schematically illustrates the steps for integrating devices based on Si NMs in stretchable, interconnected geometries with elastomeric substrates, following adapted versions of procedures described elsewhere [13, 18
]. The fabrication uses a Si wafer with a 100 nm thick coating of polymethylmethacrylate (PMMA) as a temporary substrate for the initial parts of the process.
Figure 1: Schematic illustration of the process for transfer printing an interconnected device structure from a substrate on which it is fabricated to an elastomeric sheet. (a) Interconnected sensors and electronics formed on a silicon wafer in an open mesh geometry are lifted onto the surface of a PDMS slab (i.e. stamp); (b) The back side of the mesh and the supporting PDMS stamp are coated with a thin layer of SiO2 and then pressed onto an elastomeric sheet (Ecoflex);(c) Removal of the PDMS completes the transfer; (d) Materials legend.
Figure 2: The process for fabricating a multiplexed array of
electrotactile stimulators in a stretchable, mesh geometry on the inner
surface of an elastomeric finger-tube.
(a) Casting and
curing an elastomer precursor on the finger of a model hand yields a
thin (∼ 500µm thick), closed-form membrane, i.e. a finger-tube.
(b) A PDMS stamp (here backed by a glass microscope slide) delivers the
electrotactile device to the outer surface of this finger-tube, while
compressed into a flattened geometry.
(c) Electrotactile array in the outside of the freestanding finger-tube.
(d) Turning the tube inside out relocates the array on the inner surface of the finger-tube, shown here at the midway point of this flipping process.
A layer of polyimide (PI, 1.25 µm thick) formed by spin coating a poly (amic acid) precursor and baking in an inert atmosphere at 250°C serves as the support for the devices. Electronically active materials are deposited (e.g. metallization) or transfer printed (e.g. Si NMs) onto the PI and patterned by photolithography and etching. Another layer of PI (1.25 µm thick) spin cast and cured on top of the device layers provides encapsulation and locates the devices near the neutral mechanical plane (NMP). Next, patterned reactive ion etching through the entire multilayer stack (i.e. PI/devices/PI) defines an open mesh structure. This same process removes the PI in the regions of the electrotactile stimulation electrodes, to allow direct contact with the skin. Immersion in an acetone bath washes away the underlying PMMA, thereby allowing the entire mesh to be lifted off, in a single piece, onto the surface of a flat slab of polydimethylsiloxane (PDMS), using procedures described previously [19, 20
]. Evaporation of a layer of SiO2
onto the mesh/PDMS and exposure of the silicone target substrate (Ecoflex 0030, Smooth-On, Inc.) to UV–ozone (to create reactive –OH groups on the surface) enables bonding between the two upon physical contact [21
].(Low pressures avoid contact between the PDMS and the finger-tube, thereby allowing bonding only to the mesh.) The SiO2
adhesion layer does not serve any electronic function. Removal of the stamp completes the transfer process, as shown in figure 1(c)
The electrotactile electrodes use 600 nm thick layers of Au in a concentric design, consisting of an inner disk (400 µm radius) surrounded by an outer ring (1000 µm radius) with a 250 µm wide gap between the two. The interconnects consist of 100 µm wide traces of Au in serpentine geometries (radii of curvature ∼800 µm); these traces connect the electrotactile electrodes to Si NM diodes (lateral dimensions of 225 µm x 100 µm and thicknesses of 300 nm). Two layers of Au interconnects (200 and 600 nm thick), isolated by a 1.25 µm PI layer and connected through etched PI vias, establish a compact wiring scheme with overlying interconnects. The 600 nm thick Au interconnect layer allows robust electronic contact though the PI vias. The strain gauge arrays consist of four Si NMs (strips with lateral dimensions of 1 mm x 50 µm and thicknesses of 300 nm) electrically connected by 200 nm thick, 60 µm wide Au traces patterned in serpentine shapes (radii of curvature ∼400 µm). The tactile sensors use 200 nm thick Au electrodes and interconnects in the geometry of the electrotactile arrays but with the concentric electrode pairs replaced by single, disk-shaped electrodes (radii ∼1000 µm).
The Ecoflex substrates, which we refer to as finger tubes, adopt three-dimensional forms specifically matched to those of fingers on a plastic model of the hand. The fabrication involves pouring a polymer precursor to Ecoflex onto a finger of the model and curing at room temperature for 1 h, to create a conformal sheet with ∼125 µm thickness. Pouring a second coating of precursor onto this sheet and curing for an additional 1 h doubles the thickness; repeating this process four times results in a thickness of ∼500 µm.
Removing the Ecoflex from the model and completing the cure by heating at 70°C for 2 h forms a free standing structure, i.e. a finger-tube, like the one illustrated in figure 2
. Ecoflex is an attractive material for this purpose because it has a low modulus (∼60 kPa) and large fracture strain (∼900 percent). The former allows soft, intimate contact with the skin; the latter enables the ‘flipping-over’ process referred to previously, and described in quantitative detail in section 3. Transfer printing delivers the device mesh structure to the outer surface of the finger-tube, while pressed into a flattened geometry (figure 2(b))
. The entire integrated system is then flipped inside out, to move the mesh from the outer to the inner surface of the tube, as shown in figures 2(c) and (d)
. Multifunctional devices incorporate electrotactile stimulators on the inside, and strain gauge arrays and tactile sensors on the outside.