Biomedical Engineering Reference
In-Depth Information
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3. Electrical switching mechanism in bioploymers
Biomolecules often have sensitive bio-active sites that can change under external stimuli
such as temperature, light, electrical signals, PH and chemical/biochemical reactions of their
environs. Such switchable biomolecules are of tremendous usefulness in diverse areas
including biological, medical and bio-electronic technology. Most research groups in this
field are interested in investigating new class of switchable biological systems albeit the field
is still at its infancy stage. Chu et al. (2010) reported electro-switchable oligopetides as a
function of surface potential. Oligolysine peptides exhibit protonated amino side chain at
PH-7 providing the basis of switching ''ON'' and ''OFF'' of the biological activity on the
surface upon application of negative potential. Switching initiated by PH changes has been
observed in other biomolecules and biopolymers (Zimmermann et al., 2006). Biomolecular
motors of actomyosin experience rapid and reversible on-off switching by thermal
activation (Mihajlovi et al., 2004). The most optimistic approach of integrating photo
switchable biomolecules into opto-electronic devices is provided by highly photo sensitive
bacteriorhodopsin. This molecule has shown remarkable photo sensitive switching of its
electrical properties that mimic conventional Gate transistors (Roy et al., 2010; Pandey, 2006;
Qun et al., 2004). Bottom-up approach toward building optical nano-electronic devices is
also feasible with the discoveries of switchable photoconductivity in even the smallest
structures such as quantum dots of cross-linked ligands (Lilly et al., 2011).
Electrical switching in biomolecules has wider applications in electronic industry. However,
a lucid understanding of microscopic switching mechanism in these biological systems is
still an outstanding challenge. In most investigations probing electrical switching in organic
molecules, an external electrical signal is applied to the sample sandwiched between metal
electrodes. Many materials have been reported to show hysteretic impedance switching
where a system in its high impedance state (OFF) is switched by a threshold voltage into a
low impedance state (ON) and remains in the ON state even with the reversal of applied
voltage. This phenomenon is also known as resistive switching. Switching mechanism
depend on whether the contribution comes from thermal, electronic, or ionic effects (Waser
et al., 2007). Resistive switching is generally dependent on number of mobile charges, their
mobility and the average electric field. In the case where the current is highly localized
within a small sample area, filamental conduction take place (Scott et al., 2007). This simply
involves formation of metallic bridge connecting the two electrodes. Filamental conduction
accounts for negative differential resistance (NDR) model which is the basis of many
molecular switching processes (Ren et al., 2010). Although this model was originally applied
to inorganic materials, it can also explain resistive switching in organic samples (Tseng et al.,
2006). The model assumes a trap- controlled channel where tunnelling take place in between
chains of metallic islands. Similarly a decrease in electron transport channels and weak
coupling between electrodes and the contact molecule causes NDR switching behaviour.
Electric field induced switching mechanism is common in literature (Waser et al., 2007). In
