**2. Characterization of Kapton HN films**

electrical properties as well as exceptional thermal and chemical stability. However, such films feature an inert and highly hydrophobic surface. Hydrophilic (e.g. water-based) fluids (solutions, suspensions, inks, *etc*.) will ball up on such surfaces (due to "lotus effect") [1, 2]. However, for fabrication of an entire electronic device, both organic solvent- and water-based fluids are usually needed to deposit functional materials on the same substrate surface. As a result, surface modifying polyimide substrates to reduce their inherent surface hydrophobicity and/or inertness is usually needed to allow for the continuous and uniform deposition

Traditionally, polyimide substrates are surface modified with a number of methods including plasma [3, 4] and ion-beam [5, 6] etching, UV/ozone exposure [3, 7], acid [3, 8] and/or base [9, 10] treatments, and laser ablation [11, 12]. These methods, however, usually compromise the structural integrity and the properties (such as the cohesive strength, and the thermal and chemical stability) of the polyimide substrate, since they utilize relatively harsh conditions to oxidize and/or tear out part of the surface polyimide. Additionally, the wastes and by-products (such as acrolein which is extremely irritating, strong bases and acids which are corrosive, and benzene which is carcinogenic) generated from these harsh treatments can raise serious environmental and safety issues (especially when the treatments are performed indoors and/ or in large scales). For example, incubation with a sodium hydroxide solution has been one of the most common traditional methods to tune the surface properties of Kapton polyimide substrates [9, 10, 13], but such a treatment not only generates highly corrosive strong base waste but also tears out some surface polyimide resulting in pits in the Kapton surface [14]. The defects on structurally damaged Kapton films would result in not only poor deposition quality of the device components but also weakened mechanic strength of the resulting devices. The increasingly growing of flexible electronic devices (such as flexible displays [15], electronic paper [16, 17], photovoltaic cells [18, 19], sensors [1, 2, 20–23], LEDs [24], electronic textiles [25], RF tags [26], and electrochemical devices [27], *etc*.) is calling for mild and environmentally friendly surface modification approaches which can minimize the compromise to the structural integrity and the properties of Kapton polyimide substrates while efficiently tuning the surface properties of the substrates. To take full advantage of the properties of Kapton HN films, any surface modification to such films should avoid as much as possible compromising their structural integrity and properties. For extremely thin Kapton HN films, such as Kapton 30HN (thickness 7.5 μm), 50HN (thickness 12.7 μm), and 75HN (thickness 19.1 μm), it is criti-

cal to make sure that their surface modification is non- or minimally destructive.

charges on the slip additive particles.

Kapton HN films have a slip additive incorporated in the polyimide matrix to enhance their mechanical properties [13]. The nature of the additive, however, has been very scarcely reported in the literature. Williams *et al.* has mentioned, but without providing supporting data, that the additive in Kapton HN films was calcium phosphate dibasic (CaHPO<sup>4</sup>

This chapter first describes the characterization of Kapton 500HN films particularly of their slip additive, then introduces two recently-developed mild and environmentally friendly wet chemical approaches for surface modification of Kapton HN films to allow for not only great printability of both water- and organic solvent-based inks but also strong adhesion between the inkjet-printed traces and the surface modified substrates. Unlike the aforementioned traditional Kapton surface modification methods which target, and oxidize and/or tear out part of, the surface polyimide matrix, the approaches described in this chapter target the electric

) [14].

with both organic solvent- and water-based fluids.

2 Flexible Electronics

In this section, a number of characterizations were performed on as-received Kapton 500HN films (a gift from Dupont, Wilmington, DE, USA) particularly on their slip additive. The optical microscopy analysis of the films showed particles of varying sizes which were imbedded in Kapton HN polyimide matrix (**Figure 1a**). These particles have been shown to be the slip additive to the polyimide matrix of Kapton HN [14, 28]. As shown in **Figure 1a**, the majority of the slip additive particles exuded to the substrate surface, which is consistent with a previous observation [29]. The large hump (with 2θ ranging from ˜10° to ˜35°) and the sharp narrow peaks in the X-ray diffraction (XRD) pattern of the Kapton HN films (**Figure 1b**) indicated the presence of amorphous and crystalline components, respectively. Apparently the amorphous moiety was the polyimide polymer and crystalline moiety the slip additive. While calcium phosphate dibasic (CaHPO<sup>4</sup> ) might be present in the additive as previously reported [14], the crystalline peaks in the XRD pattern of the Kapton HN films matched very well with those of calcium carbonate (CaCO<sup>3</sup> ) (ICDD reference code 04–001-7249) (**Figure 1b**) but did not match any of the CaHPO<sup>4</sup> peaks. Significant carbon and oxygen peaks and a small calcium peak showed up in the energy dispersive X-ray spectroscopy (EDX) pattern of the Kapton substrate (**Figure 1c**).

To better characterize the slip additive in Kapton HN films, efforts were made to minimize the interference from the polyimide polymer matrix. Kapton HN films were fired at 800°C for 2 hours in air (this firing treatment has been shown to be efficient to pyrolyze the entire polyimide polymer moiety in Kapton HN [30]) to remove the polyimide polymer. The remaining inorganic components were characterized with scanning electron microscopy (SEM), EDX, and XRD analyses.

**Figure 1.** Kapton HN film characterization. (a) Optical microscopy analysis of a blank Kapton HN film. (b) XRD analysis of the specimen shown in (a) (pattern ①) and reference CaCO<sup>3</sup> (pattern ②, ICDD reference code 04–001-7249) (inset: Locally enlarged XRD pattern to show the area with a 2θ of from 50° to 100°). (c) EDX analysis of the specimen shown in (a) (inset: Locally enlarged EDX pattern to show the calcium peak) [2] (licensed under creative commons attribution 4.0 international license).

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 the presence of only calcium pyrophosphate Ca<sup>2</sup> P2 O7 (ICDD Reference code 04–009-6231) (**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> and the presence of Ca<sup>2</sup> P2 O7 . The composition change was due to the multiple chemical reactions (such as the decomposition of CaCO<sup>3</sup> into CaO and CO<sup>2</sup> [31]) taken place during the pyrolyzing 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 Kapton HN was composed of CaCO<sup>3</sup> (crystalline) and one or more phosphorus-containing 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

and unknown to the public, XRD analyses showed that crystalline CaCO<sup>3</sup>

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

CaCO<sup>3</sup>

HN films.

chloride.

**modification approach**

positively charged additive particles (e.g. CaCO<sup>3</sup>

charged protamine and heparin molecules.

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

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

additive in a significant amount (**Figure 1b**). With an isoelectric point of 8.2 [32], crystalline

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

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

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.

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

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

 bears positive charges at a neutral or acidic pH. The two recently developed mild and environmentally friendly wet chemical approaches described below both target the surface electric charges borne by the additive particles imbedded in the polyimide matrix of Kapton

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

was present in the

5

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

particles) on the Kapton HN surface enabled

**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 patterns of both the ash (pattern ①) and reference calcium pyrophosphate Ca<sup>2</sup> P2 O7 (pattern ②. ICDD reference code 04–009-6231) [2] (licensed under creative commons attribution 4.0 international license).

and unknown to the public, XRD analyses showed that crystalline CaCO<sup>3</sup> was present in the additive in a significant amount (**Figure 1b**). With an isoelectric point of 8.2 [32], crystalline CaCO<sup>3</sup> bears positive charges at a neutral or acidic pH. The two recently developed mild and environmentally friendly wet chemical approaches described below both target the surface electric charges borne by the additive particles imbedded in the polyimide matrix of Kapton HN films.
