**3. A bio-enabled maximally mild layer-by-layer Kapton surface modification approach**

Protamine has been clinically used to reverse the anticoagulant effects of heparin by binding to it [33, 34]. The development of the present bio-enabled surface modification approach was inspired by the *in vivo* antagonizing interaction of these two clinically used biological molecules. In this surface modification process, negatively charged heparin and positively charged protamine were used to uniformly deposit a thin film of protamine-heparin complex on Kapton HN substrates in a layer-by-layer fashion. The surface modification process was conducted under maximally mild conditions (in aqueous solutions of clinical biomolecules, and at a neutral pH, room temperature and atmospheric pressure). During the process the positively charged additive particles (e.g. CaCO<sup>3</sup> particles) on the Kapton HN surface enabled binding of the initial heparin (negatively charged) layer via electrostatic interaction. After the initial binding of heparin, the layer-by-layer uniform deposition of the protamine-heparin complex on Kapton HN was realized by the electrostatic interaction between the oppositely charged protamine and heparin molecules.

As far as we know, the present bio-inspired method was the first to use environmentally friendly clinical biomolecules for substrate surface modification. It is also the first surface modification approach performed under maximally mild and minimally destructive conditions.

#### **3.1. Surface modification of Kapton HN films**

As shown in **Figure 2a** and **b**, the size of the ash particles varied significantly, from less than 100 nm to several microns, with the large particles probably resulting from the sintering and agglomeration of the fine ones [14]. The EDX analysis of the ash showed the presence of the elements of oxygen, calcium, and phosphorus (**Figure 2c**). The XRD analysis of the ash showed

(**Figure 2d**). Compared with the XRD pattern of the as-received Kapton HN films (**Figure 1b**), the XRD pattern of the pyrolyzed films (**Figure 2d**) indicated the disappearance of CaCO<sup>3</sup>

into CaO and CO<sup>2</sup>

ing process. Combining **Figure 1** (characterization of Kapton HN) and **Figure 2** (characterization of the Kapton HN ash resulted from pyrolysis), we can conclude that the additive in

compounds (crystalline or amorphous). Any calcium phosphate compounds, if present as previously reported [14] in the additive, must be either crystalline but in a small amount (i.e. beyond the detection limit of the diffractometer used for the XRD analyses) or amorphous, or both. While the exact nature of the additive in Kapton HN is probably proprietary

**Figure 2.** Characterization of the Kapton HN ash resulted from the pyrolysis of Kapton HN films at 800°C for 2 hours in air. (a) and (b) SEM images of the ash with low (a) and high (b) magnifications. (c) EDX pattern of the ash. (d) XRD

> P2 O7

(pattern ②. ICDD reference code

patterns of both the ash (pattern ①) and reference calcium pyrophosphate Ca<sup>2</sup>

04–009-6231) [2] (licensed under creative commons attribution 4.0 international license).

P2 O7

. The composition change was due to the multiple chemical reactions

(crystalline) and one or more phosphorus-containing

(ICDD Reference code 04–009-6231)

[31]) taken place during the pyrolyz-

and

the presence of only calcium pyrophosphate Ca<sup>2</sup>

P2 O7

(such as the decomposition of CaCO<sup>3</sup>

Kapton HN was composed of CaCO<sup>3</sup>

the presence of Ca<sup>2</sup>

4 Flexible Electronics

A small Kapton piece with appropriate dimensions (e.g., 50 mm × 50 mm) was cut from a Kapton 500HN sheet. After a brief rinse with a phosphate buffer (0.2 M, pH 7.0), the Kapton piece was incubated for 10 min with a heparin sodium solution (10 mg/ml, pH 7.0) in the phosphate buffer followed by rinsing three times with the phosphate buffer. The Kapton piece was then incubated for 10 min with a protamine sulfate solution (10 mg/ml, pH 7.0) in the phosphate buffer followed by rinsing three times with the phosphate buffer. This process (heparin/rinse/protamine/rinse) was performed for a total of 5 times. Finally, the Kapton piece was rinsed with DI water and dried in air at 60°C for 2 hours.

A control surface modification process was conducted to validate the hypothesis that the surface modification process was facilitated by the positive electric charges on the Kapton HN surface. The control process was similar to the standard process described earlier, except that the heparin solution used in each deposition cycle was supplemented with 1 M sodium chloride.

#### **3.2. Printability assessment**

**Figure 3a** and **b** show the optical images with low and high magnifications, respectively, of some proof-of-concept silver interdigitated electrode (IDE) patterns inkjet-printed on a surfacemodified Kapton substrate with an ethanediol-based silver nanoparticle ink (Sun Chemical Corporation, Parsippany, NJ, USA), while **Figure 3c** shows the optical image of some proof-ofconcept graphene patches inkjet-printed on a surface modified Kapton substrate with a cyclohexanone/terpineol-based graphene ink which had been formulated based on the procedures described by Secor *et al.* [35]. Both the silver (**Figure 3a** and **b**) and the graphene (**Figure 3c**) patterns printed with organic solvent-based inks were very similar to the designs and exhibited accurately controlled shapes with sharp edges.

A number of shapes (rectangle, circle with a 100 μm-wide gap in the center, and diamond) were then printed on both surface unmodified and surface modified Kapton HN films with a home-made water-based graphene oxide (GO) ink. As shown in **Figure 4a**–**c**, accurately controlled shapes were able to be inkjet-printed as designed on a surface modified Kapton HN film. On the other hand, the GO ink drops balled up and formed isolated small "islands" on a surface unmodified Kapton HN film (**Figure 4d**–**f**). As shown in **Figure 4g**–**i**, the GO ink that was printed on the Kapton film which had been treated with the control process (which was similar to the standard process except that the heparin solution used in each deposition cycle was supplemented with 1 M sodium chloride) balled up and exhibited isolated small "islands," similar to the patterns with surface unmodified Kapton HN shown in **Figure 4d**–**f**. This validated the hypothesis that the present surface modification method was made possible by the electric charges on the Kapton HN surface. Indeed, with such a high concentration of NaCl present in the heparin solution, the Cl¯ ions will screen the positive electric charges on the Kapton HN film surface. As a result, the initial heparin binding to the blank substrate film, and the subsequent protamine and heparin binding to the substrate, will all be drastically

reduced. Consequently, the Kapton HN film will not be properly surface modified and its

**Figure 4.** Printability assessment of a water-based GO ink on regularly surface modified (a, b, c), surface unmodified (d, e, f) Kapton HN, and Kapton HN which had been surface modified with a control process which was similar to the regular surface modification process except that 1 M NaCl was supplemented to the heparin solution (g, h, i) [2] (licensed

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7

For ink particles which are electrically charged under particular experimental conditions, the present surface modification method can enhance the uniformity of the thin films deposited on the resulting substrate. This is realized via reduction of "coffee ring effect" during the drying process. In the case of inkjet-printing, "coffee ring effect" results in an appreciable amount of more solid ink material deposited at the substrate perimeter than the other areas upon ink drying. The present surface modification method can choose to terminate the substrate surface with either negatively charged heparin or positively charged protamine. Terminating the substrate surface with opposite electric charges of the ink particles will enable local electrostatic interaction between the substrate surface and the ink particles. Consequently, the migration of the ink particles to the substrate perimeter during drying will be drastically reduced and

the uniformity of the inkjet-printed thin films significantly enhanced after drying.

A flexible multilayered gas sensor was inkjet-printed adhering to the following steps: A Kapton HN film was treated with the process described in the present work, and a water-based

**3.3. Fabrication and sensing tests of all-inkjet-printed flexible gas sensors**

surface properties were hardly tuned.

under creative commons attribution 4.0 international license).

**Figure 3.** Optical images of proof-of-concept silver IDEs and graphene patches printed on surface modified Kapton HN films with an ethanediol-based silver nanoparticle ink and a cyclohexanone/terpineol-based graphene ink, respectively. (a) and (b) Low (a) and high (b) magnification optical images of the silver IDEs fabricated by printing (for five passes) the silver nanoparticle ink on a surface modified Kapton film followed by annealing at 120°C for 3 hours. (c) Optical image of the graphene patches fabricated by printing (for five passes) the graphene ink on a surface modified Kapton film followed by drying at 100°C for 1 hour [2] (licensed under creative commons attribution 4.0 international license).

Surface Modification of Polyimide Films for Inkjet-Printing of Flexible Electronic Devices http://dx.doi.org/10.5772/intechopen.76450 7

**3.2. Printability assessment**

6 Flexible Electronics

ited accurately controlled shapes with sharp edges.

**Figure 3a** and **b** show the optical images with low and high magnifications, respectively, of some proof-of-concept silver interdigitated electrode (IDE) patterns inkjet-printed on a surfacemodified Kapton substrate with an ethanediol-based silver nanoparticle ink (Sun Chemical Corporation, Parsippany, NJ, USA), while **Figure 3c** shows the optical image of some proof-ofconcept graphene patches inkjet-printed on a surface modified Kapton substrate with a cyclohexanone/terpineol-based graphene ink which had been formulated based on the procedures described by Secor *et al.* [35]. Both the silver (**Figure 3a** and **b**) and the graphene (**Figure 3c**) patterns printed with organic solvent-based inks were very similar to the designs and exhib-

A number of shapes (rectangle, circle with a 100 μm-wide gap in the center, and diamond) were then printed on both surface unmodified and surface modified Kapton HN films with a home-made water-based graphene oxide (GO) ink. As shown in **Figure 4a**–**c**, accurately controlled shapes were able to be inkjet-printed as designed on a surface modified Kapton HN film. On the other hand, the GO ink drops balled up and formed isolated small "islands" on a surface unmodified Kapton HN film (**Figure 4d**–**f**). As shown in **Figure 4g**–**i**, the GO ink that was printed on the Kapton film which had been treated with the control process (which was similar to the standard process except that the heparin solution used in each deposition cycle was supplemented with 1 M sodium chloride) balled up and exhibited isolated small "islands," similar to the patterns with surface unmodified Kapton HN shown in **Figure 4d**–**f**. This validated the hypothesis that the present surface modification method was made possible by the electric charges on the Kapton HN surface. Indeed, with such a high concentration of NaCl present in the heparin solution, the Cl¯ ions will screen the positive electric charges on the Kapton HN film surface. As a result, the initial heparin binding to the blank substrate film, and the subsequent protamine and heparin binding to the substrate, will all be drastically

**Figure 3.** Optical images of proof-of-concept silver IDEs and graphene patches printed on surface modified Kapton HN films with an ethanediol-based silver nanoparticle ink and a cyclohexanone/terpineol-based graphene ink, respectively. (a) and (b) Low (a) and high (b) magnification optical images of the silver IDEs fabricated by printing (for five passes) the silver nanoparticle ink on a surface modified Kapton film followed by annealing at 120°C for 3 hours. (c) Optical image of the graphene patches fabricated by printing (for five passes) the graphene ink on a surface modified Kapton film followed by drying at 100°C for 1 hour [2] (licensed under creative commons attribution 4.0 international license).

**Figure 4.** Printability assessment of a water-based GO ink on regularly surface modified (a, b, c), surface unmodified (d, e, f) Kapton HN, and Kapton HN which had been surface modified with a control process which was similar to the regular surface modification process except that 1 M NaCl was supplemented to the heparin solution (g, h, i) [2] (licensed under creative commons attribution 4.0 international license).

reduced. Consequently, the Kapton HN film will not be properly surface modified and its surface properties were hardly tuned.

For ink particles which are electrically charged under particular experimental conditions, the present surface modification method can enhance the uniformity of the thin films deposited on the resulting substrate. This is realized via reduction of "coffee ring effect" during the drying process. In the case of inkjet-printing, "coffee ring effect" results in an appreciable amount of more solid ink material deposited at the substrate perimeter than the other areas upon ink drying. The present surface modification method can choose to terminate the substrate surface with either negatively charged heparin or positively charged protamine. Terminating the substrate surface with opposite electric charges of the ink particles will enable local electrostatic interaction between the substrate surface and the ink particles. Consequently, the migration of the ink particles to the substrate perimeter during drying will be drastically reduced and the uniformity of the inkjet-printed thin films significantly enhanced after drying.

#### **3.3. Fabrication and sensing tests of all-inkjet-printed flexible gas sensors**

A flexible multilayered gas sensor was inkjet-printed adhering to the following steps: A Kapton HN film was treated with the process described in the present work, and a water-based GO ink was then used to inkjet-print a GO patch on the resulting Kapton substrate. The electrically non-conductive GO patch was converted into its conductive reduced graphene oxide (rGO) counterpart by firing at 300°C for 1 hour in nitrogen. A selector layer was inkjet-printed on the resulting rGO patch with a dimethylformamide- (DMF-) based ink containing 10 mg/ ml of 2-(2-hydroxy-1, 1, 1, 3, 3, 3-hexafluoropropyl)-1-naphthol. An ethanediol-based silver nanoparticle ink was then used to print two silver electrodes. To achieve an optimum contact between the rGO patch and the silver electrodes, both electrodes overlapped the rGO patch by 1.5 mm. Finally the resulting multi-layered sensor prototype was fired at 120°C for 3 hours to remove the organic molecules coated on the silver nanoparticles and to anneal the silver nanoparticles for desired conductivity. **Figure 5a** shows an optical image of a typical multi-layered gas sensor prototype fabricated with the procedures described above. A flexible single-layered sensor was similarly fabricated except that the selector layer was not printed. Both the multi- and the single-layered sensors were flexible, ultra-lightweight (˜25 mg), and miniature sized (~1.5 cm x 1.0 cm).

square had sharp edges as designed and the GO flakes were well interconnected with no

**Figure 6.** SEM images with low (a) and high (b) magnifications of an inkjet-printed (10 passes) and dried GO film on a

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

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9

surface modified Kapton HN film [2] (licensed under creative commons attribution 4.0 international license).

A four-point bend tester (TestResources, Inc., Shakopee, MN, USA) controlled by R Controller software (TestResources, Inc.) was used to conduct the bend cycling tests. A fully inkjetprinted single-layered gas sensor was mounted on the bend tester with a home-made mounting system. The sensor was bent, with an amplitude of 20 mm and a bend rate of 1 mm/ second, to a radius of curvature of 1 cm 1000 times in tension and then another 1000 times in compression to the same radius of curvature. After the 2000 bending cycles, the resistance of the sensor was measured with a multimeter and its morphology examined with an optical microscope. It was found that the resistance of the sensor after the bend test was virtually the same as that before the test (i.e. ~14 kΩ) and no morphological changes were observed during the optical analyses. Keeping the amplitude and the bend rate the same, the radius of curvature was reduced to 0.5 cm and the sensor was bent 1000 times in tension and another 1000 times in compression, followed by conductivity and morphology examination. Again,

Peel tests were performed via a qualitative Scotch-tape peel test to evaluate the adhesion of an inkjet-printed single-layered gas sensor to the surface modified Kapton film. The adhesive side of a piece of Scotch® magic tape (3 M Company, St. Paul, MN, USA) was firmly pressed against the sensor and then peeled off [36, 37]. By visual inspection and optical microscopic analyses, all three components of the sensor (two silver electrodes and one rGO patch) remained attached to the substrate and retained their intactness after the tape had been peeled off.

**4. A computer-controlled polyelectrolyte multilayer-based layer-by-**

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

**3.4. Bend cycling tests on an all-inkjet-printed flexible gas sensor**

apparent conductivity or morphological changes were not observed.

**3.5. Peel testes on an all-inkjet-printed flexible gas sensor**

**layer Kapton surface modification approach**

observable cracks (**Figure 6a** and **b**).

Gas sensing was then performed with both multi- and single-layered sensors. A dimethyl methylphosphonate (DMMP) vapor of 2.5 ppm was generated from a DMMP permeation tube (KIN-TEK Laboratories, Inc., La Marque, TX, USA) installed in a FlexStream™ Gas Standards Generator (KIN-TEK Laboratories, Inc.) and carried by nitrogen with a flow rate of 500 sccm. The relative sensitivity of a multi-layered (black solid line) and a single-layered (red-dashed line) sensor upon exposure to 2.5 ppm DMMP is shown in **Figure 5b**. The relative sensitivity (S) is defined by the following formula:

$$\mathbf{S} = \frac{\mathbf{R} - \mathbf{R\_o}}{\mathbf{R\_o}}$$

where R<sup>0</sup> is the resistance between the two silver electrodes of a sensor before the exposure to the DMMP vapor and R that at a particular time after the exposure.

SEM analyses were performed to examine the micro-/nano-morphology of the inkjet-printed GO patterns. A GO patch inkjet-printed (ten passes) on a surface modified Kapton HN film was dried at 95°C under vacuum overnight (to remove the glycerol and the water in the GO ink) and then subjected to SEM analyses. Under a scanning electron microscope, the dried GO

**Figure 5.** (a) Optical image of a flexible, rGO-based, multi-layered gas sensor inkjet-printed on a surface modified Kapton HN film (the GO ink was printed for 10 passes, the selector ink for one pass, and the silver ink for 5 passes). (b) Relative sensitivity of the multi-layered (black solid line) and the single-layered (red dashed line) gas sensors upon exposure to 2.5 ppm dimethyl methylphosphonate [2] (licensed under creative commons attribution 4.0 international license).

Surface Modification of Polyimide Films for Inkjet-Printing of Flexible Electronic Devices http://dx.doi.org/10.5772/intechopen.76450 9

**Figure 6.** SEM images with low (a) and high (b) magnifications of an inkjet-printed (10 passes) and dried GO film on a surface modified Kapton HN film [2] (licensed under creative commons attribution 4.0 international license).

square had sharp edges as designed and the GO flakes were well interconnected with no observable cracks (**Figure 6a** and **b**).

#### **3.4. Bend cycling tests on an all-inkjet-printed flexible gas sensor**

GO ink was then used to inkjet-print a GO patch on the resulting Kapton substrate. The electrically non-conductive GO patch was converted into its conductive reduced graphene oxide (rGO) counterpart by firing at 300°C for 1 hour in nitrogen. A selector layer was inkjet-printed on the resulting rGO patch with a dimethylformamide- (DMF-) based ink containing 10 mg/ ml of 2-(2-hydroxy-1, 1, 1, 3, 3, 3-hexafluoropropyl)-1-naphthol. An ethanediol-based silver nanoparticle ink was then used to print two silver electrodes. To achieve an optimum contact between the rGO patch and the silver electrodes, both electrodes overlapped the rGO patch by 1.5 mm. Finally the resulting multi-layered sensor prototype was fired at 120°C for 3 hours to remove the organic molecules coated on the silver nanoparticles and to anneal the silver nanoparticles for desired conductivity. **Figure 5a** shows an optical image of a typical multi-layered gas sensor prototype fabricated with the procedures described above. A flexible single-layered sensor was similarly fabricated except that the selector layer was not printed. Both the multi- and the single-layered sensors were flexible, ultra-lightweight (˜25 mg), and

Gas sensing was then performed with both multi- and single-layered sensors. A dimethyl methylphosphonate (DMMP) vapor of 2.5 ppm was generated from a DMMP permeation tube (KIN-TEK Laboratories, Inc., La Marque, TX, USA) installed in a FlexStream™ Gas Standards Generator (KIN-TEK Laboratories, Inc.) and carried by nitrogen with a flow rate of 500 sccm. The relative sensitivity of a multi-layered (black solid line) and a single-layered (red-dashed line) sensor upon exposure to 2.5 ppm DMMP is shown in **Figure 5b**. The relative

R0

SEM analyses were performed to examine the micro-/nano-morphology of the inkjet-printed GO patterns. A GO patch inkjet-printed (ten passes) on a surface modified Kapton HN film was dried at 95°C under vacuum overnight (to remove the glycerol and the water in the GO ink) and then subjected to SEM analyses. Under a scanning electron microscope, the dried GO

**Figure 5.** (a) Optical image of a flexible, rGO-based, multi-layered gas sensor inkjet-printed on a surface modified Kapton HN film (the GO ink was printed for 10 passes, the selector ink for one pass, and the silver ink for 5 passes). (b) Relative sensitivity of the multi-layered (black solid line) and the single-layered (red dashed line) gas sensors upon exposure to 2.5 ppm dimethyl methylphosphonate [2] (licensed under creative commons attribution 4.0 international license).

is the resistance between the two silver electrodes of a sensor before the exposure to

miniature sized (~1.5 cm x 1.0 cm).

where R<sup>0</sup>

8 Flexible Electronics

sensitivity (S) is defined by the following formula:

<sup>S</sup> <sup>=</sup> <sup>R</sup> <sup>−</sup> <sup>R</sup> \_\_\_\_0

the DMMP vapor and R that at a particular time after the exposure.

A four-point bend tester (TestResources, Inc., Shakopee, MN, USA) controlled by R Controller software (TestResources, Inc.) was used to conduct the bend cycling tests. A fully inkjetprinted single-layered gas sensor was mounted on the bend tester with a home-made mounting system. The sensor was bent, with an amplitude of 20 mm and a bend rate of 1 mm/ second, to a radius of curvature of 1 cm 1000 times in tension and then another 1000 times in compression to the same radius of curvature. After the 2000 bending cycles, the resistance of the sensor was measured with a multimeter and its morphology examined with an optical microscope. It was found that the resistance of the sensor after the bend test was virtually the same as that before the test (i.e. ~14 kΩ) and no morphological changes were observed during the optical analyses. Keeping the amplitude and the bend rate the same, the radius of curvature was reduced to 0.5 cm and the sensor was bent 1000 times in tension and another 1000 times in compression, followed by conductivity and morphology examination. Again, apparent conductivity or morphological changes were not observed.

#### **3.5. Peel testes on an all-inkjet-printed flexible gas sensor**

Peel tests were performed via a qualitative Scotch-tape peel test to evaluate the adhesion of an inkjet-printed single-layered gas sensor to the surface modified Kapton film. The adhesive side of a piece of Scotch® magic tape (3 M Company, St. Paul, MN, USA) was firmly pressed against the sensor and then peeled off [36, 37]. By visual inspection and optical microscopic analyses, all three components of the sensor (two silver electrodes and one rGO patch) remained attached to the substrate and retained their intactness after the tape had been peeled off. optical
