Biomolecular Architecture for Nanotechnology - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 169

top of it. Then, if negatively charged biotinylated nanoparticles in solution are
placed over the structure, these concentrate above the positively charged electrodes
and bind to the streptavidin layer. A further nanoparticle layer can be added if
negatively charged streptavidin-coated nanoparticles are introduced, which bind
with the previous immobilized biotin-coated nanoparticles, and so on. A single
layer of 40-nm fluorescent particles can form in 15 s and 50 alternate layers need
only 1 h for completion, fractures appearing after more than 50 deposited layers.
The structure can then be lifted-off and float freely in the solution, if desired. More
complex structures, consisting of six layers of 40-nm particles followed by four
layers of 200-nm particles and a final six layers of 40-nm particles can be assembled
in this way.

5.5 Inorganic Scaffolds for Biomolecules

CNTs are ideal for scaffolds for tissue regeneration due to their mechanical strength,
flexibility, and low density compared to other ceramic-based or metallic scaffolds.
In addition, CNTs are not biodegradable, so that can be used as long-terms implants,
and have the right dimension for scaffolds aimed at bone or neural regeneration. A
review of CNT and carbon nanofiber applications in regenerative medicine and, in
particular, in bone and tissue regeneration, can be found in Tran et al. ( 2009 ).
The length and diameter of a single-walled CNT, with values between 100 and
300 nm and, respectively, 0.5 and 1.5 nm, are comparable to the size of collagen
fibrils and thus can easily mimic collagen as a scaffold on which hydroxyapatite
(HA), the major component of bone, grows. However, to attract calcium cations that
initiate HA crystallization, and thus mineralizes the bone matrix proteins produced
by osteoblasts, CNTs must first be functionalized.
Analogously, CNT functionalized with bioactive molecules is appropriate for
neural regeneration due to its electrical conductivity and dimensions similar to
that of small nerve fibers. As extracellular scaffold for neural growth, CNTs can
also control neurite branching and outgrowth if properly chemically modified. In
particular, the surface charge of CNTs affects neurite control. At a physiological pH
of 7.35, hippocampal neuronal cultures from rats grew well on neutral, positively
and negatively charged multiwalled CNTs, and zwitterionic CNTs, with almost the
same concentration of positive and negative charged groups, no difference being
observed in the number of neuritis per neuron ( Hu et al. 2004 ). However, the neurite
branching and outgrowth, characterized by the number of growth cones and neurite
length, is most developed in positively charged CNTs, moderate in zwitterionic and
neutral CNTs and least developed in negatively charged CNTs. The surface charge
density of chemically modified single- and multiwalled CNTs influence also the rate
of cell growth and the osteoblast morphology when CNTs are used as scaffolds for
proliferation of osteosarcoma rat cells and formation of bone-like tissues ( Zanello
et al. 2006 ).