beam lithography. Previously discussed mechanics-driven self-organization created
a random or regular network of 3D features that mimic the size scale and mor-
phology of submicron ECM features, such as thefibrous meshwork of brain ECM
[ 36 ]. Open mesh nanoES (Fig.4.5b) were made by photolithography with a regular
structure, similar to the ECM of the ventricular myocardium [ 37 , 38 ]. The planar
design and initial fabrication of these 3D nanoES use existing capabilities devel-
oped for conventional planar nanoelectronics, and could enable integration of
additional device components (for example, memories and logic gates) [ 39 , 40 ] and
substantial increases in device number and overall scaffold size.
Reconstructed 3D confocalfluorescence images of typical 3D self-organized
scaffolds viewed alongy- andx-axes (Fig.4.6a, I and II respectively) showed that
the framework was 3D with a highly curvilinear and interconnected structure. The
porosity (calculated from the initial planar device design and thefinal 3D construct
volume) was >99.8%, comparable to that of hydrogel biomaterials [ 6 – 8 ]. Scanning
electron microscopy (SEM) of the 3D self-organized nanoES (Fig.4.6b) revealed
sub-micrometer feature sizes of individual device. Kinked nanowires and metallic
interconnects contained within the SU-8 backbone have 80 nm and 0.7μm feature
sizes in diameter, which are comparable to those of synthetic and natural ECMs [ 3 ,
8 ], and are several orders of magnitude smaller than those for electronic structures
[ 23 ] that is used to directly insert into tissue.
Figure4.6c shows the 3D distribution of nanowire FET devices (Fig.4.6a, II)
within the scaffold. They are spanned separations of 7.3– 324 μm in 3D (Fig.4.6c),
and can be made closer together (for example, <0.5μm) by printing denser nano-
wires on the substrate [ 40 ] to improve spatial resolution of nanoelectronic sensors.
We evaluated the performance of devices through water-gate measurements for the
nanowire FET elements in the 3D scaffolds in phosphate buffered saline (PBS). The
results show device yields (80%), conductances (1.52±0.61μS; mean±SD)
and sensitivities (8.07±2.92μS/V), comparable to measurements from planar
devices using similar nanowires [ 18 ].
3D mesh nanoES were realized by manual folding and rolling of free-standing
device arrays as discussed in previous chapters. Mesh structures (Fig.4.5a, II) were
fabricated such that the nanoES maintained an approximately planar configuration
following relief from the substrate. A typical 3.5 cm1.5 cm 2 μm mesh
nanoES, was approximately planar with 60 nanowire FET devices in regular array
with a 2D open porosity of 75% (Fig.4.7a). This mesh porosity is comparable to
that of honeycomb-like synthetic ECM engineered for cardiac tissue culture [ 38 ]. In
Fig. 4.5 Macroporous andflexible nanowire nanoES. Device fabrication schematics.I:3D
self-organized nanowire FET devices.II: mesh nanowire FET devices. Light blue: silicon oxide
substrates, blue: nickel sacrificial layers, green: nanoES, yellow dots: individual nanowire FETs
4.3 Results and Discussion 47