(3.3 103 – 1.3 105 )[ 10 ]. Therefore the design of injectable mesh electronics
exhibits significantly more mechanicalflexibility to cells and tissues showing
unparalleled advantages as an implantable electronics.
5.2.6 Electrical Measurement..........................
ACF bonding for electrical measurement: Mesh electronics were injected
through syringe into solution, soft matters, biomaterials or tissues, with I/O part
injected outside the target materials. DI water and other solvents (PBS, culture
medium, hexane, etc.) were introduced to facilitate unfolding the I/O region, after
which the I/O region was rinsed and dried with ethanol (Fig.5.4). For the con-
nection to measurement setup, the unfolded and dried I/O region of injectable
electronics was bonded to the flexible cable (FFC/FPC Jumper Cables
PREMO-FLEX, Molex, Lisle, IL) through an anisotropic conductivefilm (ACF,
AC-4351Y, Hitachi Chemical Co. America, Ltd., Westborough, MA). ACF was
1.2 mm wide with conductive particles* 3 lm in diameter. Firstly, an ACF with
protective layer was positioned on the I/O region, and presealed after being heated
to 90° and a pressure of 1 MPa for 1 min with a homemade hot bar or commercial
bonding system (Fineplacer Lambda Manual Sub-Micron Flip-Chip Bonder,
Finetech, Inc., Manchester, NH) to tack it on I/O part with protective layer
removed. Then theflexible cable was placed on the ACF and aligned. At last, the
endsealing was made with a temperature of 190–210 °C in ACF and a pressure of
4 MPa on the top for 5 min applied by homemade hot bar or commercial bonding
system. In order to demonstrate the adhesion strength of the interface between I/O
pads andflexible cable, the structure was peeled from the substrate and examined
by optical microscopy.
The connection resistance of ACF was measured to investigate the influence of
bonding on electrical properties of devices (Fig.5.4c, d). The conductance of each
device was measured by probe station asR 0 andR 1 before and after ACF bonding,
respectively. The connection resistance for each I/O pad (100lm diameter) was
calculated as (R 1 −R 0 )/2, illustrated in Fig.5.4c. The calculated connection
resistance after ACF bonding with commercial bonder and homemade bonding is
ca. 21.2Xand ca. 33.7Xrespectively (Fig.5.4d), below 0.05% of typical nano-
wire resistance and 0.03% of MEA device. The insulation resistance between I/O
pads without circuits was over 10^10 X. These measurements and calculation results
demonstrated that ACF bonding had little influence on electrical properties of
injectable electronics, which ensured reliable measurement of injectable electronics
in many kinds of applications afterwards.
Yield of injection: To obtain the yield of electronics after injection, conductance
of nanowire devices before and after injection through needles was compared as
following procedure: (1) As-made 2D electronics was partially immersed in etchant
solution (Nickel Etchant TFB, Transene Company Inc., Danvers, MA) for 3–4hat
25 °C to firstly release nickel layer under I/O region of electronics. Then,
74 5 Syringe Injectable Electronics