Biomedical Engineering Reference
In-Depth Information
optical and microwave low loss, organic field effect transistors, organic light emitting diodes
(LED) ( Hagen et al., 2006). Nonlinear optical polymer electro-optic modulators fabricated
from biopolymers have demonstrated better performance compared to those made from
other materials (Piel et al., 2006).Naturally occurring Guar gum biopolymer chemically
modified with polyaniline exhibits electrical conductivity in the range of 1.6 × 10
-2
S/cm at
room temperature (Ashutosh & Singh 2008). Mallick & Sakar (2000) investigated electrical
conductivity of gum arabica found in different species of Acacia babul [
Acacia Arabica
]
(Boutelje, 1980) and found that its electrical properties are similar to that of synthetic
conducting polymer doped with inorganic salt and are proton conducting in nature.
Charge transport in biomolecular materials takes place mainly through two processes: Super
exchange transport and hopping transport. Super exchange is a chain mediated tunneling
transport. In this process electrons or holes are indirectly transferred from a donor to
acceptor group through an energetically well- isolated bridge, where the bridge orbitals are
only utilized as coupling media. In the hopping mechanism, the electron temporarily
resides on the bridge for a short time during its passing from one redox center to the other,
but in the super-exchange, the conjugated bridge only serves as a medium to pass the
electron between the donor and acceptor (Tao et al., 2005). Tunnelling is a process process
that decays exponentially with the length of the molecule. A simple tunneling model
assumes a finite potential barrier at the metal-insulator interface. It describes free electron
flow for a short distance into the sample from the metal contact. At low voltages the charge
transfer is described by Simmons relations (DiBenedetto et al., 2009) but at higher applied
voltages the tunneling is determined by Fowler-Nordheim process. Superexchange process
can either be coherent or non-coherent. Coherent tunneling process is whereby a charge
carrier moves from a donor to acceptor fast enough such that there is no dephasing by
nuclear motions of the bridge (Weiss et al., 2007). Consequently, charges do not exchange
energy with the molecules. However this process does not take place at significantly long
distances. Incoherent superexchange on the other hand is a multi-step process in which a
localized charge carrier interacts with phonons generated by thermal motion of the
molecules (Singh et al., 2010)
As opposed to superexchange, hopping transport in biopolymers is a weakly distance
dependent incoherent process. Generally superexchange is a short range transfer of charges
in a spatial scale of a few Å while hopping transport takes place over a longer distance
greater than 1nm. The exact mechanism of tunneling and hopping is not fully understood
but it is known to be influenced by several factors. First, type of charge carriers in
biopolymers which can either be holes, electrons or even polarons influences charge
transport. Hole transfer is initiated by photo-oxidation of the donor groups attached to the
terminus of the molecule whereas electron transfer occurs by chemical reduction of the
acceptor group. A direct reduction of the molecule in contact with the metal electrode occurs
when the voltage is applied. The interplay between donor acceptor and coupling fluctuation
in biological electron transfer has also been observed (Skourtis et al., 2010). Secondly, band
structure and hoping sites also influence charge transport. Although there are no band
structures in biomolecules, energy gaps exist due to different hybridized electronic states.
These energy gaps provide hopping sites through which charges propagate. Finally,
conformation and spatial changes for the conducting state may overlap and hence create
hopping sites as described by variable range hopping (Shinwari et al., 2010). Variable range
hopping mainly describes transport mechanism in solid-state materials, but has also been
observed in biopolymers (Mei Li et al., 2010). Similar to superexchange, electron hopping in
