By Dr. S. S. VERMA; Department of Physics, S.L.I.E.T
Conventional electronics are typically made of rigid, brittle & non-degradable materials and don’t function well in a wet environment. Current medical devices are very limited by the fact that the active electronics have to be ‘canned,’ or isolated from the body, and are on rigid silicon. By building thin, flexible silicon electronics on silk substrates, researchers have made electronics that almost completely dissolve inside the body. Imagine building biocompatible, biodegradable and cheaper electronics on a variety of substrates — materials like plastic, paper, or fabric. Mother nature has in course of evolution processed the most effective catalysts (enzymes), and biomolecules of optimal recognition and binding capabilities that lead to highly selective and specific biopolymer complexes. Similarly, biology provides the fastest and most complex computing and imaging systems where optical information is processed and stored in the form of three-dimensional memorable images. The tremendous biochemical and biotechnological progress in tailoring new biomaterials by genetic engineering or bioengineering provides unique and novel means to synthesize new enzymes and protein receptors, and to engineer monoclonal antibodies or aptamers for nonbiological substrates and DNA-based enzymes. All these materials provide a broad platform of functional units for their integration with electronic elements. The bioelectronic devices may operate in dual directions: in one configuration, the biological event alters the interfacial properties of the electronic element, thus enabling the readout of the bioreaction by monitoring the performance of the electronic unit such as the readout of the potential, impedance, charge transport, or surface resistance of electrodes or field-effect transistors, or by following the resonance frequencies of piezoelectric crystals. The second configuration of bioelectronic systems uses the electronic units to activate the biomaterials toward desired functions.
Bioelectronics is a recently coined term for a field of research that works to establish a synergy between electronics and biology. The emerging field of bioelectronics seeks to exploit biology in conjunction with electronics in a wider context encompassing; for example, electronics waste management, biological fuel cells, bionics and biomaterials. A key aspect is the interface between biological materials and micro- and nano-electronics. The research field gained the buzzword ‘‘bioelectronics’’ aimed at highlighting that the world of electronics could be combined with biology and biotechnology. The integration of biomolecules with electronic elements to yield functional devices attracts substantial research efforts because of the basic fundamental scientific questions and the potential practical applications of the systems. Bioelectronics deals with the coupling of the worlds of electronics and biology, and this coupling can go both ways. The natural ability for “recognition” in the biological world, such as between two complimentary DNA strands, can be combined with the awesome power of microelectronics to process signals to build powerful new biosensors. At the same time, electronic devices can help “guide” biological events, for example cell growth, thereby creating new tools for biomedical research. This cross-fertilization between the two disciplines improves our understanding of life processes and forms the basis for advanced disease detection and treatment. Tools generated in this arena, such as medical diagnostics and bioelectronic implants, will dominate the future of healthcare and help increase the span and quality of our lives. They will also play a dominant role in modernizing agriculture and in protecting animal health, our food supply, and the environment.
Fundamentals of technology
Key to these new technologies is a fundamental understanding of the interface between electronic materials and biology. Organic electronics – an emerging technology that relies on carbon-based semiconductors and promises to deliver devices with unique properties – seems to be ideally suited for the interface with biology. The “soft” nature of organic materials offers better mechanical compatibility with tissue than traditional electronic materials, while their natural compatibility with mechanically flexible substrates suits the non-planar form factors often required for biomedical implants. More importantly, their ability to conduct ions in addition to electrons and holes opens up a new communication channel with biology. Our Department combines expertise in organic electronics and biology. Our research aims to elucidate the fundamentals of the electronic materials/biology interface and to launch new bioelectronic technologies.
A fundamental requirement of any bioelectronic system is the existence of electronic coupling and communication between the biomolecules and the electronic supports. Special methods to immobilize biomolecules on solid supports while preserving their bioactive structures were developed. Ingenious methods to structurally align and orient biomaterials on surfaces in order to optimize electronic communication were reported. Although impressive advances in the functional tailoring of biomolecule electronic units–hybrid systems were accomplished, challenging issues await scientific solutions. The miniaturization of the bioelectronic systems is a
requisite for future implantable devices, and these types of applications will certainly introduce the need for biocompatibility of the systems. The miniaturization of the systems will also require the patterned, dense organization of biomolecules on electronic supports. Such organized systems may lead to high throughput parallel biosensing and to devices of operational complexity. The development of methods to address and trigger specific biomolecules in the predesigned arrays is, however, essential.
The understanding of charge transport phenomena through biological matrices attracted in the past decades, and continues to evolve, intensive theoretical and experimental work. The understanding of superexchange charge transfer theory, and the definition of superior tunneling paths in proteins had a tremendous impact on the understanding of biological processes such as the electron transfer in the photosynthetic reaction center, or the charge transport in redox-proteins that are the key reactions for numerous electrochemical and photoelectrochemical biosensing systems. A continuous feedback between elegant experimental work employing structurally engineered proteins and theoretical analysis of the results led to the formulation of a comprehensive paradigm for electron transport in proteins. Charge transport through DNA is anticipated to play a key role in the electrical detection of DNA and in the analysis of base mismatches in nucleic acids, in the use of DNA. The electrical contacting between biomolecules and electrodes is an essential feature for most bioelectronic systems. Numerous redox enzymes exchange electrons with other biological components such as other redox-proteins, cofactors or molecular substrates. The exchange of electrons between the redox-centers of proteins and electrodes could activate the biocatalytic functions of these proteins, and may provide an important mechanism for numerous amperometric biosensors. Nonetheless, most of the proteins lack direct electron transfer communication with electrodes, and the lack of electrical communication between the biomaterials and the electronic elements present one of the fundamental difficulties of bioelectronic systems.
Developments
The major activities in the field of bioelectronics relate to the development of biosensors that transduce biorecognition or biocatalytic processes in the form of electronic signals. Other research efforts are directed at utilizing the biocatalytic electron transfer functions of enzymes to assemble biofuel cells that convert organic fuel substrates into electrical energy. Exciting opportunities exist in the electrical interfacing of neuronal networks with semiconductor microstructures. The excitation of ion conductance in neurons may be followed by electron conductance of semiconductor devices, thus opening the way to generating future neuron-semiconductor hybrid systems for dynamic memory and active learning. The recent progress in nanotechnology and specifically in nanobiotechnology adds new dimensions to the area of bioelectronics. Metal and semiconductor nanoparticles, nanorods, nanowires, and carbon nanotubes represent nano-objects with novel electronic properties. Recent studies revealed that the integration of these objects with biomolecules yields new functional systems that may yield miniaturized biosensors, mechanical devices and electronic circuitry.
A team of physicists and microbiologists at the University of Massachusetts (USA) says their discovery “a bacterium called Geobacter sulfurreducens contains microbial nanowires that can efficiently transmit electricity” could lead to cheaper, nontoxic nanomaterials for biosensors and solid state electronics that interface with biological systems. Researchers say the bacterial filaments, known as microbial nanowires, can move charges as efficiently as synthetic organic metallic nanostructures – and over remarkable distances, thousands of times the bacterium’s length. The ability of protein filaments to conduct electrons in this way is a paradigm shift in biology and has ramifications for our understanding of natural microbial processes as well as practical implications for environmental clean-up and the development of renewable energy sources. The discovery could lead to a range of new conducting nanomaterials that are living, naturally occurring, nontoxic, easier to produce and cheaper than man-made technologies. They could even allow the use of electronics in water and moist environments. The bacterial nanowires are said to be tunable in a way not seen before. It’s possible to manipulate their conducting properties by simply changing the temperature or regulating gene expression to create a new strain and by introducing a third electrode, a biofilm can act like a biological transistor, able to be switched on or off by applying a voltage. Another advantage with the invention is its ability to produce natural materials that are more eco-friendly and cheaper than man-made versions, many of which require rare elements. Some of the main landmarks achieved in this direction are:
- Development of a memory device that is soft and functions well in wet environments can open the door to a new generation of biocompatible electronic devices.
- Biodegradable circuits could enable better neural interfaces and LED tattoos.
- Development of flexible, stretchable silicon circuits whose performance matches that of their rigid counterparts.
