431599_Print.indd

4.2.2 Scaffold Mechanical Properties.....................

The effective bending stiffness per unit width of the mesh scaffold,D, can be
estimated by [ 17 ]

D¼asDsþamDm ð 4 : 1 Þ

whereasandamare the area fraction of the single-layer polymer (SU-8) ribbon
(without metal layer and top polymer passivation layer) and three-layer ribbon
(bottom polymer layer, metal layer and top passivation layer) in the whole mesh
structure.Ds=Esh^3 /12 is the bending stiffness per unit width of the single-layer
polymer, whereEs= 2 GPa andhare the modulus and thickness of the SU-8.Dmis
the bending stiffness per unit width of a three-layer structure, which can be cal-
culated by [ 27 ]

Dm¼

Embmh^3 m

12 b

þ

Es

b

ðÞbbmðÞ 2 hþhm^3

12

þ

1

6

bmh^3 þ 2 bmh

h

2

þ

hm

2

! 2

ð 4 : 2 Þ

whereEm= 121 GPa andhmare the modulus and thickness of the palladium,bis
the width of the single-layer ribbon and the total width of the three-layer ribbon,bm
is the width of the palladium layer. In addition, the chromium layers are so thin
(1.5 nm) that their contribution to the bending stiffness is negligible. Whenhm=
75 nm,h= 0.5μm,b=10μm,bm=5μm,as= 2.51% andam= 3.57%, we can
calculateD= 0.006 nN m. Whenhm=75nm,h=2μm,b=40μm,bm=20μ
m,as= 10.06% andam= 13.31%, we can calculateD= 1.312 nN m.
To calculate the strain in tubular constructs, we used the equatione=y/R, where
yis the distance from the neutral plane, andRis the radius of curvature [ 27 ]. For the
symmetric mesh scaffold, since the neutral plane is the middle plane, the maximum
strains of metal and SU-8 appear aty=hm/2 andy=hm/2 +h, respectively. When
hm= 75 nm,h=2μm,R= 0.75μm−^1 , the maximum strains of metal and SU-8
are 0.005 and 0.272%, respectively.
The self-organized structure of nanoES was simulated by the commercialfinite
element software ABAQUS following the same parameters and procedure as
described in Chap. 2.

4.2.3 Cell Culture

Neuron culture: Device chips were cleaned by oxygen plasma, andfixed onto a
temperature controlled chamber with double-sided tape (Fig.4.2a). A 1 mm thick
polydimethylsiloxane (PDMS) membrane was placed over the device area, followed
by wire-bonding of individual devices (Fig.4.2b). An autoclaved glass ring was
placed andfixed over this PDMS chamber (Fig.4.2c). The whole chip was sterilized

42 4 Three-Dimensional Macroporous Nanoelectronics...