**4. A computer-controlled polyelectrolyte multilayer-based layer-bylayer Kapton surface modification approach**

This is a computer-controlled, facile, environmentally friendly, low-cost, readily scalable, and layer-by-layer deposition process. This process used two weak polyelectrolytes, poly (acrylic acid) (PAA) and polyethylenimine (PEI), to apply polyelectrolyte multilayers (PEMs) on flexible Kapton HN films in an alternating, layer-by-layer fashion under controlled pH and ionic strength. Compared to strong polyelectrolytes, weak electrolytes have the advantage of controlling the PEM properties more systematically and simply [38]. To our knowledge, this work is the first to use only weak polyelectrolytes to surface modify Kapton substrates.

### **4.1. Computer-controlled polyelectrolyte multilayer-based surface modification of Kapton HN films**

Small pieces of Kapton 500HN with appropriate dimensions (e.g. 95 mm x 95 mm) were cut from a Kapton 500HN sheet and cleaned by sonication, first with a 10 g/L suspension of Powdered Precision Cleaner (Alconox, Inc., White Plains, NY, USA) in DI water for 10 min and then with acetone for 10 min, in an ultrasonic cleaner (Model 2510. Branson Ultrasonics, Danbury, CT, USA). The cleaned Kapton films were rinsed three times with DI water, placed in the sample chamber of a custom-built, computer-controlled, automated deposition system, and then subjected to a layer-by-layer PEM deposition process. The PEM deposition process involved alternating exposure to two solutions of oppositely charged, relatively small polyelectrolyte molecules, polyethylenimine (PEI. Branched, M.W. 1800 Dalton; Alfa Aesar, Ward Hill, MA, USA) and poly (acrylic acid) (PAA. Average M.W. ~1800 Dalton; Sigma-Aldrich, St. Louis, MO, USA). In a typical deposition cycle, the cleaned Kapton pieces were incubated for 10 min in a PAA aqueous solution (10 mg/ml. pH adjusted to 5.1 with NaOH) containing 0.5 M NaCl followed by rinsing three times with a 0.5 M NaCl aqueous solution. The Kapton pieces were then incubated for 10 min in a PEI aqueous solution (10 mg/ml. pH adjusted to 2.5 with HCl) containing 0.5 M NaCl followed by rinsing three times with a 0.5 M NaCl aqueous solution. Such a cycle was repeated to deposit the desired number of PEM layers on the Kapton pieces. To automate the deposition process, a peristaltic pump was used to deliver each of the polyelectrolyte solutions and the 0.5 M NaCl rinse solution to the sample chamber of the system. A two-way drain valve was used to remove the polyelectrolyte or the rinse solution to a waste chamber after a given incubation or rinsing step had been completed. A properly programmed microprocessor was used to operate the peristaltic pumps and the drain valve. After the desired number of deposition cycles has been finished, the resulting Kapton films were rinsed with DI water and dried in air at 60°C for 2 hours. deposition

average contact angles of water and the water-based GO ink were reduced to 41.0° and 55.5°, respectively, after the deposition of only one layer of PEM, whereas the contact angles of the organic solvents (DMF and ethanol) and the ethanediol-based silver ink were essentially not changed (**Table 1**). It has been shown that the wetting of fluids onto sequentially adsorbed PEM layers was affected primarily by the outermost layer [39, 40]. In agreement with this previous observation, when additional PEM layers were deposited on the Kapton film, all the inks and the solvents examined in this work (water-based GO ink, ethanediol-based silver ink, DMF, ethanol, and water) exhibited little further changes in their contact angles (**Table 1**).

**Table 1.** Contact angles of different solvents/inks on surface unmodified and PEM-modified Kapton HN films [1] (with

25.2 ± 1.8 23.6 ± 0.3 24.1 ± 1.1 26.4 ± 1.6

**Kapton with 1 PEM** 

**Kapton with 3 PEM** 

**Kapton with 4 PEM** 

11

**layers**

http://dx.doi.org/10.5772/intechopen.76450

**layers**

Surface Modification of Polyimide Films for Inkjet-Printing of Flexible Electronic Devices

**layer**

Water 76.6 ± 3.9 41.0 ± 2.4 39.8 ± 1.9 37.4 ± 1.2 Ethanol 10.4 ± 2.5 12.1 ± 1.4 10.4 ± 1.9 12.2 ± 4.8 DMF 11.5 ± 1.3 12.5 ± 1.5 12.9 ± 0.1 13.3 ± 2.6 Water-based GO ink 72.4 ± 3.5 55.5 ± 2.6 53.7 ± 1.9 51.5 ± 1.2

Two organic solvent-based inks (a commercial ethanediol-based silver nanoparticle ink (Sun Chemical Corporation, Parsippany, NJ, USA) and a cyclohexanone/terpineol-based graphene ink (home-formulated based on the procedures described by Secor *et al.* [35])) and a waterbased GO ink were examined for their inkjet printability on Kapton HN films before and after

**Figures 7** and **8** show high-resolution scanned images of silver IDE patterns and graphene patches, respectively, that were inkjet-printed on surface unmodified Kapton HN and Kapton HN which had been deposited with 1, 3 and 4 layers of PEMs. All of the silver IDEs and graphene patches exhibited uniform morphologies and precisely controlled shapes with sharp edges, as designed, irrespective of whether they had been printed on surface unmodified or

**Figure 9a** shows a square that was printed (five passes) with the GO ink on a surface unmodified Kapton KN film. The ink drops balled up to form isolated small "islands." On the other hand, after 1, 3, or 4 PEM layers had been deposited on a Kapton HN film, precisely controlled ink squares with sharp edges were able to be printed as designed. A typical GO ink square printed on a Kapton HN film that had been deposited with 4 PEM layers is shown in **Figure 9b**. For electrically charged ink particles, similar to the bio-enabled method described above, this PEM-based surface modification method can also enhance the uniformity of the inkjetprinted thin films (by reducing the "coffee ring effect" during drying) compared with tradi-

**4.3. Printability assessment**

ethanediol-based silver ink

the PEM-based surface modification.

**Unmodified Kapton**

permission from the Royal Society of Chemistry).

PEM-modified Kapton HN films.

tional Kapton surface modification methods.

#### **4.2. Contact angle measurements**

Contact angle measurements were conducted (on a Rame-Hart goniometer equipped with a CCD camera (Rame-Hart Instrument Co., Succasunna, NJ, USA)) to evaluate the wetting of water, organic solvents, and inkjet inks on surface unmodified and PEM-modified Kapton HN films. **Table 1** shows that water and the water-based GO ink (which contained 60 wt% of glycerol for viscosity adjustment) exhibited an average contact angle of 76.6° and 72.4°, respectively, on a surface unmodified Kapton HN film. Organic solvents (ethanol and DMF), on the other hand, had quite small contact angles (<13°) on the same film. The commercial ethanol-based silver ink, with the presence of additional components such as a binder and a stabilizer, exhibited an average contact angle of 25.2° which was slightly higher than those of the two organic solvents. After PEM deposition on the Kapton HN film, the contact angles of both water and the water-based GO ink were significantly and reproducibly reduced. The


**Table 1.** Contact angles of different solvents/inks on surface unmodified and PEM-modified Kapton HN films [1] (with permission from the Royal Society of Chemistry).

average contact angles of water and the water-based GO ink were reduced to 41.0° and 55.5°, respectively, after the deposition of only one layer of PEM, whereas the contact angles of the organic solvents (DMF and ethanol) and the ethanediol-based silver ink were essentially not changed (**Table 1**). It has been shown that the wetting of fluids onto sequentially adsorbed PEM layers was affected primarily by the outermost layer [39, 40]. In agreement with this previous observation, when additional PEM layers were deposited on the Kapton film, all the inks and the solvents examined in this work (water-based GO ink, ethanediol-based silver ink, DMF, ethanol, and water) exhibited little further changes in their contact angles (**Table 1**). contact

#### **4.3. Printability assessment**

(acrylic acid) (PAA) and polyethylenimine (PEI), to apply polyelectrolyte multilayers (PEMs) on flexible Kapton HN films in an alternating, layer-by-layer fashion under controlled pH and ionic strength. Compared to strong polyelectrolytes, weak electrolytes have the advantage of controlling the PEM properties more systematically and simply [38]. To our knowledge, this work is the first to use only weak polyelectrolytes to surface modify Kapton substrates.

Small pieces of Kapton 500HN with appropriate dimensions (e.g. 95 mm x 95 mm) were cut from a Kapton 500HN sheet and cleaned by sonication, first with a 10 g/L suspension of Powdered Precision Cleaner (Alconox, Inc., White Plains, NY, USA) in DI water for 10 min and then with acetone for 10 min, in an ultrasonic cleaner (Model 2510. Branson Ultrasonics, Danbury, CT, USA). The cleaned Kapton films were rinsed three times with DI water, placed in the sample chamber of a custom-built, computer-controlled, automated deposition system, and then subjected to a layer-by-layer PEM deposition process. The PEM deposition process involved alternating exposure to two solutions of oppositely charged, relatively small polyelectrolyte molecules, polyethylenimine (PEI. Branched, M.W. 1800 Dalton; Alfa Aesar, Ward Hill, MA, USA) and poly (acrylic acid) (PAA. Average M.W. ~1800 Dalton; Sigma-Aldrich, St. Louis, MO, USA). In a typical deposition cycle, the cleaned Kapton pieces were incubated for 10 min in a PAA aqueous solution (10 mg/ml. pH adjusted to 5.1 with NaOH) containing 0.5 M NaCl followed by rinsing three times with a 0.5 M NaCl aqueous solution. The Kapton pieces were then incubated for 10 min in a PEI aqueous solution (10 mg/ml. pH adjusted to 2.5 with HCl) containing 0.5 M NaCl followed by rinsing three times with a 0.5 M NaCl aqueous solution. Such a cycle was repeated to deposit the desired number of PEM layers on the Kapton pieces. To automate the deposition process, a peristaltic pump was used to deliver each of the polyelectrolyte solutions and the 0.5 M NaCl rinse solution to the sample chamber of the system. A two-way drain valve was used to remove the polyelectrolyte or the rinse solution to a waste chamber after a given incubation or rinsing step had been completed. A properly programmed microprocessor was used to operate the peristaltic pumps and the drain valve. After the desired number of deposition cycles has been finished, the resulting

**4.1. Computer-controlled polyelectrolyte multilayer-based surface modification of** 

Kapton films were rinsed with DI water and dried in air at 60°C for 2 hours.

Contact angle measurements were conducted (on a Rame-Hart goniometer equipped with a CCD camera (Rame-Hart Instrument Co., Succasunna, NJ, USA)) to evaluate the wetting of water, organic solvents, and inkjet inks on surface unmodified and PEM-modified Kapton HN films. **Table 1** shows that water and the water-based GO ink (which contained 60 wt% of glycerol for viscosity adjustment) exhibited an average contact angle of 76.6° and 72.4°, respectively, on a surface unmodified Kapton HN film. Organic solvents (ethanol and DMF), on the other hand, had quite small contact angles (<13°) on the same film. The commercial ethanol-based silver ink, with the presence of additional components such as a binder and a stabilizer, exhibited an average contact angle of 25.2° which was slightly higher than those of the two organic solvents. After PEM deposition on the Kapton HN film, the contact angles of both water and the water-based GO ink were significantly and reproducibly reduced. The

**4.2. Contact angle measurements**

**Kapton HN films**

10 Flexible Electronics

Two organic solvent-based inks (a commercial ethanediol-based silver nanoparticle ink (Sun Chemical Corporation, Parsippany, NJ, USA) and a cyclohexanone/terpineol-based graphene ink (home-formulated based on the procedures described by Secor *et al.* [35])) and a waterbased GO ink were examined for their inkjet printability on Kapton HN films before and after the PEM-based surface modification.

**Figures 7** and **8** show high-resolution scanned images of silver IDE patterns and graphene patches, respectively, that were inkjet-printed on surface unmodified Kapton HN and Kapton HN which had been deposited with 1, 3 and 4 layers of PEMs. All of the silver IDEs and graphene patches exhibited uniform morphologies and precisely controlled shapes with sharp edges, as designed, irrespective of whether they had been printed on surface unmodified or PEM-modified Kapton HN films.

**Figure 9a** shows a square that was printed (five passes) with the GO ink on a surface unmodified Kapton KN film. The ink drops balled up to form isolated small "islands." On the other hand, after 1, 3, or 4 PEM layers had been deposited on a Kapton HN film, precisely controlled ink squares with sharp edges were able to be printed as designed. A typical GO ink square printed on a Kapton HN film that had been deposited with 4 PEM layers is shown in **Figure 9b**.

For electrically charged ink particles, similar to the bio-enabled method described above, this PEM-based surface modification method can also enhance the uniformity of the inkjetprinted thin films (by reducing the "coffee ring effect" during drying) compared with traditional Kapton surface modification methods.

#### **4.4. Adhesion sustainability tests after chemical functionalization**

For sensing applications, GO films normally need to be surface functionalized for the purpose of sensitivity and/or selectivity enhancement. Such surface functionalization includes reduction/oxidation and introduction and/or amplification of particular surface chemical groups and so on. The adhesion between the inkjet-printed GO films and the substrates has to be

Surface Modification of Polyimide Films for Inkjet-Printing of Flexible Electronic Devices

http://dx.doi.org/10.5772/intechopen.76450

13

To assess the adhesion of inkjet-printed rGO-based flexible sensors on Kapton HN substrates after such chemical functionalization, Kapton HN films were first surface modified via three approaches: UV/ozone treatment (traditional method), plasma treatment (traditional method), and the standard PEM-based deposition process described in this work. The UV/ ozone treatment was conducted on a 95 mm x 95 mm Kapton HN film with a UVO Cleaner (Jelight Company Inc., Irvine, CA, USA) for 5 min. The plasma treatment was performed on a 95 mm x 95 mm Kapton HN film with a plasma cleaner (model PDC-001. Harrick Scientific Corp., Ossining, NY, USA) in air for 20 min with the RF power set to the "high" level. GO patches were then inkjet-printed on the resulting surface modified Kapton films with the water-based GO ink. The inkjet-printed GO traces were then fired in nitrogen for one hour to be reduced to their rGO counterparts, followed by chemical treatments to introduce various surface chemical functional groups (hexafluoroisopropyl, amine, acrylate, and hydroxyl

After the reduction or a GO film functionalization process, the adhesion between the rGO patches and the surface-modified Kapton substrates was evaluated by visual inspection while slowly bending (to a radius of curvature of ˜1 cm) the rGO-on-Kapton structure. The structures were first bent 150 times in tension and then another 150 times in compression. **Table 2** summarizes the results of such adhesion sustainability bend tests. The GO films inkjet-printed on all the surface-modified Kapton HN substrates remained attached to the substrates after the thermal reduction and such bending. After the thermal reduction followed by each of the film functionalization treatments, the GO films printed on the 3-PEM-layer-modified or 4-PEM-layer-modified Kapton HN film remained attached upon bending (**Table 2**). On the other hand, the adhesion of the inkjet-printed GO films on the UV/ozone-, plasma-treated, or 1-PEM-layer-modified Kapton HN substrates was not as universally robust after the thermal reduction followed by the various film functionalization treatments. As shown in **Table 2**, the adhesion sustainability of the inkjet-printed GO films increased with increasing number of PEM layers on the Kapton substrates, which is probably due to the following reasons: each time after a polyelectrolyte (PAA or PEI) had bound to a Kapton HN substrate surface or to the oppositely charged polyelectrolyte which had previously bound to the substrate, there still existed nonoccupied binding

**Figure 9.** Optical images of GO squares printed (5 passes) with a water-based GO ink on surface unmodified (a) and

4-PEM-layer-modified (b) Kapton substrates [1] (with permission from the Royal Society of Chemistry).

strong enough to survive such chemical functionalization.

groups) to the resulting rGO patches.

**Figure 7.** Scanned high-resolution images of silver IDE patterns printed on surface unmodified and PEM-modified Kapton HN films with a commercial ethanediol-based silver nanoparticle ink. (a), (b), (c) and (d) are low magnification images of silver IDEs inkjet-printed on surface unmodified, 1-PEM-layer-modified, 3-PEM-layer-modified and 4-PEMlayer-modified Kapton HN films, respectively. (e), (f), (g) and (h) are the high magnification counterparts of (a), (b), (c) and (d), respectively. All these silver IDE patterns were fabricated by inkjet-printing for 5 passes the silver ink on the appropriate Kapton films followed by drying at 120°C for 3 hours [1] (with permission from the Royal Society of Chemistry).

**Figure 8.** Scanned high-resolution images of graphene patches printed on surface unmodified and PEM-modified Kapton HN films with a cyclohexanone/terpineol-based graphene ink. (a), (b), (c) and (d) are low magnification images of graphene patches inkjet-printed on surface unmodified, 1-PEM-layer-modified, 3-PEM-layer-modified and 4-PEMlayer-modified Kapton HN films, respectively. (e), (f), (g), and (h) are the high magnification counterparts of (a), (b), (c), and (d), respectively. All these graphene patches were fabricated by inkjet-printing for 5 passes the graphene ink on the appropriate Kapton films followed by drying at 100°C for 1 hour [1] (with permission from the Royal Society of Chemistry).

and so on. The adhesion between the inkjet-printed GO films and the substrates has to be strong enough to survive such chemical functionalization.

**4.4. Adhesion sustainability tests after chemical functionalization**

For sensing applications, GO films normally need to be surface functionalized for the purpose of sensitivity and/or selectivity enhancement. Such surface functionalization includes reduction/oxidation and introduction and/or amplification of particular surface chemical groups

**Figure 7.** Scanned high-resolution images of silver IDE patterns printed on surface unmodified and PEM-modified Kapton HN films with a commercial ethanediol-based silver nanoparticle ink. (a), (b), (c) and (d) are low magnification images of silver IDEs inkjet-printed on surface unmodified, 1-PEM-layer-modified, 3-PEM-layer-modified and 4-PEMlayer-modified Kapton HN films, respectively. (e), (f), (g) and (h) are the high magnification counterparts of (a), (b), (c) and (d), respectively. All these silver IDE patterns were fabricated by inkjet-printing for 5 passes the silver ink on the appropriate Kapton films followed by drying at 120°C for 3 hours [1] (with permission from the Royal Society of

**Figure 8.** Scanned high-resolution images of graphene patches printed on surface unmodified and PEM-modified Kapton HN films with a cyclohexanone/terpineol-based graphene ink. (a), (b), (c) and (d) are low magnification images of graphene patches inkjet-printed on surface unmodified, 1-PEM-layer-modified, 3-PEM-layer-modified and 4-PEMlayer-modified Kapton HN films, respectively. (e), (f), (g), and (h) are the high magnification counterparts of (a), (b), (c), and (d), respectively. All these graphene patches were fabricated by inkjet-printing for 5 passes the graphene ink on the appropriate Kapton films followed by drying at 100°C for 1 hour [1] (with permission from the Royal Society of

Chemistry).

Chemistry).

12 Flexible Electronics

To assess the adhesion of inkjet-printed rGO-based flexible sensors on Kapton HN substrates after such chemical functionalization, Kapton HN films were first surface modified via three approaches: UV/ozone treatment (traditional method), plasma treatment (traditional method), and the standard PEM-based deposition process described in this work. The UV/ ozone treatment was conducted on a 95 mm x 95 mm Kapton HN film with a UVO Cleaner (Jelight Company Inc., Irvine, CA, USA) for 5 min. The plasma treatment was performed on a 95 mm x 95 mm Kapton HN film with a plasma cleaner (model PDC-001. Harrick Scientific Corp., Ossining, NY, USA) in air for 20 min with the RF power set to the "high" level. GO patches were then inkjet-printed on the resulting surface modified Kapton films with the water-based GO ink. The inkjet-printed GO traces were then fired in nitrogen for one hour to be reduced to their rGO counterparts, followed by chemical treatments to introduce various surface chemical functional groups (hexafluoroisopropyl, amine, acrylate, and hydroxyl groups) to the resulting rGO patches. a

After the reduction or a GO film functionalization process, the adhesion between the rGO patches and the surface-modified Kapton substrates was evaluated by visual inspection while slowly bending (to a radius of curvature of ˜1 cm) the rGO-on-Kapton structure. The structures were first bent 150 times in tension and then another 150 times in compression. **Table 2** summarizes the results of such adhesion sustainability bend tests. The GO films inkjet-printed on all the surface-modified Kapton HN substrates remained attached to the substrates after the thermal reduction and such bending. After the thermal reduction followed by each of the film functionalization treatments, the GO films printed on the 3-PEM-layer-modified or 4-PEM-layer-modified Kapton HN film remained attached upon bending (**Table 2**). On the other hand, the adhesion of the inkjet-printed GO films on the UV/ozone-, plasma-treated, or 1-PEM-layer-modified Kapton HN substrates was not as universally robust after the thermal reduction followed by the various film functionalization treatments. As shown in **Table 2**, the adhesion sustainability of the inkjet-printed GO films increased with increasing number of PEM layers on the Kapton substrates, which is probably due to the following reasons: each time after a polyelectrolyte (PAA or PEI) had bound to a Kapton HN substrate surface or to the oppositely charged polyelectrolyte which had previously bound to the substrate, there still existed nonoccupied binding

**Figure 9.** Optical images of GO squares printed (5 passes) with a water-based GO ink on surface unmodified (a) and 4-PEM-layer-modified (b) Kapton substrates [1] (with permission from the Royal Society of Chemistry).

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

**4.5. Fabrication and bend testing of all-inkjet-printed flexible gas sensors**

(**Figure 10b**–**d**).

**5. Conclusions**

Single-layered, rGO-based gas sensors were fabricated on a 4-PEM-layer-modified Kapton HN substrate following the procedures described in Section 3.3. A such fabricated sensor, as shown in **Figure 10a**, was subjected to bend testing. The sensor was bent, to a radius of curvature of ~1 cm, 1000 times in tension followed by another 1000 times in compression. During the bending process, conductivity measurements were performed on the sensor after every 50 times of bending. The conductivity of the sensor was found to be virtually the same (i.e. a resistance of ~6 kΩ) throughout the whole bend testing as that before the bending. Optical and SEM microscopic analyses were conducted after such repeated bending to examine the morphology of the sensor. No apparent cracks on either the silver electrodes or the rGO patch were observed and the sensor remained the same morphology as that before the bending

Surface Modification of Polyimide Films for Inkjet-Printing of Flexible Electronic Devices

http://dx.doi.org/10.5772/intechopen.76450

The slip additive in Kapton HN films contains a significant amount of crystalline CaCO<sup>3</sup>

targeted the electric charges borne by the additive particles.

and were found insensitive to repeated bending to a small 0.5 cm radius.

another 1000 times in compression) bending to a radius of curvature of ~1 cm.

tions associated with inkjet-printing of Kapton-based flexible electronic devices.

printed films via reduction of the "coffee ring effect" during drying.

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

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

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

For electrically charged ink particles, both methods can enhance the uniformity of the inkjet-

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 pollu-

.

15


**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 the Royal Society of Chemistry).

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

#### **4.5. Fabrication and bend testing of all-inkjet-printed flexible gas sensors**

Single-layered, rGO-based gas sensors were fabricated on a 4-PEM-layer-modified Kapton HN substrate following the procedures described in Section 3.3. A such fabricated sensor, as shown in **Figure 10a**, was subjected to bend testing. The sensor was bent, to a radius of curvature of ~1 cm, 1000 times in tension followed by another 1000 times in compression. During the bending process, conductivity measurements were performed on the sensor after every 50 times of bending. The conductivity of the sensor was found to be virtually the same (i.e. a resistance of ~6 kΩ) throughout the whole bend testing as that before the bending. Optical and SEM microscopic analyses were conducted after such repeated bending to examine the morphology of the sensor. No apparent cracks on either the silver electrodes or the rGO patch were observed and the sensor remained the same morphology as that before the bending (**Figure 10b**–**d**).
