**5. Conclusions**

sites (point charges) on the substrate surface [39]. One PEM layer deposited on the substrate would probably be able to cover most (but not all) of the blank Kapton HN surface, which allowed for reasonably good printability for the GO ink. As more PEM layers were deposited, the uncovered substrate surface gradually decreased and the surface positive charge density gradually increased. As a result, when the GO ink particles (negatively charged) were inkjet-printed on, and bound to, such surface modified substrate, the adhe-

**UV/ozone Plasma 1 PEM layer 3 PEM layers 4 PEM layers**

**Figure 10.** Morphological analyses of a single-layered rGO-based sensor printed on a 4-PEM-layer-modified Kapton HN substrate before and after the bend testing. The water-based GO ink and the ethanediol-based Ag ink were printed for 60 and 5 inkjet passes, respectively. (a) and (b) optical images of the sensor before and after the bend testing, respectively. (c) Low and high (inset) magnification SEM images of a silver IDE of the sensor after the bend testing. (d) SEM image of

ink

the rGO patch of the sensor after the bend testing [1] (with permission from the Royal Society of Chemistry).

sion between the inkjet-printed GO film and the substrate gradually increased.

Thermal reduction **+ + + + +** Hexafluoroisopropyl addition — — **+ + +** Amine addition — — **+ + +** Acrylate addition NA NA — **+ +** Hydroxyl addition NA NA NA **+ +**

**Table 2.** Adhesion sustainability of GO films on surface-modified Kapton HN substrates upon thermal reduction followed by a number of surface group-introducing reactions ("+": the adhesion survived the corresponding chemical treatment; "—": the adhesion did not survive the corresponding chemical treatment; "NA": the corresponding chemical treatment was not performed since a prior step had peeled the GO film off from the substrate) [1] (with permission from

**Modification to GO Modification to Kapton**

the Royal Society of Chemistry).

14 Flexible Electronics

The slip additive in Kapton HN films contains a significant amount of crystalline CaCO<sup>3</sup> . Taking advantage of the electric charges borne by the additive particles at a neutral or acidic pH, two mild and environmentally friendly wet chemical approaches have been recently developed to surface modify Kapton HN films. The resulting surface modified films allowed for not only great printability of both water- and organic solvent-based inks (thus facilitating the full-inkjet-printing of entire flexible electronic devices) but also strong adhesion between the inkjet-printed traces and the substrate films. Different from the traditional Kapton surface modification approaches which target the surface polyimide matrix, these two mild methods targeted the electric charges borne by the additive particles.

The bio-enabled method, which utilized two clinical biomolecules and was conducted in aqueous salt solutions at a neutral pH, room temperature, and atmospheric pressure, was maximally mild and minimally destructive. The flexible rGO-based gas sensors fully inkjetprinted on the resulting surface modified Kapton HN films survived a Scotch-tape peel test and were found insensitive to repeated bending to a small 0.5 cm radius.

The computer-controlled PEM-based method involved the use of only weak polyelectrolytes (to enable systematic and simple control of the PEMs formed via adjustment of the pH of the polyelectrolyte solutions). The adhesion sustainability increased with increasing number of PEM layers. The rGO-based sensors printed on the resulting surface modified (with 4 layers of PEM layers) Kapton HN substrate was insensitive to repeated (1000 times in tension and another 1000 times in compression) bending to a radius of curvature of ~1 cm.

For electrically charged ink particles, both methods can enhance the uniformity of the inkjetprinted films via reduction of the "coffee ring effect" during drying.

The two methods have not only introduced new means to tune the surface properties of Kapton HN films thus allowing for the full-inkjet-printing of flexible and robust electronic devices but also brought forth solutions to significantly reduce of the environmental pollutions associated with inkjet-printing of Kapton-based flexible electronic devices.
