WO2011117357A2

WO2011117357A2 - Flexible biofuel cell, device and method

Info

Publication number: WO2011117357A2
Application number: PCT/EP2011/054554
Authority: WO
Inventors: Tautgirdas Ruzgas; Sergey Shleev; Thomas Arnebrant
Original assignee: Tautgirdas Ruzgas; Sergey Shleev; Thomas Arnebrant
Priority date: 2010-03-24
Filing date: 2011-03-24
Publication date: 2011-09-29
Also published as: WO2011117357A3

Abstract

This invention relates to a flexible biofuel cell for extraction of electricity from a biological system, and to a flexible device comprising the biofuel cell. The biofuel cell and device may be used in conjunction with biological fluids, e.g., blood, sweat and ophthalmic liquid. Furthermore a method for manufacture of said biofuel cell and said device is disclosed.

Description

FLEXIBLE BIOFUEL CELL, DEVICE AND METHOD

Field of the Invention

This invention pertains in general to the field of fuel cells. More particularly the invention relates to a biofuel cell for extraction of electricity from a biological system, and to a device comprising the biofuel cell, and even more particularly to a method for manufacture of said biofuel cell and said device.

Background of the Invention

The recent progress in nanotechnology enables design of totally new device elements or entire devices of nano- and micro-dimensions, which can carry out very powerful data processing and communication functions. Such devices in combination with knowledge gained in the fields of cell and molecular biology, as well as medicine open up an opportunity for new types of biomedical devices, which could operate in proximity to the body and perform biochemical, chemical or physical monitoring or actuation. In this context the energy requirements for attachable or adherent nanotechnology-based biomedical devices becomes very important. In many applications of both simple and complicated biomedical devices, biofuel cells could provide the electrical power.

Such cells are usually made of two electrodes contacting a fluid containing oxidisible and reducible molecules. The material of the electrodes often is a metal or carbon. The surface of the electrodes is usually modified with a layer of cells, enzymes or other catalysts embedded into a polymer matrix containing bound or freely diffusing redox mediators. The role of redox mediators is to provide electron coupling of the catalysts to the electrodes. The catalysts, e.g., enzymes, carry out the reactions of oxidation of biofuel molecules at the electrodes. The electrode where the biofuel molecules, e.g., glucose, are oxidised is called anode and the electrode where the compounds, specifically, biooxidants, e.g., oxygen, of the biological fluids are reduced, is called cathode. A semi permeable membrane usually divides the anode and cathode parts of the cell to prevent a short-circuiting between the oxidation and reduction reactions. The proposed or described biofuel cells are usually very bulky or rigid, and may contain redox mediators, which may leak from the fuel cell. Biofuel cells based on oxidases, e.g., glucose oxidase, always produce some amount of hydrogen peroxide, which inactivates the enzymes of biofuel cells reducing their stability or behaves as a toxic compound to living biological cells. Used redox mediators may be toxic, which is a great drawback in a biological system. Bulky or rigid biofuel cell designs also disturb fluxes of metabolites and, thus, might limit the performance of biological systems. These construction characteristics impose restrictions for the use of biofuel cells.

For example, US 6,294,281 discloses fuel cells capable of operation by electrolyzing compounds in a biological system. However, the fuel cells according to US 6,294,281 are bulky and rigid, and are made to be permanently or semi-permanently implanted into the biological system where they are intended to operate.

US 7,238,440 discloses biofuel cells containing free diffusing redox mediators. These mediators interfere with biological functions of eukaryotic and prokaryotic cells. In contact with biological tissue the mediators can cause diseases such as cancer. Some redox mediators also produce highly active radical species that can damage biological cells.

Some patents address mechanical (not rheological) compatibility problems between biological body/tissues and the construction of biofuel cells by proposing the use of flexible substrates (base) on which catalytic and other biofuel cell components should be built. E.g., patents US 2008213631 Al, WO 2007030943 Al, US 6294281 Bl, and US 2008160384 Al . All these inventions brilliantly point to the problem however does not solve it, since proposed and illustrated flexible embodiments usually are described as laminate or layered structures for which a term of flexibility means a bendable property. All these constructions lack plastic properties which would allow very intimate contact between biofuel cells electrodes and biological tissues in the body.

Hence, an improved biofuel cell and device would be advantageous and in particular a biofuel cell and device allowing for increased flexibility, cost-effectiveness, environmental friendliness and/or ease of use would be advantageous.

Summary of the Invention

Accordingly, the present invention preferably seeks to mitigate, alleviate or eliminate one or more of the above-identified deficiencies in the art and disadvantages singly or in any combination and solves at least the above-mentioned problems by providing a biofuel cell, a device and a method according to the appended patent claims.

In an aspect, a biofuel cell (10) for extraction of electricity from a biological system is provided. The biofuel cell comprises an anode (11), comprising a first catalyst. The first catalyst oxidises a reducing agent, and delivers electrons onto a conducting matrix of the anode when the anode has an anode surface in contact with the reducing agent. The biofuel cell also comprises a cathode (12), comprising a second catalyst. The second catalyst reduces an oxidising agent and delivers electrons from the cathode when the cathode has a cathode surface in contact with the oxidising agent. The anode (11) and cathode (12) are operatively connected in an electrical circuit (13). Thus, the anode (11) and cathode (12) delivers an electrical current and electrical potential. The biofuel cell is flexible due to the elastic and plastic properties of the cathode and the anode materials with an elastic modulus in the range of 0.02 to 1 MPa and with a viscosity in the range 0.01 to 200 Pa*s.

In another aspect, a device (20) comprising the biofuel cell is provided. The device further comprises a sensor array (21) for measuring the level of several processes and elements in the biological system, where at least one measurement, such as characteristics of the biological body or tissue, is used for referencing the analytical result. The device also comprises an actuator (22) for indicating the result of said measurement. The biofuel cell (10), sensor array (21) and actuator (22) are operatively connected and incorporated into a single unit. The device (20) surface, i.e. interface between the body of interest and the is flexible (viscoelastic) with an elastic modulus in the range of 0.02 to 1 MPa and the viscosity in the range from 0.01 to 200 Pa*s.

In yet another aspect, a method for manufacture of the biofuel cell or the device is disclosed. The method comprises a first step of preparing a colloidal dispersion of self assembling molecules, with colloidal or multimolecular structures for controlling oxygen permeability and viscoelasticity of the biofuel cell electrodes. Next, the method comprises the step of destabilize the colloidal dispersion to induce a collapse of the same, thereby inducing self assembly of the self assembling molecules. The method then comprises the step of letting the self assembling molecules assemble for 2 to 24 hours, resulting in a 3D structure.

Further embodiments of the invention are defined in the dependent claims.

The present invention has the advantage over the prior art that it is flexible and thus allow an easy shape adjustment of the biofuel cell and/or device for adherent or attachable forms of application. Because of this, it is possible to match the

viscoelasticity of the biofuel cell and/or device to that of biological system. This capability is essential in reducing limitations/restrictions imposed on biological systems by external, e.g., attachable, implantable, or semi-implantable devices including biofuel cells and/or devices. Furthermore, it is environmentally friendly. Also, it is easy to use and operate. The biofuel cell of the device is made from non-leaking and non-toxic materials posing minimal restrictions on permeability of oxygen through the device. Furthermore, an advantage with the present invention is that it may be applied as a single packaged, self-powered device which facilitates utility and usability of the device in a great number of applications.

Brief Description of the Drawings

These and other aspects, features and advantages of which the invention is capable of will be apparent and elucidated from the following description of

embodiments of the present invention, reference being made to the accompanying drawings, in which

Fig. 1 is a schematic illustration of a biofuel cell according to one embodiment; Fig. 2 is an illustration of an embodiment comprising a biofuel cell, a sensor array and an actuator;

Fig. 3 is a representation of an embodiment with a perforated biofuel cell electrode structure;

Fig. 4 is an illustration showing an embodiment with an electrical device on a contact lens;

Fig. 5 is a schematic illustration showing an embodiment with an electrical device on an adhesive patch;

Fig. 6 is an illustration showing an embodiment with a micro or nano electromechanical device on an adhesive patch; and

Fig. 7 is an illustration of an embodiment with adhesive patch with a biofuel cell and an indicator for analysis and presentation of biological information, wherein Fig. 7A is a top view and Fig. 7B is a bottom view including anode and cathode of biofuel cell; and

Fig. 8 is a plot with performance data for a biofuel cell according to an embodiment.

Description of embodiments

Several embodiments of the present invention will be described in more detail below with reference to the accompanying drawings in order for those skilled in the art to be able to carry out the invention. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. The embodiments do not limit the invention, but the invention is only limited by the appended patent claims. Furthermore, the terminology used in the detailed description of the particular embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention.

The following description focuses on an embodiment of the present invention applicable to a biofuel cell and in particular to a device comprising the biofuel cell.

As mentioned above, scientific progress enables that biofuel cells could be easily adapted or matched to the shape, elastic and plastic properties of a biological system, such as different rheo logical properties of biological tissues, forms or shapes of biological body. Below it is shown how adherent, attachable, elastic and viscous electrodes of biofuel cells may be constructed. In addition, several embodiments of fully integrated biomedical devices driven by the power from biofuel cells are disclosed. With a small size and low weight, the biomedical devices according to some embodiments may be very important in addressing bioanalytical or actuation function in different organs such as eyes, brain, teeth, etc. Miniature biofuel driven biomedical devices may provide a tremendous improvement in clinical and animal studies, environmental monitoring, food monitoring, biomedical monitoring or actuation, fitness management, etc.

A fuel cell generally differs from a battery by consuming reactant, which must be replenished, whereas batteries store electrical energy chemically in a closed system. Reactants of a biofuel cell utilizing enzymes as catalysts to extract electrical energy may be all biological fluids, such as ophthalmic liquid, perspiration liquid, saliva, blood, plasma or serum, intestinal fluid, cerebrospinal fluid, interstitial fluid, etc.

In an embodiment of the invention according to Fig. 1 a biofuel cell 10 comprising an anode 11 and a cathode 12 is disclosed. The anode 11 and a cathode 12 extract electricity, via catalysts, from the bio electrolyte surrounding the biofuel cell device 10. This is done by using a three-dimensional matrix of carbon nano-tubes or gold nanoparticles, preferably gold nanoparticles, such as gold nanoparticles with a diameter of 40 nm modified with, such as impregnated with, enzymes according to the method disclosed below. The anode 11 comprises gold nanoparticles modified with a first catalyst Corynascus thermophuilus cellbiose dehydrogenase. The cathode 12 comprises gold nanoparticles modified with a second catalyst Myrothecium verrucaria bilirubin oxidase.

Gold nanoparticles modified with the enzymes according to above are produced by gently mixing an enzyme solution of the desired enzyme with nanoparticle dispersion for 2 hours, in concentration ratio that allows 10-100 area % coverage of nanoparticle by the enzyme, assuming that all of the enzyme binds to the surface of nanoparticles, making an adsorbed enzyme monolayer. In some cases gold nanoparticles might be initially modified with compounds (e.g., cistamine, mercaptoethanol, etc) that regulate surface properties of gold nanoparticles (e.g., charge and hydrophobicity), which facilitate electronic coupling of an enzyme with the surface of nanoparticle.

The gold nanoparticles modified with the respective enzyme according to above are self-assembled into three-dimensional matrix by layer-by-layer deposition of twenty or more gold nanoparticle-enzyme layers. In this embodiment, this is done by exposing an electrically conducting wire or conducting surface to a 0.1 mg/mL solution of polylysine in water for about 10 minutes and then washing the surface with water. After that the surface is exposed to, such as immersed in, to the dispersion of gold nanoparticles modified with an appropriate enzyme, as described above, for about 30 minutes followed by washing with water. After that, the step with polylysine and gold nanoparticles can be repeated for a required number of times resulting in a 3D assembly of gold nanoparticles modified with an anode or cathode enzyme. The described modification is done at room temperature. To make a process quicker, mixing or flow of the solutions could be used. A similar method of making 3D anodes or cathodes can be applied with other nanoparticles, e.g., carbon nanotubes. The biofuel cell's 10, e.g. produced from 3D assembly of gold nanoparticles according to above, contact to the biological body is flexible with an elastic modulus in the range of 0.02 to 0.2 MPa and with a viscosity in the range from 0.01 to 10 Pa*s, as measured by Quartz Crystal

Microbalance with Dissipation (QCM-D). Viscoelastic characteristics can be varied in this case by addition of salts, e.g., KC1, to screen repulsive and attractive electrostatic interaction between the components of the three-dimensional assembly of the cathode and anode. Positively or negatively charges polyelectrolytes, of varying molecular mass, multivalent ions, or cross-linkers might also be used to achieve desired viscoelastic properties of the biofuel cell electrodes. As mentioned above, the viscoelastic properties can be determined by the measurement of the energy dissipation in the material of biofuel electrodes, QCM-D.

This is advantageous, since the biofuel cell may thus be adapted to fit in different contexts, when used in conjunction with a biological system, such as on a contact lens or on an adhesive patch, such as a band-aid.

In use, the surface whereon the fuel cell 10 has been printed is brought into contact with a biological fluid, comprising glucose and oxygen. The catalysts immobilised on or enclosed by the three-dimensional gold nanoparticle assembly of the anode 11 and cathode 12 trigger a redox reaction, which generates electricity from glucose and oxygen. The electricity is lead through an electric circuit 13, said anode 11 and cathode 12 thus delivering an electrical current and electrical potential.

In an embodiment, the bio fuel cell, or components thereof, has an oxygen permeability between 0.1 and 200 Barrer (one Barrer is 10 -^"11 (cm 3 0₂ ) cm cm -^"2 s-^"1 mmHg^"1). This is advantageous, since the fuel cell in use does not limit the oxygen diffusion into the biological system, with which it is operating.

Fig. 8 is showing a diagram 80, specifically, of the power output 81 in relation to the voltage 82 for the biofuel cell 10, with a maximum 83. The electrical voltage generated by the cell in blood is approximately 0.65 V and the current density is about 2.7 μW/cm . The performance of the flexible biofuel cells 10 in physiological fluids (blood, saliva, and tears) were studied in a microcell made from a glass capillary with the total volume of 10 μΐ.

In addition to the catalysts, additional arrangements may be made to transfer the electrons between the enzymes and the surface of the electrodes. Preferably, the electrons between the enzymes and the electrodes are transferred by means of soluble or surface confined redox mediators. The electrons may also be directly transferred between the active site of the enzymes and the electrode surface, if the distance between the active site of the enzyme and the surface of the electrode is short and a special electron transfer pathway exists.

In particular the energy is obtained from biological fluids including body fluids such as blood or blood plasma, saliva, ophthalmic liquid, urine, fluids in/of the intestinal tract, sweat, cerebral or cerebrospinal fluid or fluids in contact with biological systems such as cell culture media, different foods etc. As a biological energy source for the biofuel cell, biological materials in other forms than fluids may also be used, such as carbohydrates, proteins or lipids or their mixtures in form of crystals or in amorphous states. Components that may be oxidized are e.g. glucose or other sugars, cofactors, amino acids, alcohols, lipids, or other bio molecules and their metabolites. Reducible compounds that may be used in the biofuel cell are dissolved or complex bound oxygen, as well as other compounds with high redox potential present in biological fluids, e.g., peroxides.

The biofuel cells are flexible, which means that it capable of response to stimuli (e.g., mechanical) and adapt to the form or shape accordingly. In materials science flexibility is often characterised in terms of viscoelasticity, meaning that it can be described by its plasticity or viscosity and elasticity. Plasticity/viscosity means that the material may undergo non-reversible changes of shape. Examples of plastic materials are clay, moulded glass at high temperatures, etc. In common terms a plastic material may be seen as lacking "memory" for the shape. Elasticity means that the material may undergo change of shape, but once the stimulus is removed the material returns to its initial form. So, it has a complete "memory" for the initial shape.

Examples of elastic entities are metals in form of springs, rubber, etc.

Depending on the application, the biofuel cell electrodes or some devices (e.g., sensor arrays) described herewith are plastic or viscous. This means that the biofuel cell electrodes may fill in cavities, adapt to the shape of bone, etc. without breaking or otherwise become damaged. Viscosity is a measure of the resistance against deformation of the material and is often characterised by a coefficient of viscosity or simply viscosity.

Elasticity is characterised by elastic modulus. The biofuel cell electrodes or device disclosed herewith are able to match the elasticity of the materials to which the cell will be attached, such as a contact lens or a patch, such as a band-aid.

Electrodes

As stated above, a biofuel cell according to one embodiment comprises two electrodes, an anode 1 1 and a cathode 12. The flexible biofuel cells are fabricated using gold micro wires (0.1 mm in diameter) modified with gold nanoparticles (40 nm in diameter). The modification was done by layer-by-layer assembly as described above, i.e., by repeated immersing of the microwire to polylysine solution and gold

nanoparticle dispersion. Electrodes comprise catalysts, usually enzymes. The electrodes are separated by e.g. a membrane to avoid short-circuiting. The separation membrane may be passive or active. A passive membrane is usually assembled from

(bio)polymers, e.g., layer-by-layer deposited (polyelectrolytes), negatively charged polylactic acid and positively charged chitosan. An active membrane contains catalysts, e.g., enzymes for hydrolysis of biopolymers producing smaller metabolites, which are further oxidized or reduced by catalysts of anode and cathode electrodes, respectively. The anode 1 1 and cathode 12 may also be separated by a space. If so, no membrane is required.

The electrodes of a biofuel cell, or the device, may be formed from

semiconducting or conducting materials in form of structures of nano- and/or micro size, e.g., microchips, nanoparticles, nanotubes, colloidal particles, molecular assemblies, composites or different material phases, e.g., conducting crystals, which in the electrode preparation, e.g., by mixing or self-assembly, result in electron conducting electrode structures. In an embodiment, flexible electrodes based on gold nanoparticles are made according to the following.

First, gold nanoparticles are prepared as a basis for making conducting 3D nanostructures by synthesis from 1 mM HAuCl₄ solution with addition of 1 % trisodium citrate, which gives gold nanoparticles with a diameter of 40 nm. The dimension of nanoparticles is regulated by the amount of added citrate, which is well known to a person skilled in the art. Depending on the amount of added citrate this allows synthesis of gold nanoparticles with nanometer dimensions, such as 5 to 150 nm in diameter. The resulting dispersion of gold nanoparticles is usually dialysed against water to give a stable colloid, which is well known to a person skilled in the art.

Next, the cathodic (e.g. bilirubin oxidase) or anodic (e.g. cellobiose dehydrogenase) catalysts, i.e. enzymes, are added into the gold nanoparticle dispersion to form a colloidal dispersion of gold nanoparticles modified with the appropriate enzyme. This is done by gently mixing the enzyme solution with nanoparticle dispersion for 2 hours. The enzyme/nanoparticle concentration ratio is chosen to allow 10-100% coverage of the nanoparticle surface by the enzyme, assuming that all enzyme binds to the surface of nanoparticles making an adsorbed enzyme monolayer. In some cases gold nanoparticles might be initially modified with compounds (e.g., cistamine, mercaptoethanol, etc) that regulate surface properties of gold nanoparticles (e.g., charge and hydrophobicity), which facilitate electronic coupling of an enzyme with the surface of nanoparticle. The dispersion is then mixed with an inert polymer (e.g. silicone, which might constitute up to 40% of the colloid's dry weight). This solution is poured into the place where the electrode of the biofuel cell should be situated and allowed to dry for 2- 24 hours (depending on the amount per surface area and air humidity) to make a hydrogel structure. The hydrogel is stabilized due to electrostatic, hydrogen or other intermolecular forces. The hydrogel might be cross linked by different cross-linkers

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(e.g., diglycidylether) for stability. Divalent and trivalent ions (e.g. Ca and Al^JT) m concentration ranging from 1 to 100 mM might also be used for this purpose instead or together with polymers. The degree of intermolecular interaction as well as cross-links (both of these variables determine viscoelastic properties of the biofuel cell electrodes) can be regulated by choice of polymer, multivalent inorganic ions or cross-linkers and their concentrations.

Also, the reaction time may affect the final result. For example, 24 hours drying time might be required to produce 3-D assembly using 10 mM multivalent ion

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concentration (Ca ) as nanoparticle cross-linker. Viscoelastic properties of biofuel cell electrodes can be determined by measurement of the dissipation of vibration energy in the electrode material by using QCM-D method. In general, the difference in cathode or anode preparation is just that appropriate enzyme should be used. In this example, when bilirubin oxidase and cellobiose dehydrogenase are used, the electrical voltage generated by the cell will be approximately 0.65 V in blood and the power density about 2.7 μW/cm in unstirred solution.

Both voltage and power of the biofuel cell strongly depend on many factors, such as enzymes used to modify electrodes, content and pH of biological fluids or type of electrode material and its preparation etc. Theoretically, the voltage of the device will be determined by the redox potential difference of the anode and the cathode enzymes, by the efficiency of heterogeneous electron transfer reactions, and possible non- enzymatic reactions, which might occur at the electrode surfaces. In practice, the voltage values may vary from 0.15 V for biofuel cells operating in human serum, for devices based on cellobiose dehydrogenase-modified anodes and bilirubin oxidase modified cathodes, up to 0.8 V for biofuel cells operating in human acidic saliva, for devices designed from glucose oxidase-modified anodes and fungal laccase-modified cathodes. The power of devices might vary from 10^"6 Watts per cm² for biofuel cells based on planar well-polished metal electrodes, which are placed in non agitated liquids, up to 10 -^"3 - 10 -^"4 Watts per cm 2 for biofuel cells placed in mixing solutions under optimal conditions of their operation for devices designed from three-dimensional electrodes.

Advantages with the electrodes according to embodiments herein are that the assembled electrodes, comprising an electrically conducting matrix, such as a viscoelastic electrically conducting matrix, wherein with said catalyst dispersed within said electrically conducting matrix, enables flexible changes of the form in nano-, micro-, or millimetre dimensions. The flexibility of the electrode material is ensured by choosing appropriate binder, optimising a cross-linker concentration, adjusting volume fraction of particles and avoiding the use of bulky solid materials in the electrode construction.

In an embodiment, the flexibility is of elastic character. This means that the biofuel cell deforms under stress (e.g. external forces), but returns to its original shape when the stress is removed.

In another embodiment, the flexibility is of plastic or viscous character. This means that the biofuel cell deforms under stress and remains in the deformed state even after the stress is removed. In this embodiment the amount of cross-linker should be minimized or avoided. In the last case the stability of bio fuel cell electrodes can be achieved by non-covalent intermolecular interaction which can be regulated by surface modification of nanoparticle and enzymes and the choice of polyelectrolytes bearing different mass, branching and density of active groups, e.g., charge groups.

In yet another embodiment, the flexibility is of viscoelastic character. This means that the biofuel cell has both elastic and viscous properties.

The advantage of the described electrode structure is that a conducting matrix provides a three-dimensional (3-D) network with increased loading of catalyst into electrodes and thus ensures high electrical power production per area or volume of the electrode. This decreases the overall size of the electrode. The semiconducting or conducting materials, which may form a 3-D conducting matrix of the electrode, may be nano- or micro-particles, crystals, chips, etc. of different shapes made from carbon, noble metals, solid, liquid or polymeric semiconductors or conductors.

When particles are used for the electrode construction a flexibility or viscoelasticity of the resulting conducting matrix at nano-, micro- or millimetre dimensions is ensured by flexible binding media consisting of polymers, biopolymers, cross-linkers, and/or lipids. When particles are mixed with binding media the resulting 3-D electrode structures form viscoelastic gels, plastic pastes or similar. Viscoelastic characteristics of the resulting electrode material can be determined by QCM-D, more specialised mechanical oscillation rheometers and microrheology analyzers based on measurements of dynamic light scattering. QCM-D should preferably used to characterise nanometer thin, such as 10-400 nm biofuel cell electrodes.

Preferred conducting or semiconducting materials of biofuel cell electrodes are metal nanoparticles, e.g., gold nanoparticles, carbon powder, carbon black, carbon nanohorns, nanocones, nanotubes, or other differently shaped semiconducting and conducting carbon structures according to the art, including single-walled or multi- walled structures. In most cases these carbon structures are chemically pre-treated according to processes well known to a person skilled in the art to introduce charged or polar surface groups, e.g., carboxylic groups. Polyaniline, polypyrrole, polythiophene or similar conjugated polymeric structures may be exploited as polymeric conductor and semiconductor materials for construction of biofuel cell electrodes. Particles or other structures with highly developed 3D surfaces areas containing deposited metals (e.g., gold layer deposited on chemically or electrochemically etched silicon) for making them electrically conducting might be also exploited as a material of biofuel cell electrodes. Preferred binding media materials for the electrode construction are gels or hydrogels based on epoxy resins, silicone polymers and elastomers, polyethyleneimine (PEI), polyvinyl alcohol (PVA), polymeric acrylates such as poly(methyl acrylate) PMA, poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate) (pHEMA), polydimethylsiloxane (PDMS) and their derivatives, halogenated polymers including fluorinated. Synthetic or natural surfactants, lipids and oils or their liquid crystalline phases, etc. may also be used as a binder. Additionally, natural or synthetic polypeptides, albumins, mucins, polysaccharides, DNA and RNA structures, etc might serve as a binding media for making flexible biofuel cell electrodes.

To ensure electric conductivity between conducting or semiconducting particles mixed with binding media the particles must produce percolating 3D material. This means that the concentration of the particles should be above a special threshold called percolation concentration. Practically, this threshold is about 7-10 mass % of the electrode material. Binding media holds elastic or plastic structure by long-ranged (electrostatic) or short ranged (van der Waals) forces, inter-entanglement of branched (bio)polymeric chains, hydrogen bonds, electrostatic interactions such as divalent or

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higher valence ion interaction (e.g., Ca cross-linked hydrogels) or other complexes (e.g., exploiting histidine tags or disulphide bridges), or by covalent bonds. Covalently bound hydrogels or gels are prepared by using cross-linkers such as glutaraldehyde, polyethyleneglycol diglycidylether (PEGDGE), or other chemical structures containing more than two active groups for chemical cross-linking. To regulate elasticity of the electrode the electrode structures might be chemically or mechanically perforated, prepared by using dissolvable templates or synthesised following principals of imprinted synthesis, producing mesh or sponge-like flexible and soft structures.

Biofuel cell electrodes in contact with biological systems or tissues described herewith may have an elastic modulus in the range of 100 Pa to 10, such as 0.02 to 1 MPa. In an embodiment, the biofuel cells described herewith have an oxygen permeability, Dk, in range of 0.1 to 200 10 -^"11 (cm 3 0₂ ) cm cm -^"2 s-^"1 mmHg -^"1 (Barrer).

Preferred flexibility of the electrodes can be explained by elastic and plastic characteristics of the electrode structure. The choice of these characteristics is strongly dependent on the area of application. In case of biofuel cells for contact lenses, stickers, patches, plasters, band-aids or other attachable or adhesive applications to soft tissues the elasticity of biofuel cell electrodes should be preferably characterised by elastic modulus in the range of 0.02 to 1 MPa and with a viscosity in rage of 0.01 to 10⁶, such as 0.01 to 200 Pa*s. In the majority of cases a final desired elasticity and plasticity of biofuel cell electrodes should match the elasticity of the soft tissues where the biofuel cell is attached and have sufficient plasticity to adjust or take a form of the substrate, where the device will be attached. To meet these requirements the amount of cross- linkers as well as the proportion in the particle/polymer or particle/oil phase should be regulated. It should be pointed out that mentioned viscoelastic properties are of the highest importance at the interface between the electrode of the biofuel cell and the biological tissue. It means that entire biofuel cell might be inhomogeneous, however, it is desired that the electrode in contact biological body matches rheological properties of the tissue.

To facilitate diffusion of biofuel molecules into 3-D structure of the biofuel cell electrodes, a hydrogel systems or liquid crystalline phases are preferred as binding media. The preferred water content of hydrogels is about 20-70% of hydrogel mass. Preferred oxygen transmissibility of biofuel cell electrodes should satisfy efficient oxygen supply into 3-D structure of cathodes and anodes as well as it should not become a limiting factor for underlying biological systems and their functions.

Specifically, lack of oxygen supply to eye cornea might provoke corneal swelling and oedema if oxygen transmissibility to cornea will be less than 87xl0^"9 (cm ml 0₂)/(sec ml mmHg). To satisfy this requirement the hydrogel structures of biofuel cell electrodes should preferably be built using, e.g., silicone chemistry and/or halogenated monomers or polymers.

In an embodiment according to Fig. 3, an electrically conducting perforated biofuel cell electrode structure 30 is shown. A first 31, second 32, third 33 etc. 2D perforated and electrically conducting structures may be arranged to build 3D flexible electrodes, e.g., by manual stacking, using self-assembling means, or using dissolvable templates for 3D synthesis. In the later case surfaces of 2D perforated structures should be pre-treated to acquire different charges or affinity motives. Treatment of 2D perforated structures with charged polymers or surfactants could be one way to make self-assembly driven stacking. Furthermore, interdigitation of these perforated structures is possible and might be useful to enhance equilibration of mass transfer balances for chemical and electrochemical reactions of biofuel cells, such as the balance of proton flux. The number of layers may be any number suitable to generate the desired electric potential. The first 31, second 32, third 33 etc 2D perforated and electrically conducting structures are linked together in an electrical circuit 13. The first 31, second 32, third 33 perforated and electrically conducting structures may be mesh structures. In an embodiment, an anode 11 or a cathode 12 is built by such electrically conducting perforated electrode construction structures with catalyst dispersed within said electrically conducting perforated structures. Catalysts

The catalyst of the anode 11 may be a single enzyme or a mixture or combination of several enzymes, which oxidises bioorganic molecules and delivers electrons onto a conducting matrix of the anode. One such enzyme belongs to the enzyme class of oxido-reductases. Examples of anode enzymes are glucose oxidase, glucose dehydrogenase, cellobiose dehydrogenase, oligosaccharide dehydrogenase, fructose dehydrogenase, pyranose dehydrogenase, pyruvate oxidase, lactate oxidase or dehydrogenase, succinate dehydrogenase, alcohol oxidase or dehydrogenase, amino acid oxidase, etc. Oxidases and dehydrogenases can alone oxidize biofuel or perform oxidation in combination with other enzymes hydrolyzing bioorganic molecules, e.g., enzymes such as lactase, sucrase, invertase, amylase, i.e., carbohydrate hydrolysis enzymes, different proteases, etc. Dehydrogenase enzymes are preferred since they usually do not use oxygen as electron acceptor and thus do not produce hydrogen peroxide, which is toxic as well as it deactivates the enzyme reducing stability of biofuel cell electrodes.

The catalyst of the cathode 12 may be an enzyme or combination of several enzymes, which reduces oxygen or other reactive oxygen species, preferably to water. Reactive oxygen species are e.g., organic peroxides and hydrogen peroxide. Usual cathode enzymes are peroxidase, blue multicopper oxidases (laccases, bilirubin oxidase, ceruloplasmin, ascorbate oxidase, Fe -oxidase, tyrosinase) or other oxidoreductases. Possible are also enzyme combinations, e.g., glucose oxidase-peroxidase combination.

To optimize electron transfer between the enzymes of cathode 12 or anode 11 and the conducting electrode matrix, the contact enzyme - conductor surface (e.g., enzyme-gold nanoparticle, enzyme-carbon nanotube) may be specially designed or a special combination of the enzymes and the electrode materials is selected. E.g., from known prior art it is well known that carbon surfaces rich with phenolic and carboxylic surface groups provide good electronic coupling to some of laccases, peroxidises and cellobiose dehydrogenases. To enrich carbon materials with these functionalities they might be pre-treated with hot acids, peroxide or other chemical or physical means imposing oxidation or opening of conjugated surfaces carbon structures. The main feature of the electron transfer (ET) is direct (mediatorless) electron transfer (DET) between the surface of conducting material and the enzyme. An essential characteristic of the DET contact, which is desirable, is a rapid ET between the active site of the enzyme and the electrode. A preferred ET rate is higher than 1 s^"1. This or higher DET rate may be realized by selecting enzyme-conducting particle combination, e.g., laccase and carbon black, bilirubin oxidase and carbon nanotubes, cellobiose dehydrogenase and thiol modified gold nanoparticles. The biofuel cell electrodes described above comprises cellobiose dehydrogenase and bilirubin oxidase enzymes adsorbed on gold nanoparticles of 40 nm diameter. This enzyme - nanoparticle arrangement allows 0.1 to 50 s^"1 electron transfer rates between the enzyme and the nanoparticle. The

heterogeneous electron transfer rate can be regulated by optimising coupling between the enzyme and the nanoparticle. One of the methods could be a simple change of the nanoparticle diameter. For example, in case of bilirubin oxidase, the heterogeneous electron transfer increases more than twice when choosing 20 nm diameter gold nanoparticles instead of 100 nm.

According to an embodiment, the biofuel cell has a DET process between the electron conducting electrode surface of 3D matrix and the enzyme. This means that the ET does not require artificial redox mediators and proceeds directly between the enzyme and a conducting surface of the electrode. Additional means to facilitate DET is to use so called ET -promoters. The promoter function might be multiple. First, the promoter helps to properly orient the enzyme towards the surface of conducting electrode. This process results in a pathway of bonds (covalent, hydrogen), considerably facilitating electronic coupling of an active site of the enzyme in relation to the surface of the conducting electrode. Such electron promoters may be charged (amino or carboxylic groups), polar, hydrophilic or hydrophobic functionalities, enzyme substrate or inhibitor analogues, natural interaction partners or their mimics, or genetically engineered biochemical structures. Secondly, the function of the promoter may also diminish effect of inhibitors on the enzymes used in biofuel cells. Electronic coupling of the enzyme in relation to the electrode by molecular structures, e.g., conjugated chemical structures; restrict entrance of inhibitors into an active centre of the enzyme. ET promoters with these functions may be aromatic compounds, such as phenolic and polyphenolic compounds, i.e., aromatic substrates of polyphenol oxidizing enzymes (e.g., laccases, peroxidases, bilirubin oxidases, etc.). In general, the promoter structures might be chosen to shield the active site of the enzyme from inhibitors by realising steric, charge-charge, or other molecular interactions between the promoter and the enzyme. The incorporation of the enzyme into a 3-D conducting electrode structures of a cathode 12 and anode 11 may be made by different ways. Many of them are known from making biosensor electrodes based on carbon pastes, screen-printing or in-jet inks, hydrogels or layer-by-layer deposition on the surface of solid electrodes. Not yet exploited are methods that use intermolecular or inter-particle forces (sometimes called colloidal forces) to self assemble 3-D enzyme modified electron-conducting materials, specifically, 3-D anodes or cathodes of bio fuel cells. The essence of this method is the exploitation of forces such as surface tension, adhesion, electrostatic double-layer, van der Waals, hydrophobic, gravity, osmotic, electrokinetic, magnetic, etc. to self assemble ordered 3-D bioanodes and biocathodes. This is possible by choosing conditions when a stable dispersion is destabilised a bit and it starts to collapse, i.e., precipitate, flocculate, aggregate. Such If we would destabilise a lot (rapid process of aggregation), e.g., by changing pH, ionic strength, addition of specific divalent ions, etc. the dispersion will collapse producing very nonhomogeneous 3D structure, which is bad from the point of controlling the flexibility and catalytic properties of 3D structure. These properties might be accidentally very good, but it will be hard to reproduce. To provide a more ordered assembly, an extended assembly time may be required. For example, 24 hours might be required to produce 3-D assembly using 10 mM multivalent ion concentration

2__|_

(Ca ) as nanoparticle cross-linkers. The enzyme molecules or their layer should be first immobilised on nano- or micro-particles (generally colloidal particles) and then the assembled enzyme-particle conjugates should be dispersed in to a stable colloid dispersion/solution. The assembly of 3-D conducting material from a colloid can be regulated by changes in temperature, pH, ionic strength of buffers, additives, etc. All these experimental measures are aimed to slightly shift the balance of colloidal forces; in other words, to slightly destabilise the colloidal dispersion. A shift from force equilibrium in the colloidal dispersion should be tailored to enable slow 3-D material formation due to appropriate driving forces that enables 3-D ordered self assembly. E.g., nanoparticles consisting of carbon nanotubes, chitosan and enzyme make a stable colloid dispersion at pH 4.5 due to electrostatic repulsion between the nanoparticles. Positively charged chitosan at pH 4.5 ensures the repulsion. However, if pH is slowly changed to more neutral, say pH 7.5, the electrostatic repulsion might be not sufficient for repulsion between particles. The particle will slowly start to flocculate (aggregate) thereby self-assembling into an ordered 3-D material.

Another way could be impregnation of 3-D conducting matrixes with enzyme, which is suitable for perforated or chemically synthesized 3-D conducting structures. Below follows an account of how to specifically produce biofuel cells according to some embodiments. In this way, they may be made into a device of adherent, adhesive or attachable capsule, body, e.g., in form of contact lenses, stickers, patches or band-aids. They may also be deposited as a paste, ink, or viscoelastic material taking a shape of the substrate to which the biofuel cell device is attached. For efficient functioning of the biofuel cell, biological compounds have passive or facilitated diffusion towards the electrodes of the biofuel cell. Such facilitation may be realized by exploiting the environment where the biofuel cell is attached, e.g., ophthalmic liquid is mixed by blinking eye or by using polymers, which pre-concentrate or extract biofuels into 3-D structure of the biofuel cell. E.g., silicone polymers have higher affinity to oxygen if compared to cellulose-based polymers. Adhesive or attachable properties of the biofuel cell may be realized by using adhesive packaging of biofuel cells. Such adhesives may be made from polymers, e.g., chitosan, lipid mixtures, e.g., monoleine based cubic phases, or biologically resistant or biodegradable glues.

In an embodiment according to Fig. 2, the anode 11 and cathode 12 power an electrical device 20 comprising a sensor array 21 and an actuator 22, in order to perform operations, such as according to embodiments shown below. The anode 11, cathode 12, sensor array 21 and actuator 22 are bound together by electrically conducting leads thus forming an electrical circuit 13. The sensor array 21 may consist of a biosensor for measuring the level of at least one element in said biological system, such as any amperometric or potentiometric biosensor. The array 21 also comprises at least one additional sensor, preferably physical sensor, the signal from which is used to reference the analytical data. An example of such a sensor is an impedance measuring device for assessing permeability properties of biological body or tissue, or an optical oxymeter for determining blood circulation, in the place where this embodiment is placed or attached.

Several combinations of such sensors are possible and would be beneficial for improving accuracy and precision of analytical data by using their signal as a referencing baseline. The actuator 22 may be a pump, quartz crystal or polymeric actuator or an indicator for indicating the result of said measurement, such as any electrically driven output-input device, such as a photodiode, liquid crystals, an electrochromic polymer, a video-transferring unit, such as a photodiode array, a wireless transmitter, etc. The biofuel cell 10, sensor array 21 and actuator 22 are operatively connected and incorporated into a single unit. The biofuel cell device may additionally comprise energy storage and voltage multiplying units, in order to allow storage of energy or increased voltage levels. These additional featured might be required to operate wireless communication devices, miniature processors, etc. Furthermore, the device 20 has flexible biofuel cell electrodes with an elastic modulus in the range of 0.02 to 1 MPa and viscosity in the range 0.01 to 200 Pa*s.

In an embodiment, the device 20 has oxygen permeability between 0.1 and 200 Barrer.

In an embodiment, the sensor array 21 has a glucose biosensor, suitable for measuring glucose levels in the electrolyte surrounding of the biofuel cell. This may allow for easy measuring of glucose levels within the biological system where the biofuel cell device is placed. The preferred glucose biosensors are made in a similar manner as the anode 11 of the biofuel cell 10 described herewith. This also means that viscoelelastic properties might be retained in sensing parts of the biosensors, especially, in the current conducting parts, which connect the biofuel cell to the biosensor.

Additional requirements on sensitivity, selectivity and linearity of the sensor might need to be regulated by exploiting polymeric membranes and adjusting the size of the sensing electrode.

In another embodiment, the sensor array 21 has a lactic acid sensor.

In yet another embodiment, the sensor array 21 has an amine sensor.

In an embodiment, the sensor array 21 has a catecholamine sensor.

In use, the device 20 may be contacted by a biological system chosen from the group consisting of blood, saliva, perspiration, ophthalmic liquid, urine and mucus.

A further embodiment of the invention is illustrated in Fig. 4. Here, a contact lens 400 is disclosed. The contact lens 400 has an inner and an outer surface. The contact lens is equipped with a biofuel cell device 40 comprising an anode 11 and cathode 12. The anode 11 is located on the inner surface of the contact lens 400. The cathode 12 is located on the outer surface of the contact lens 400. This arrangement of cathode realises a well known design of "breathing" or "floating" cathode which maximises oxygen flux to the cathode. Furthermore, a sensor array 21 arrangement and actuator 22 arrangement are shown, which may be varied according to the desired use of the device 40. In use, the contact lens 400 is placed in the eye; with the inner surface facing towards the cornea and the outer surface facing from the cornea. Electricity is generated between the anode 11 and the cathode 12, from biological molecules present in the ophthalmic liquid. The electricity may be used to drive any kind of sensor in array 21 or actuator 22. The mentioned placement of the biofuel cell electrodes on the inner or outer surface is just to provide an illustration. In general the placement of the biofuel cell electrodes on contact lenses can be freely chosen including the surface and the bulk of the lens.

In another embodiment of the invention according to Fig. 5, an adhesive patch 500 comprising a biofuel cell device 50 is shown. The device 50 comprises an anode 11 and cathode 12, a sensor 51 and actuator 52. Furthermore, the device comprises a transistor 53. The transistor 53 may be any amplifying device such as an operational amplifier. In use, electricity is generated between the anode 11 and the cathode 12, by e.g. enzymes coating the surfaces. The electricity may be used to drive any kind of sensor 51 or actuator 52 as described herewith.

In an embodiment according to Fig. 6, a device 60 comprising an electro mechanical system 63, such as a micro electro mechanical system (MEMS) or a nano electro mechanical system (NEMS), is provided in combination with a sensor 61 and an actuator 62 on patch 600. An anode 11 and a cathode 12 power the device. A

microprocessor 64 may carry out amplification, detection or management of the electro mechanical system 63 for pre-concentration of bio analytes and their detection, or biosensor dependent delivery of chemical or electrical stimulations. In an embodiment, the sensor 61 is a biosensor, such as a stripping voltammetry sensor for detection of pre- concentrated analytes. An advantage with this device is that a low analyte concentration can be extracted and measured. This may provide favourable signal to noise ratio for detecting relevant compounds, e.g., disease markers such as catecholamines.

In an embodiment according to Fig. 7, a patch 70 is shown. Fig. 7 A is showing a top view of the patch, fitted with presentation means 71 for information, such as a screen for presentation of biological parameters, help in navigation, learning, entertainment etc. Fig. 7 B is showing a bottom view of the patch, with anode 11 and cathode 12 positioned so that they, when the patch is in use, are pressed against the skin of a person for extracting electrical energy from biological fluids, such as body fluids. In use, electricity is generated between the anode 11 and the cathode 12, by e.g.

enzymes coating the surfaces. The anode 11 and cathode 12 are arranged to form an electrical circuit 13 with said presentation means 71. The electricity may be used to drive any kind of sensor 13 or actuator 14.

In an embodiment, a single packaged device comprising the fuel cell may be manufactured according to the following. Ink is prepared according to above. This ink may be printed on a suitable surface, e.g. a contact lens. The ink may be printed by any method known in the art, such as screen-printing or ink-jet printing. Several layers may be deposited to achieve about 1-10 μιη thickness. Electro chromic material, as an actuator unit, may also be deposited by ink-jet printing as well as connecting conductors between the actuator and bio fuel cell electrodes. The conductors may be electrically insulated by polymerizing material similar to the material of the hosting surface (for silicone based contact lenses it should be silicone). The insulating layer may be applied by any method known in the art, such as writing, stamp printing or ink-jet printing.

In the case of a contact lens, the surface may additionally be treated with mucin solution or other suitable (bio)polymer, to reduce the friction of bio fuel cell electrodes and the tissue of eyelid. The cathode is preferably deposited on outer (distal) side of the lens surface. The anode electrode is preferably deposited on the inner (proximal) part of the lens for continuous supply of biofuel, e.g., glucose or lactate from the ophthalmic or tear liquid. By adjusting the characteristics of anode electrode, e.g., dimensions, amount of the enzyme, etc. the biofuel cell - actuator arrangement may function as a monitor of glucose or lactate, depending on the enzyme of the anode.

Applications and use of the above described biofuel cell device according to the invention are various and include different fields as exemplified by contact lenses equipped with biofuel cell, including indicator, (bio) sensor, actuator, and/or wireless communication unit or their arrays. Application areas of such devices are analysis of biochemical parameters such as level of glucose, alcohol or neurotransmitters

(adrenaline, catecholamines, etc), active delivery of biologically active substances, display of information for help in navigation, learning or entertainment.

Furthermore, the biofuel cell device may be used in conjunction with different rigid or flexible packages, such as food packages, equipped with biofuel cell, indicator, (bio) sensor or other bioanalytical device (e.g., chromatographic column), and/or a wireless communication unit or their arrays. The freshness of food may thus be monitored by measuring changes in acidity, concentration of oxygen, amino acids or other amines. For this a specific sensor or sensor arrays known in the art may be adjusted into a flexible or attachable structure of the devices described herewith.

Additionally, biofuel cells can be imprinted into a packaging material, such as paper and with an appropriate sensor or their combinations could be used to monitor simple or complicated characteristics important for food management, e.g., temperature fluctuations, changes in humidity and pressure, etc.

Furthermore, stickers or adhesive labels, plasters, or band-aids or adhesive patches may be equipped with a biofuel cell, indicator, (bio)sensor or other bioanalytical device (e.g., chromatographic column), actuator, and/or wireless communication unit or their arrays. Application areas of such devices may be (i) analysis of biological parameters, e.g., concentration of bioanalytes, wound healing characteristics, (ii) drug delivery, (iii) extraction, preconcentration and analysis of drug metabolites or biomarkers as well as display of information to aid navigation, learning, entertainment etc. Application areas of such devices are bioanalysis of biologically important compounds, such as levels of glucose, alcohol, and neurotransmitter (adrenaline, catecholamines, urea, drugs and drug metabolites, etc) in biological fluids, such as blood, plasma, serum, saliva, in ophthalmic liquid and sweat, inside the intestinal tracts, vesicles, capillaries, veins, and arteries.

Advantages of the biofuel cell device according to some embodiments are that environmentally friendly and flexible materials are used in the manufacture. Thus, the resulting product is safe to adhere or attach to a biological system, without causing damage. Most commonly, the device is made so that it in itself adhere to skin, mucosa surfaces of eyes and mouth, intestinal and urinary tract, as well as surface of bone, teeth or other body or technical surfaces, which are in contact with biological material (e.g., cell cultivation plate, wall of fermentation vessel including process equipment, etc) or other liquid or solid materials containing or consisting of (bio)organic substances with a low (reducing) and a high (oxidising) redox potentials.

The device according to some embodiments may thus stay adhered or glued to the surface of biological or artificial tissue or material for a desired time and may simply be pealed off when the operation of the device is finished or no longer needed. In the majority of cases the adhesion is ensured by electrostatic, hydrophobic and van der Waal's forces, capillary or other colloidal forces between the surface of the device and the surface of biological or artificial tissue or material.

Another advantage with the device according to some embodiments is that the single packaged device is plastic enough to take a shape needed for its attachment to any biological tissue or artificial material. The elastic properties of the device are achieved by designing viscoelastic properties of biofuel cell electrodes as described in this invention. The electrodes are the bulkiest part of a device consisting of biofuel cell. Viscoelastic biofuel cell electrodes allow the device to stay in an intimate contact with biological or artificial tissue, thus not limiting the performance of the tissue as well as ensuring an appropriate chemical and biochemical communication between the device and the tissue.

Although the present invention has been described above with reference to specific embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the invention is limited only by the accompanying claims and, other embodiments than the specific above are equally possible within the scope of these appended claims.

In the claims, the term "comprises/comprising" does not exclude the presence of other elements or steps. Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly advantageously be combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. In addition, singular references do not exclude a plurality. The terms "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example and shall not be construed as limiting the scope of the claims in any way.

Claims

1. A bio fuel cell (10) for extraction of electricity from a biological system, said biofuel cell comprising

an anode (11), comprising a first catalyst, which catalyst oxidises a reducing agent, and delivers electrons onto a conducting matrix of the anode, said anode having an anode surface in contact with the reducing agent;

a cathode (12), comprising a second catalyst, which catalyst reduces an oxidising agent and delivers electrons from the cathode, said cathode having a cathode surface in contact with the oxidising agent;

said anode (11) and cathode (12) being operatively connected in an electrical circuit (13), said anode (11) and cathode (12) thus delivering an electrical current and electrical potential;

wherein the material of the anode or the cathode or both electrodes, of the biofuel cell (10) is flexible with an elastic modulus in the range of 0.02 to 1 MPa and has a viscosity in the range 0.01 to 200 Pa*s.

2. Biofuel cell (10) according to claim 1, wherein the biofuel cell electrodes have oxygen permeability between 0.1 and 200 Barrer.

3. Biofuel cell according to any of claims 1 or 2, wherein the anode (11) and cathode (12) are viscoelastic electrically conducting two- or three-dimensional matrices with said catalyst dispersed within said viscoelastic electrically conducting matrices.

4. Biofuel cell according to any of claims 1 or 2, wherein the anode (11) and cathode (12) are electrically conducting perforated structures with said catalyst dispersed within said electrically conducting perforated structures.

5. Biofuel cell according to any of the preceding claims, wherein the material of the anode and cathode comprises gold nanoparticles modified with an enzyme.

6. Biofuel cell according to claim 5, wherein the diameter of the gold nanoparticles is between 5 nm and 100 nm.

7. Biofuel cell according to any of the preceding claims, wherein the first catalyst is bilirubin oxidase and the second catalyst is cellobiose dehydrogenase.

8. A device (20) comprising the biofuel cell according to any of the above claims, further comprising

a sensor array (21) for measuring characteristics of the biological body or tissue which are be used for referencing of analytical data; and

an actuator (22) for indicating the result of said measurement,

wherein said biofuel cell (10), sensor array (21) and actuator (22) being operatively connected and incorporated into a single unit,

wherein the interface between the body of interest and the device (20) is flexible with an elastic modulus in the range of 0.02 to 1 MPa and with a viscosity range from 0.01 to 200 Pa*s.

9. Device (20) according to claim 8, wherein the material of the biofuel cell electrodes of the device (20) has oxygen permeability between 0.1 and 200 Barrer.

10. Device according to any of claims 8 or 9, wherein said at least one element is glucose.

11. Device according to any of claims 8 or 9, wherein said at least one element is lactic acid.

12. Device according to any of claims 8 or 9, wherein said at least one element is amine.

13. Device according to claim 12, wherein the amine is catecholamine.

14. Device according to any of claims 8 to 13, wherein said biological system is chosen from the group consisting of blood, plasma, serum, saliva, perspiration, intestinal fluid, interstitial fluid, ophthalmic liquid, cerebral fluid, cerebrospinal fluid, urine and mucus.

15. Method for manufacture of the biofuel cell according to any of claims 1 to 7, comprising the steps of: preparing a colloidal dispersion of self assembling molecules, with colloidal or multimolecular structures for controlling oxygen permeability and viscoelasticity of the biofuel cell electrodes;

destabilize the colloidal dispersion to induce a collapse of the same, thereby inducing self assembly of the self assembling molecules;

letting the self assembling molecules assemble for 2 to 24 hours, resulting in a 3D structure.

16. Method according to claim 15, wherein the destabilizing is done by changing pH or ion strength of the colloidal dispersion, or by adding divalent ions colloidal dispersion.

17. Method according to claim 15 or 16, wherein the colloidal dispersion comprises gold nanoparticles modified with an enzyme.

18. Method according to any of claims 15-17, wherein the colloidal dispersion comprises silicone, fluorinated polymers and/or halogenated polymers.