Biomedical Engineering Reference
In-Depth Information
biopolymers is a multi-step process. According to this mechanism, the overall distance
between primary electron donor and final electron acceptor is split into a series of short
electron transfer steps. The essential difference is the existence of bridge units (oxidized or
reduced species) that function as relays system and the fact the hopping process has a weak
distance dependence (Cordes & Giese, 2009).Both of the two processes can take place at the
same time and have been observed by several experimental groups. Treadwaya et al. (2002)
noted that DNA assemblies of different lengths, sequence, and conformation may allow
tunneling, hopping, or some mixture of the two mechanisms to actually dominate.
From measurements that probed changes in oxidized guanine damage yield with response
to base perturbations, Armitage et al. (2004) noted that charge transfer through base-base of
DNA molecules takes place through hopping via the π-π bond overlap. Tao et al. (2005)
reported electron hopping and bridge-assisted superexchange charge transfer between
donor and an acceptor groups in peptide systems. The charge and dipole of the peptide
play an important role in the electron transfer (Amit et al., 2008). Galoppini & Fox (1996)
demonstrated the effect of electric field generated by the helix dipole on electron transfer in
Aib-rich α helical peptides and found out that other than the effect from secondary structure
(α helix and β sheet), dipole and hydrogen bonding, the solvent also has a marked influence
on the study of the electron transfer. Due to complexity of peptides, the importance of
individual amino acids in controlling electron transfer is not yet understood in detail.
Similar studies in proteins have concluded that electron transfer can occur across hydrogen
bonds and that the rate of such transfer is greatly increased when the electron motions are
strongly coupled with those of the protons (Ronald et al., 1981). While studying energy
transport in biopolymers, Radha & Rossen (2003) suggested, based on the experimental
results, that a soliton in biopolymers is an energy packet (similar to the “conformon” which
is the packet of conformational strain on mitochondria) associated with a conformational
strain localized in region much shorter than the length of a molecule. It was also noted by
the same group that as the soliton (localized curvature) moves on the polymer, it could trap
an electron and drag it along. This mechanism may be important in understanding charge
transport in biological molecules, where curvatures abound. Studies on charge transport in
ethyl cellulose- chloranil systems have also been done, (Khare et al., 2000) where the space
charge limited current (SCLC) was found to be the dominant mode of electrical conduction
at high field in these systems.
Mechanisms leading to charge conduction in metal-polymer-metal configuration have been
the subject of intensive study in the past two decades. Much of these studies have focused
on doped and undoped synthetic polymers where the commonly discussed high-field
electronic conduction mechanism for various films are Fowler-Nordheim tunneling, Poole-
Frenkel (P-F), Richardson-Schottky (R-S) thermionic emissions, space charge limited
conduction and variable range hopping. Based on the same sample geometry it is reasoned
that the mechanism mentioned above could also contribute to detectable current flow in
biopolymers sandwiched between metal electrodes. These mechanisms are discussed
hereunder and in section 5, experimental results based on cutin biopolymer are presented
and discussed in reference to the charge transport mechanism mentioned above.
2.1 Fowler-Nordheim tunneling
In Fowler-Nordheim tunneling the basic idea is that quantum mechanical tunneling from
the adjacent conductor into the insulator limits the current through the structure. Once the
carriers have tunneled into the insulator they are free to move within the material.
