431599_Print.indd

smaller than 200μm. the two correspondingfitting relation are 0.499/rand 0.473/
r. For the needle diameter around 100μm, the maximal principle strain can be
extrapolated as 0.998 and 0.946% respectively, which are both smaller than the
critical breaking strain of SU-8 for bulk materials. The stress intensity factorKfor a

thinfilm under pure bending has the following scaling relation [ 30 ]K*Ee

ffiffiffi

h

p
,
whereEis the Young’s odulus of the material, andhis the thickness of ribbon. The
ribbon breaks whenKreaches the toughness of the materialKc.Kcis usually on the
order of 100KPa

ffiffiffiffi

m

p
,[ 31 ] andEfor SU-8 is around 1 GPa. Therefore, for a device
with thickness several hundred nanometers, the fracture strainecis on the order of
several percent. In fact, with our current structure, experiment demonstrates that
SU-8 ribbon can sustain the bending with curvature larger than 0.1μm−^1 that
corresponds to the curvature of 20-μm ID needle.
Last, we tested the injection by using thinfilm electronics with same thickness
(Fig.5.12a). For ca. 400-μm inner diameter needle, only 1.5 mm thinfilm elec-
tronics can go through (Fig.5.12b). These results further demonstrate the unique
design of the mesh electronics for injection.

5.3.3 Syringe-Injectable Electronics for Soft Matters.........

Mesh electronics can be co-injected with various materials into cavities with a small
injection site and unfold to distribute sensors (Fig.5.6c). We mixed 15-mm-wide
electronics containing nanowire strain sensors with PDMS monomer diluted in
hexane. We injected this mixture through a 20-gauge (603μm, ID) needle into a
cavity constituted by two pieces of cured PDMS (Fig.5.13a), and ejected inter-
connects and I/O pads outside for bonding. Mesh electronics inside the cavity can
gradually unfold and cover the 15 mm
20 mm area (Fig.5.13b, c). Micro-CT
3D reconstructed imaging of mesh electronics shows that, due to theflexibility,
mesh electronics can cover the 3D step-like structures inside the cavity (Fig.5.13c,
II). The conductance changes from 11 silicon nanowire devices on the mesh
electronics can be recorded as strain sensors. We monitored the response of
nanowire device when a point load in z-direction has been introduced to PDMS to
create a non-uniform strain distribution (Fig.5.13d). The conductance changes
from nanowire device are consistent with strain distribution and our previous report
[ 14 ]. This result proves that injected electronics can be delivered into soft system
with small damage and used to measure strain distribution inside soft materials to
the external mechanical deformation.
We further extended this co-injection concept through the co-injection of mesh
electronics with embryonic neural cells into tissue scaffold (Fig.5.14). Embryonic
rat hippocampal neurons were mixed with mesh electronics and uncured
MatrigelTM. We inject the composite into cured MatrigelTM (Fig.5.14a).
3D-reconstructed confocal micrographs from two-week culture showed that neu-
rons with high-density outgrowth neurites interpenetrating in the mesh structure of

5.3 Results and Discussion 85