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Biosensors and Bioelectronics 26 (2011) 2742–2745

Contents lists available at ScienceDirect

Biosensors and Bioelectronics

journa l homepage: www.e lsev ier .com/ locate /b ios

hort communication

hin and flexible bio-batteries made of electrospun cellulose-based membranes

.C. Baptistaa,∗, J.I. Martinsb, E. Fortunatoa, R. Martinsa, J.P. Borgesa, I. Ferreiraa,∗

CENIMAT/I3N, Departamento de Ciência dos Materiais, Faculdade de Ciências e Tecnologia, FCT, Universidade Nova de Lisboa and CEMOP/UNINOVA, 2829-516 Caparica, PortugalUniversidade do Porto, Faculdade de Engenharia, Departamento de Engenharia Química, 4200-465 Porto, Portugal

r t i c l e i n f o

rticle history:eceived 20 July 2010eceived in revised form0 September 2010ccepted 30 September 2010

a b s t r a c t

The present work proposes the development of a bio-battery composed by an ultrathin monolithic struc-ture of an electrospun cellulose acetate membrane, over which was deposited metallic thin film electrodesby thermal evaporation on both surfaces. The electrochemical characterization of the bio-batteries wasperformed under simulated body fluids like sweat and blood plasma [salt solution – 0.9% (w/w) NaCl].Reversible electrochemical reactions were detected through the cellulose acetate structure. Thus, a sta-

vailable online 8 October 2010

eywords:io-batterieslectrospinninghin films

ble electrochemical behavior was achieved for a bio-battery with silver and aluminum thin films aselectrodes. This device exhibits the ability to supply a power density higher than 3 �W cm−2.

Finally, a bio-battery prototype was tested on a sweated skin, demonstrating the potential of applica-bility of this bio-device as a micropower source.

© 2010 Elsevier B.V. All rights reserved.

ellulose acetatelectrochemistry.

. Introduction

Research applications in biomedical science and technol-gy usually require advanced, efficient, portable, wearable andmplantable devices that can be used in biological and biomedicalystems. Most of the medical electronic devices, such as cardiacacemakers, require low-power batteries (in the microampereange) with long active life. Thus, the development of portableicropower sources became a demanding and a challenging goal

Linden and Reddy, 2002; Xian Gao et al., 2007).In order to power electronic medical implants, power-supply

ystems must be able to operate independently over a prolongederiod of time, without the need of external recharging or refu-ling (Kerzenmacher et al., 2008). Furthermore, it is important toxpand the design of new batteries to improve the flexibility ofnergy storage/source devices.

Using a simple and cheap technology, the paper batteries (orellulose-based devices) are already being used for clinic diagnosis.Lee, 2005) has developed human urine activated paper batteriess a power source to drive the on-board biosensors for healthcare

creening of urine. Also Pushparaj (Pushparaj et al., 2007) and hisesearch team demonstrated that electrode, separator, and elec-rolyte, can all be integrated into single contiguous nanocompositenits to build blocks for a variety of thin mechanically flexible

∗ Corresponding authors.E-mail addresses: ana.baptista@fct.unl.pt (A.C. Baptista), imf@fct.unl.pt

I. Ferreira).

956-5663/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.bios.2010.09.055

energy storage devices. Recently, a research team of Uppsala Uni-versity has made a flexible battery using two common ingredients:cellulose and salt, to power cheap medical diagnostics devices orsensors on packaging materials or embedded into fabric (Nyströmet al., 2009).

The work previously reported (Nyström et al., 2009) consists incomposite structures fabricated by laminating multiple stacks ofindividual layers. Although flexible, these cellulose-based devicesdo not take full advantage of their thin and porous structure.

The present work proposes the development of a monolithicstructure in which the separator and the electrodes are physicallyintegrated into an ultrathin and flexible polymeric structure. Ahighly porous structure is produced by electrospinning to work asa bio-battery after the deposition of metallic layers (electrodes) ineach one of the faces. Electrospinning is a process for drawing fiberswith sub-micrometer diameters through the action of electrostaticforces (Canejo et al., 2008). The large surface area of the nanofibrousnetwork enhances ion conductivity, thus polymer batteries or fuelcells comprising nanofiber membranes may improve energy den-sity per weight as compared with conventional devices (Ferreiraet al., 2004; Cho et al., 2008).

Unlike biofuel cells, that need to deal with the instability ofthe enzymes, the structure of the proposed battery is based in anultrathin monolithic structure that can achieve good physical and

chemical stability as a micropower density energy source. In thisway thin and flexible batteries were developed envisaging theirapplication in biomedicine (such as in implantable microdevices) orin food industry (such as for food quality control). This study takesinto account the different electrodes couples in the electrochemi-

d Bioelectronics 26 (2011) 2742–2745 2743

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al behavior of membranes using a salt solution, which intends toimulate body fluids like sweat or blood plasma. A prototype wasuccessfully fabricated and the conversion from sweat to energyas demonstrated.

. Experimental methods

.1. Production of the cellulose acetate membrane bylectrospinning

A cellulose acetate (CA) solution [15–21% (w/w) – Mw ∼ 61,000 –0% acetyl groups] in acetone and ethanol (0–30% (w/w) of ethanol)as loaded into a 5 ml syringe. A syringe pump was used to squeeze

ut the polymer solution at a speed between 0.1 and 1.5 ml/hhrough a needle. A voltage of 5–30 kV was applied between theeedle and a grounded collector at 10–25 cm distance.

.2. Membranes characterization

The thickness of membranes (45–55 �m) was measured with aicrometer and their density (�) was calculated from the respec-

ive mass and volume. The porosity was determined using theollowing equation:

Porosity =(

1 − �electrospun membrane

�nonporous membrane

)× 100 (1)

.3. Thin film deposition and morphological characterization

Thin films of Al, Cu and Ag were deposited on the electrospunellulose acetate membrane by thermal resistive vacuum evapo-ation technique using tungsten crucibles, located at about 30 cmistance from the substrate, at pressures <10−3 Pa. The CA mem-rane is placed in the substrate holder and for controlling thehickness of the deposited metals a glass substrate is placed nearhe membrane. The films thickness varied between 50 and 500 nmnd was taken by a profilometer – Ambios XP-Plus Series in the filmeposited onto the glass substrate.

The surface of CA membranes was observed by scanning elec-ron microscopy (SEM – model Hitachi SU-70), before and after thinlms deposition.

.4. Electrochemical measurements

The electrochemical experiments were carried out using aotentiostat (Reference 600TM – Gamry Instruments) to provide

nformation about electrochemical reactions and electron-transferinetics.

The samples (around 1 cm2) were impregnated with a dropf the electrolytic solution – 0.9% (w/w) NaCl – and sandwichedetween two gold contacts. The measurements were performedsing a cell with two-electrode configuration at room temperature25 ◦C) and standard atmospheric pressure conditions. The refer-nce electrode (RE) and the counter electrode (CE) are merged toe one electrode in a two-electrode system and the potential of theorking electrode (WE) vs. the RE (also as the CE) was measured

nd controlled. The electrochemical measurements were carried

ut at a scan rate of 40 mV/s.

Using the same cell configuration, the open circuit voltage waseasured during 1 h. For that purpose, the device was soaked in the

lectrolyte solution for 20 min to guarantee the device wettabilityuring the measurement.

Fig. 1. Comparison of the electrochemical behavior of the electrospun membranewith the one of a nonporous membrane. Inset figure shows a SEM image of theas-spun cellulose acetate membrane.

2.5. Prototype development

A prototype was developed in order to prove the bio-batteryconcept. A monolithic structure (dimension of 1 cm × 2 cm and53 �m of thickness) is formed by the CA membrane covered with athin film of Al in one face and Ag in the opposite face. The open cir-cuit voltage (Voc) and short circuit current (Isc) were measured witha multimeter directly connected to the carbon conductive wireswhich are in contact with the electrodes. A medical tape (Mefix®)was used to keep the device in contact with the sweated skin andto fix the carbon electrodes wires.

3. Results and discussion

3.1. Cellulose based membranes

The cellulose acetate membranes were produced by electrospin-ning forming a matrix composed of sub-micrometric fibers, witha porosity of 70–80%. Cellulose acetate is a well known polymeralready used in biomedical applications, for instance as a mem-brane for hemodialysis process.

A scanning electron microscopy analysis confirmed a randomlyfiber mesh like structure characterized for being porous and, conse-quently, with a high surface area. The fibers have a smooth surfaceand a narrow diameters distribution.

Owing to cyclic voltammetric measurements, previous work(Baptista et al., 2010) has demonstrated a change in the elec-trochemical behavior of the membranes when a salt solution(electrolytic solution) is added – 0.9% (w/w) NaCl. This solution aimsto simulate body sweat which is mainly composed of water andionic species such as Na+, Cl− and K+. When compared with a dryas-spun membrane, the voltammogram obtained shows anodic andcathodic waves due to electrochemical reactions during the scan ofpotential, which was not observed for the dry as-spun membrane,Fig. 1 (inset Fig. 1, the SEM image of electrospun CA fibers). Takingalso into account the displayed values of current can be concludedthat the ionic transport is favored in the electrospun membrane incomparison with the compact (nonporous) membrane.

3.2. Device structure and characterization

By depositing thin metal films electrodes onto the faces ofCA electrospun membranes, their flexibility and superficial areawere preserved leading to a highly flexible and foldable mono-lithic device (see Fig. S1 of Supplementary material). During the

2744 A.C. Baptista et al. / Biosensors and Bioe

Fig. 2. (A) Electrochemical measurements of 10 voltammetric cycles (at 40 mV/s),fitc

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materials used. The bio-battery will be also able to work with other

or a device having silver and aluminum as electrodes. Inset figure shows the SEMmage of the surface fibers of the device covered with a metal thin film forminghe electrode. (B) P–V (power density–voltage) and J–V (current density–voltage)haracteristics for the tested device.

eposition of the metal films the outer fibers of the CA membranere homogenously covered. The metal penetrates the membraneoughly up to the fifth layer of fibers. Instead of a continuous filmormed on the top of membrane, the fibers are covered by a conduc-ive layer and due to interconnection between fibers, the electronsre efficiently collected. Thus the resistivity at the surface of mem-ranes is low. One face of the CA membrane was covered with Agnd the other with Al thin layers.

The metals used as electrodes in the study of the electrochem-cal behavior of the membranes were Al, Cu, and Ag because theyre cheaper, abundant and easier to deposit by thermal evapora-ion. The Ag was also used for its properties of biocompatibilityFujita et al., 1984; Kattfält et al., 2007). In the Fig. S2 (see Sup-lementary material) is shown the stability of the bio-battery (Agnd Al electrodes) soaked with the electrolytic solution throughhe Voc measurements (Voc ∼= 0.3 V) during about 1 h. Additionally,yclic voltammetry measurements were taken introducing just arop (less than 0.1 ml) of NaCl solution inside the device. Fig. 2Ahows the voltammogram obtained with Ag electrode connect tohe WE. Inset figure shows the SEM image of CA fibers covered withthin Al layer to form the anode of the bio-battery. This lightweightonolithic structure has a thickness of 53 �m and after 10 suc-

essive cycles, the voltammogram shows a stable electrochemicalehavior demonstrating the charged and discharged reversibility.uring the forward potential sweeping, a peak is observed at 0.10 V

ollowed by an anodic wave at 0.4 V with an inflection around to.85 V, while the backward sweeping discloses a broad cathodiceak at 0.12 V followed by a cathodic peak at −0.12 V. Accordingo the results gathered from the voltammograms and open circuit

lectronics 26 (2011) 2742–2745

potentials of silver and aluminum in distilled water and 0.9% NaCl,the anodic wave at 0.12 V is attributed to reaction of the workingelectrode with the medium:

Ag + 2Cl− � AgCl2−,Eo = 0.503 Vvs.SHE (2)

Given the structure of cellulose and the interaction of adjacentfibers to metals used as electrodes is possible that the anodic andcathodic peaks, respectively, at 0.1 V and −0.12 V may be relatedwith redox reactions at C-6 or C-2/C-3 positions in the celluloseacetate structure, according to the following reactions:

(3)

(4)

However, this device provides energy in the context of ametal–air battery. So, the cathodic reaction is the oxygen reduc-tion on the silver and the anodic reaction the aluminum oxidation.The power density of the device was taken from the J–V (currentdensity–voltage) curve analysis (Fig. 2B). The maximum powerdensity achieved was 3.38 �W cm−2 for a current density (Jsc) of24.54 �A cm−2 and a voltage of 0.13 V. This power density valueis a promising achievement, since a typical power required for apacemaker operation is around 1 �W (Linden and Reddy, 2002). Asimilar electrochemical stable behavior was observed for the Cu/Alelectrode couple, but lower current densities were obtained.

Table 1 shows the main results obtained by cyclic voltammetryconcerning the Voc and Jsc for two structures (Ag/CA-membrane/Aland Cu/CA-membrane/Al) keeping constant the quantity and typeof electrolyte solution.

A comparison of the monolithic structures with a laminatedstacked structure (Nyström et al., 2009) using an electrolyte solu-tion concentration one order of magnitude higher, is also shown inTable 1. The lower Voc recorded in this work is related to the differ-ent concentration of the electrolyte solution and the metal used aselectrode. The proposed devices are thinner and lighter comparedto the composite structures reported by Nyström and co-workers.

Optimization studies concerning the influence of the amountand concentration of electrolyte, and the electrodes couples on theelectrochemical behavior of these monolithic structures are underway. However, the preliminary results here presented are verypromising allowing foreseeing the future development of new andattractive devices for a wide range of applications going from bio-to microelectronic system applications.

3.3. Bio-battery application

To demonstrate the concept of applying a bio-battery, a mono-lithic structure was place in contact with sweated skin as shownin Supplementary material (Fig. S3). The Voc ∼= 0.32 V (Fig. S3A) andIsc ∼= 0.1 mA (Fig. S3B) were measured directly with a multimeter.The Al/CA-membrane/Ag (Ag in contact with skin) device as wellas the carbon wires connected to the multimeter probes were fixedto the skin with a medical adhesive. Higher voltage values can beachieved throughout the integration of several cells in series, whichcan be performed by proper interconnection of external contacts.

Ongoing studies are being performed to optimize the electriccharacteristics of the bio-battery and the biocompatibility of the

human body fluids, for instance blood plasma. Small implantabledevices, such as pacemakers, biochemical monitoring systems andartificial human muscles stimulation mechanisms, can all be fore-seen as potential field of applications where it is desirable this kind

A.C. Baptista et al. / Biosensors and Bioelectronics 26 (2011) 2742–2745 2745

Table 1Main results obtained for the two developed monolithic structures – with the different electrodes couples: silver/aluminum and copper/aluminum. Comparison of thoseresults with the ones obtained for a laminated stacked structure (composite structure) produced by Nyström and co-workers. Voc: open circuit voltage; Jsc: short-circuitcurrent density.

Device Voc Jsc Device thickness (�m) Weight/area (mg cm−2) Electrolyte Reference

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Linden, D., Reddy, T.B., 2002. Handbook of Biobatteries, 3rd ed. McGraw-Hill.Nyström, G., Razaq, A., Strömme, M., Nyholm, L., Mihranyan, A., 2009. Nanoletters 9

CA with thin films of Ag and Al 0.28 52.21 53CA with thin films of Cu and Al 0.32 12.41 53Ppy/cellulose composite 1 – 2000

f implantable micro power sources. In the field of food quality con-rol, a similar device might be used not as primary energy source buto detect meat or fruit degradation using their own organic fluid.

In conclusion, the obtained results suggest good perspectives foruture applications of the proposed bio-battery, taking advantagef its thin, lightweight, low cost and flexible structure.

. Conclusions

The present work demonstrates the development of an ultrathinnd flexible monolithic structure as a bio-battery able to generatelectrical energy from physiological fluids to supply small biomed-cal devices and biosensors for health care diagnostics.

The results seem to confirm the interaction of CA fibers withhe electrolytic solution (0.9% (w/w) NaCl) through redox reactionsround ±0.1 V. The mesh-like obtained membrane formed bylectrospining CA fibers has very high porosity which is a greatdvantage for electrochemical device applications. It was demon-trated that metal electrodes can be efficiently deposited on thebers as making them suitable for electron transport, keeping theexibility of the membranes. The bio-batteries tested reached aower density higher than 3 �W cm−2. The concept here proposed

s quite promising to produce a competitive power source devicehat can be easily adapted to limited shape and space requirements.

cknowledgments

The authors would like to thank Marta Ferro from CICECO-A for the SEM images (Project: REDE/1509/RME/2005). This

2.1 <0.2 M NaCl This work2.1 <0.2 M NaCl This work

75 2 M NaCl (Nyström et al., 2009)

work is partially financed by FCT-MCTES through CENIMAT/I3Nand by the projects PTDC/CTM/73943/2006, PTDC/EEA-ELC/64975/2006, PTDC/SAL-BEB/098125/2008, PTDC/FIS/74274/2006 and PTDC/EEA-ELC/74236/2006.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.bios.2010.09.055.

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