fluid, the glucose concentration was lower than the initial glucose concentration (11 mM). This lower concentration is attributed to the consumption of nutrients by the cells. The coupled device (cell culture lab on a chip þ I-ABmFFC) was evaluated using polarization curves at a flow rate of 0.3 mL h1. The materials used as the anode and cathode were Au/C and Pt/C, respectively, and 0.3 M KOH was used as the catholyte solution. These experiments took place over two days. In the last day a chrono-amperometric evaluation at 200 mV was performed in order to observe the catalyst stability. All the electrochemical experiments were performed in a Potentiostat Bio-
Logic VSP with a variable load. 3. Results and discussion For the I-ABmFFC operating under ideal conditions, was employed 5 mM Glucose in 0.3 M KOH and 0.3 M KOH as the anolyte and catholyte, respectively. As expected, the performance of these solutions was superior to sterile cell culture medium, Fig. 2. Under ideal conditions, the OCV (open circuit voltage) was 0.82 V and 13.42 mA cm2 of maximum current density. In RPMI medium, the OCV was 0.68 V and the maximum current density was 9.29 mA cm2. The maximum power output in RPMI medium (2.00 mW cm2) was about half the power output under ideal conditions (4.40 mW cm2). This reduction in performance could be attributed to changes in pH (from 13.5 to 7.4), which seriously affected the oxidation reaction carried out in the anode electrode. In addition, others components present in RPMI, such as amino acids and vitamins, could be adsorbing onto the gold electrode surface, decreasing the active catalytic sites [22]. The result of the blank experiment is showed in Fig. S1. An H-ABmFFC (containing an enzymatic anode and an inorganic cathode [20,23]) and a B-ABmFFC (containing both enzymatic electrodes [24,25]) were also fabricated and tested. The operating pH of these ABmFFCs is 7.4, which is the optimum pH to oxidize glucose by the glucose oxidase enzyme. In addition, it has been reported that enzymatic electrodes are capable of working with biological media (environment where the inorganic electrodes are easily poisoned [26]), and they are more selective to their substrate than inorganic electrodes. The performance of the H-ABmFFC and BABmFFC for the real and ideal conditions are summarized in Table 1 and Fig. 2. For the H-ABmFFC, the OCV obtained under ideal conditions (5 mM glucose in PBS 7.4 as anolyte and 0.3 M KOH as catholyte) was 0.76 V, higher than the real condition medium (RPMI) at just 0.34 V. However, the maximum current density obtained in RPMI was 4.55 mA cm2, compared to 9.80 mA cm2 for the ideal conditions. These results demonstrate the benefits of using a hybrid microfluidic cells, because the enzyme is a better catalyst than the inorganic catalysts (Au, Pt or Cu) under biological conditions, whereas platinum is a very good catalyst for the oxygen reduction reaction in alkaline media [16]. For the waste RPMI the maximum current density was decreased by 35% (6.11 mA cm2); in this case the glucose concentration could have been decreased but also the waste cell products could have been adsorbed on the electrode, inhibiting the efficiency of the glucose oxidase enzyme [27]. The performance for the B-ABmFFC operating under ideal and real conditions is very similar: 0.44 V vs 0.43 V; 2.82 mA cm2 vs 3.96 mA cm2; and 0.43 mW cm2 for both conditions. There is no practical difference between the two because the pH for both anolyte solutions is 7.4. For the waste RPMI medium our results show a decrease in 50e80% of the parameters (voltage, current
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Fig. 2. Polarization curves of the ABmFFC with anolyte being either (a) RPMI sterile medium or (b) RPMI waste medium.
Table 1 Parameters obtained from I-ABmFFC, H-ABmFFC and B-ABmFFC at 0.3 mL h1 of flow rate.
InorganicABmFFC
Hybrid ABmFFC
Bio ABmFFC
Anode
Cathode
Fuel
Oxidant
OCV (V)
Current (mA)
Power (mW)
Au/C
Pt/C
112.90
37.04
Pt/C
0.68
78.03
16.80
Au/C
Pt/C
0.62
60.48
11.17
GOx
Pt/C
0.76
38.22
7.98
GOx
Pt/C
0.34
82.07
9.32
GOx
Pt/C
0.40
51.49
6.13
GOx
Lac-ABTS/C
0.44
23.69
3.61
GOx
Lac-ABTS/C
0.43
33.26
3.61
GOx
Lac-ABTS/C
Air 0.3 M KOH Air 0.3 M KOH Air 0.3 M KOH Air 0.3 M KOH Air 0.3 M KOH Air 0.3 M KOH Air PBS pH 5 Air PBS pH 5 Air PBS pH 5
0.82
Au/C
5 mM Glucose in 0.3 M KOH (Ideal conditions) Glucose (11 mM) e RPMI (Real conditions-RPMI) Glucose (<11 mM) e RPMI
0.23
13.02
0.84
5 mM Glucose in PBS pH 7.4 (Ideal conditions) Glucose (11 mM) e RPMI (Real conditions-RPMI) Glucose (<11 mM) e RPMI 5 mM Glucose in PBS pH 7.4 (Ideal conditions) Glucose (11 mM) e RPMI (Real conditions-RPMI) Glucose (<11 mM) e RPMI
density and power density). These results are similar to those previously reported for biofuel cells. The low power density obtained could be related to a poor electron transfer between the enzyme and the electrode surface [28]. The result of the blank experiment for the B-ABmFFC is shown in Fig. S1. The results of the three types of ABmFFC evaluated are summarized in Table 1. It is obvious the better performance of I-ABmFFC, which we selected to operate in tandem with the cell-culture microfluidic system. 3.1. WTE from a microfluidic cell-culture system The waste-to-energy setup consists of an air-breathing microfluidic fuel cell (ABmFFC) connected in tandem to a cell-culture microfluidic chip, as shown in Fig. 3. The cancer cells are initially flowed into the microfluidic device and allowed to attach to the surface. The outlet of the cell-culture chip is connected to the inlet of the ABmFFC using short plastic tubing. Culture media previously perfused to the cells is fed into the ABmFFC without further treatment. Next, having selected the I-ABmFFC as the fuel cell of choice, its performance using waste generated in real time from a microfluidic
cell-culture system was evaluated. The microfluidic culture-system consists of eight parallel microfluidic chambers with a total volume of 36.65 mL (see design in Fig. S2). The output of this microfluidic system was connected to the anode electrode inlet of the I-ABmFFC. Lung cancer epithelial cells (C57BL/6) were seeded in the microfluidic device and allowed to attach. Cells morphology before attaching is initially rounded (Fig. S3a) but after the second day the cells became elongated, an indication that they have attached to the surface of the microfluidic chambers (Fig. S3b). During the first day of cell culturing the output power of the IABmFFC was measured to be 0.51 mW/cm2, but increased slightly to 0.63 mW cm2 on the second day, Fig. 4a. The low value measured during the first day could be attributed to an increase in glucose consumption by the cells (cells consume high quantities of glucose to grow and reproduce [29]). It is also possible that cell debris or unattached cells are dragged into the electrodes, blocking their surface and thus decreasing their performance. We observed that the amount of cells not adhered to the glass substrate on the second day was lower than the first day. In the half-cell study (no cells) we observed that the potential at which the oxidation of glucose in media RPMI sterile and RPMI waste shifted to more positive potentials (0.05 V), which could be attributed to a change in pH,
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Fig. 3. (a) Photograph of the cell culture microfluidic device connected to the air-breathing microfluidic fuel cell (ABmFFC) coupled with the lab-on-a-chip platform. (b) Bright-field and (c) fluorescent image of the cells cultured on the chip.
Fig. 4. (a) Polarization curve comparison for the microfluidic fuel cell: Real conditions-RPMI vs coupled conditions (day 1, red; day 2, green). (b) Chronoamperometric curve of the coupled fuel cell (10 min, 200 mV). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. S4. Indeed, the colour on the pH indicator of the medium changed from an alkaline to an acid state. A chrono-amperometric evaluation at 200 mV on the second day was performed Fig. 4b. Performance was steady, which
suggests that even when operating continuously the electrodic materials are not poisoned by organic molecules present in the culture media. After 10 min of testing, the power harvested was 2.72 mAh with a maximum power of 3.36 mW, enough energy to
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power a microsensor device [30]. On the third day we performed the same evaluation and measured a total energy extracted of 16 mJ. Also, the cell viability to test for any contamination resulting from back-pressure of the I-ABmFFC was evaluated. Cells were stained with fluorescein diacetate (FDA) to determine their viability. The results indicate that the cells remained viable after three days, confirming that even for sensitive assays (such as cell culturing), there was no contamination from the ABmFFC electrodes (Fig. S5). When compared to other miniaturized fuel cells with our IABmFFC has a reasonable performance (OCV 0.82 V), despite glucose (210 kJ mol1) is thermodynamically less energetic than methanol (702 kJ mol1) or formic acid (286 kJ mol1), see Table S1. Furthermore, under real conditions (RPMI media), the uncoupled configuration performed better than methanol/air fuel cells [30,31] even in the absence of strong oxidizers, such as H2O2 or KMnO4, which could be used to achieve similar power levels [32,33]. This could be attributed to the use of the air breathing technology in fuel cells, which allows the oxygen to be readily available to react at the cathode electrode. 4. Conclusions From the analysis of the results of each configuration of ABmFFC, it was observed that the I- ABmFFC got the better performance in the cell culture medium and waste medium reuse. However, the hybrid and biofuel cell have potential application as power source from waste medium reuse, because of the glucose selectivity of the glucose oxidase. This study could lead to the development of truly energy self-sufficient lab-on-a-chip devices. In summary, we have demonstrated that electric power can be indeed generated from a microfluidic device using the cell culture medium waste as fuel in a mFFC. The performance of the coupled device was comparable to other microfluidic fuel cells that, nevertheless, are not integrated to other microsystems. One of the advantages is the cell culture medium waste does not require a pretreatment and thus could be used directly in the mFFC. The energy harvested with our mFFC dongle can find many applications, such as powering small sensors that could in turn monitor reactions inside the microfluidic device, or the energy produced could simply be stored in batteries. Acknowledgements The authors thank the Mexican Council for Science and Technology (CONACyT) for financial support through the Fondo Sectorial AEM-CONACyT, Grants 248511 and 262771, “Fondo Sectorial SEP- CONACYT” Grant CB-256097, and the project “Fondo de Forn”, Grant FIN201618. Part of this work talecimiento a la Investigacio was performed in the Laboratorio Nacional de Micro y Nanofluídica, pez-Gonza lez, A. Moreno-Zuria and R.J. Grant 280485. B. Lo nez-Valde s acknowledge CONACYT for the doctoral Jime scholarships. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.05.118. References [1] C. Acar Dincer, A review on clean energy solutions for better sustainability, Int. J. Energ. Res. 39 (2015) 585e606. [2] R. Kothari, V.V. Tyagi, A. Pathak, Waste-to-energy: a way from renewable energy sources to sustainable development, Renew. Sust. Energ. Rev. 14 (2010) 3164e3170.
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