Biomedical Engineering Reference
In-Depth Information
Fig. 7.8 Current-time
patterns in a fractal memory
I
in 1
out 1
t
t
I
in 2
out 2
t
t
I
in 3
out 3
t
t
Such a fractal memory photodiode can be implemented with four molecules:
ferrocene, which acts as electron donor, flavin as the sensitizer, viologen as the relay,
and, finally, the protein cytochrome c as the electron acceptor. These are deposited
sequentially in this order on a substrate in a metal/insulator/metal configuration,
the top electrode being an aluminum layer ( Choi et al. 2003 ). The electron
transport in this biomolecular photodiode mimics the photosynthesis process. The
state of the photodiode can be preset by irradiation with a laser pulse and can
be reset by applying a bias in dark/absence of light. The fractal memory effect
was tested by transient photocurrent measurements for trains of input pulses that
simulate the ASCII codes “U,” “T,” “Q,” and “P,” different responses/decay profiles
corresponding to different inputs.
7.6
Logical Drug Delivery and In Vivo Computation
Biomolecular logic computation can be used in biomedicine for controlled delivery
of drugs. In such a device ( Shlyahovsky et al. 2009 ), the 0 and 1 Boolean bits are
encoded as presence and absence, respectively, of aptamer substrates or a ssDNA
molecule, such that for each gate a set of aptamer substrates or DNA strands
representing the output release a catalytic nucleic acid that yield a fluorescent
signal. Logic gates combine two DNA translators that mediate the release of certain
output nucleic acid strands when the DNA input strand does not match the output.
This operation, which decouples the input sequence from useful output sequences,
allows information translation from one code to another. The operation of the device
is based on strand displacement instigated by formation of a duplex in the loop
region (see Fig. 7.9 ). This mechanism is based on the fact that duplex formation
is favored by base-pairing of loop-bridged complementary sequences A and B, and
that stability of the duplex region is dictated by the extent of base pairing.
More specifically, if one duplex region is separated by the creation of an aptamer-
substrate complex or by strand displacement due to favored interactions/superior
base pairing, the other weakened double-stranded region is separated in its turn in a
subsequent process. The DNA logic device consists of four footholds hybridized
to the same DNA scaffold that acts as a track. The outer footholds, hybridized
with mediator strands, sense the input and, for appropriate input aptamers or DNA
strands, translocate the mediator strands to the inner footholds. Then, the nucleic
acids linked to the inner footholds are displaced by the mediator strands and form
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