Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 221

Fig. 8.11 Biomimetic
nanopore in the ( a ) opened
and ( b ) closed state.

a b

length of 11:7 mm at 23 ı C, and becomes convergent for higher temperatures,
reaching 22.8 mm at 47 ı C. The response time is about 20 s. This principle can be
used for implementation of lenses that autoadjust at variations of other parameters.
For example, for pH-sensitive AA hydrogel rings, which contracts for pH <5:5
and expands for pH >5:5, the divergent focal length changes from 6 mm to
2:5mm when the pH changes from 2 to 8 and remains almost constant for a further
increase in the pH; the response time in this case is about 12 s. Arrays of microfluidic
lenses can be fabricated that resemble the compound eyes of Drosophila insects, in
which each ommatidium senses one area in space and can track spatial movements
across the field of view of the eye. For example, two liquid microlenses with
different hydrogel rings, which have opposite responses to pH, for example, such
that one expands and the other contracts at a pH change, mimic two compound eyes
with possible overlapping field of views, FOVs, and tunable focal lengths, which
autoadjust at specific environmental conditions. An array consisting of two such
microlenses is shown in Fig. 8.10 b.
Biomimetic nanopores that resemble biological ion channels have also been
fabricated. A stimuli-activated change in conformation of molecules can close or
open solid-state nanopores, preventing or allowing ions to pass through ( Yameen
et al. 2009 ). If the surface of a conical nanopore etched in a polyimide membrane
is covered with temperature-sensitive N-isopropylacrylamide (NIPAM) polymer
brushes covalently tethered at one end, as illustrated in Fig. 8.11 , the small opening
of the nanopore remains open at temperatures higher than the critical solubility
temperature of the polymer since the brushes are in a collapsed state (Fig. 8.11 a).
On the contrary, at lower temperatures, in particular at room temperature, NIPAM
polymerizes, takes a swollen/stiffer form and the small opening, with a diameter of
about 8 nm, becomes partially or even completely obstructed, and the ionic current
through the nanopore decreases significantly compared to the previous situation (see
Fig. 8.11 b). In particular, the effective diameter of the small opening decreases 3.4
times if the temperature changes from 40 ı Cto23 ı C. NIPAM can be considered as
thermally driven molecular gate that controls the ionic current through the nanopore
in a physiologic range of temperatures. The change in diameter/current is not abrupt
since the polymer brushes exhibit intermediate conformational states between
collapsed and swollen; the conformational changes are completely reversible.