Flexible, Transparent Electronics for Biomedical Applications

Michael Klopfer, Chris Cordonier*, Koutoku Inoue*, G.-P. Li, Hideo Honma*, Mark Bachman** University of California, Irvine (Irvine, California, USA)

* Kanto Gakuin University (Yokohama, Japan) ** Corresponding Author: Mark Bachman (mbachman@uci.edu)

Abstract The development and integration of flexible

biocompatible electronics is of considerable interest in the biomedical community. Electronic and fluidic based monitoring and therapeutic platforms can be contoured into comfortable, low profile devices suitable for implanting in the body or for wearing on the body or in clothing. Truly integrated bioflexible devices would incorporate electronics, optics, photonics, wireless, fluidics, mechanical components, and power systems on a single flexible biocompatible substrate. Continued development of applications of this technology requires further development of biocompatible, flexible films with integrated electronics which can be mass produced at low cost.

In this work, we demonstrate the fabrication of flexible printed circuits on Cyclo Olefin Polymer (COP) thermoplastic as a substrate material. COP is an attractive polymer for integrated bioflexible devices due to demonstrated biocompatibility and excellent material properties, such as high transparency over a wide band of wavelengths, low water absorption, and good mechanical properties.

On significant challenge to building bioflxible devices on polymer films (such as COP) is the need for metalizing and patterning traces on these materials at low cost, and in a scalable manner. In this paper, we report work that utilizes a new technique for patterning metals on polymers that can result in low cost manufacturing of electronic circuits on biocompatible films such as COP. The process uses high intensity UV light that is directed through a quartz mask to selectively irradiate a film of COP. After processing, the material can be treated and subsequently metalized using electroless plating techniques. The great benefit of this approach is that no photoresists steps are needed--no coating, exposing, developing, etching, or stripping is required for the creation of the final device. Furthermore, the process can pattern traces at high resolution (<2 microns) and can coat the insides of through hole vias, allowing multi-layer electronics to be produced. This greatly simplifies the manufacture of the circuits and reduces production cost considerably, when compared to conventional processes such as sputtering and etch.

We demonstrate the production of electronic circuits on COP for the purpose of making bioelectronic devices and characterize some of the main properties of the device. We discuss the advantages of this approach and identify some of the manufacturing pitfalls.

Introduction There is great interest in the development of thin, flexible

integrated electronic devices that can also support non-electrical technologies, such as fluidics, optics, and sensing.

Such devices hold promise for truly portable, implantable, or wearable biomedical systems that can collect fluids from the body and perform analysis on them, or provide therapeutic relief[1-7]. The so-called “smart bandage” could be placed on a wound to provide care and then disposed of afterwards. Or such devices may find utility as patches for drug delivery or physiological monitoring. Alternatively, the devices may be placed into clothing, such as an insert into footwear or undergarments. Such devices will need to be flexible and biocompatible, and readily manufactured at low cost.

Current methods for producing such devices (e.g., PDMS casting, flex circuit technology) either do not lend themselves to low cost integration[8-10], or are not particularly well suited to biomedical applications[11, 12]. Some work has progressed in the development of circuit boards using conductive inks. These must be silk-screened (or stenciled) on to the surfaces and cured[13]. Lithographic techniques for building devices are desirable owing to he ability to print very fine features, do very high alignment, and produce circuits in batch.

Figure 1: Direct selective metallization of clear COP plastic can be used to produce traces, pads and through hole vias for flexible, transparent printed circuit boards.

We have explored the use of COP as a potential substrate

material for making integrated biomedical circuits. COP is an ideal candidate for such applications owing to its favorable properties of low moisture absorption, low dielectric constant, high transparency, low autofluorescence, low cytotoxicity, and easy processability. COP is already used for medical grade products and as a packaging material for food and medical products. It finds application as high moisture barrier flexible packaging for medical and pharmaceutical applications including IV bags, TPN bags, blister packaging, and overwrap bags. COP is typically produced as an

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extremely pure material with low out-gassing and residual metals (less than 0.02 ppm). COP exhibits low adsorption of drugs as compared to polypropylene, and has significantly less protein adsorption than polypropylene, making it optimal for biomedical applications that must handle protein rich fluids, or perform protein analysis on fluids. In addition, COP exhibits high transparency (92% in visible range 400-800 nm), good moldability (for embossing microfluidics), low fluorescence (less than polycarbonate or polystyrene), low birefringence (one third of polycarbonate), low water absorption (less than 0.01% per 24 hours), high heat resistance (100 °C to 160 °C), and good chemical resistance to acids, bases, alcohols and low adsorbtion of biofluids. Electrical properties are similar to PTFE (Table 1)[14].

In order to produce circuits, conductive layers must be

created and patterned on the substrate. COP can be metalized by conventional physical vapor deposition (e.g., sputtering), but in order to realize large volume production of bioflexible circuits at low cost, low cost metallization techniques should be developed, as well as low cost, high resolution patterning of the metals traces. Conventional patterning of metal films utilizes a subtractive process. This process uses a photoresist layer (usually laminated on the surface) which is lithographically patterned. This requires that the photoresist layer be carefully deposited on the surface, and the resulting film laminate exposed to light through a mask. Following this, the resist must be developed to yield the protective pattern. The metal is then etched in a corrosive solution, and the protective photoresist layer is stripped. This is a time consuming, wasteful process with multiple steps, many of them resulting in the generation of hazardous materials that must be managed.

In this paper, we demonstrate the ability to quickly and cheaply produce high resolution circuits made from thin metal films that does not require a photoresist and etch step. This process allows for both sides of a COP film to be metalized and patterned in a single process. Moreover, the same process allows for vias to be metalized, allowing for low cost, batch production of two layer circuits on COP.

Fabrication Methods

Films (0.5 mm thick) of COP (Zeonex™) were provided by ZEON Corporation, Tokyo, Japan for use in this research[14]. The films were cut to panel sizes and thru-holes were drilled at known via locations and at alignment points using a CNC controlled drill. These films were then cleaned

and dried in preparation for the metallization and patterning process.

Lithographic masks were prepared using quarts plates that were patterned with high resolution UV blocking material (AZ® 4620, AZ Electronic Material Services Ltd, Middlesex, UK). Masks were created as a negative image (dark field) of the traces we wished to pattern on the COP. The COP films were surface treated by exposing them to high intensity, short wave UV light through the masks. This surface modification results in a physical and chemical change to the surface of COP, allowing for good bonding of metals to the surface in subsequent electroless plating.

The quartz mask was placed on the COP films and aligned with the via holes using the drill hole alignment holes. The film was then exposed to short wave light source utilizing a high power, low pressure mercury lamp and oven provided by Koto Electric Co., Ltd, Tokyo, Japan. This lamp has the unique ability to produce intense UV light at low wavelengths, which is needed for the surface modification process (Figure 2).

After exposing to UV through a first mask, the film was

then turned over and a second mask was aligned with the via holes, and the exposure process repeated. This exposure process allowed both sides of the film to be exposed, and also

Characterization COP

PTFE Flouropolymer

Specific Gravity 1.01 2.14~2.20 Water adsorption (%) <0.01 <0.01

Dielectric constant [1.0 GHz] 2.3 2.1

Dielectric dissipation factor [1.0 GHz] 0.0003 0.0002

Table 1: Some properties of Cyclic Olefin Polymer.

Figure 2: Short wave UV light passed through a pattern mask is used to pattern the catalyzation process, and eventual plating pattern onto the substrate. Our process relied on a low pressure mercury light source (manufactured by Koto Electric Co., Ltd, Tokyo, Japan).

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allowed light exposure to occur in the via holes, effectively modifying the surfaces of the via holes.

Following exposure to UV light, the films were then cleaned and processed for electroless deposition. No further patterning work was performed and there was no need to develop or strip any materials. After cleaning and processing, the films were placed in an electroless plating bath which allowed metal films to form on the surface where the UV had been exposed, effectively patterning the metal on the polymer substrate. The entire process is shown in Figure 3 below. Our experiments used two masks and six thru via holes, as shown in Figure 4.

The process of metallization at select sites on the polymer

(and the resulting excellent adhesion) is believed to occur through two mechanisms. Deep UV (short wavelength) light exposed to the COP surface results in breakage of polymer chains and the formation of radials. This process is also assisted by the generation of ozone (O3) from ambient oxygen immediately above the surface of the COP. These allow for a number of chemical moieties to form on the surface when the electroless process is initiated. In addition the UV exposure is known to create a thin layer of nanopores in the surface which act as a physical anchors for catalyst particles and the resulting metal that is formed (Figure 5). Since the pores are very tiny (few tens of nanometers), the resulting surface of the film remains smooth and metals can be patterned at high resolution (Figure 6).

The surface modification areas of the COP surface that promote plate-out of metal in the area of sensitization using an electroless copper or nickel plating process. In general, increased plating time results in increased thickness for plated metal on the COP substrate, up to a maximum thickness of a few micrometers. If electrical contact is provided to the electroless coated metal, a second layer of metal may be electrolytically coated on top of the COP. Commonly nickel is deposited first followed by copper for electrical circuits. From our experience, a thickness of 0.5 to 1 micron for the full metal coat thickness allows uniform conductivity across the trace while allowing flexation without buckling or delamination of the metal layer from the COP substrate. The electroless process overview is shown in table 2.

If no mask is used, the entire surface of the COP may be

coated with metal, and this layer may be used as a seed layer for subsequent plating steps allowing wide assortment of metals to be coated on the polymer. The use of quartz masks allows the electroless layer to be patterned directly into traces without the need for resists and etching. However, this also results in traces that may not be readily accessible to make electrical contact for subsequent plating. This, the direct patterning process, which highly beneficial for producing low cost, patterned metal traces on COP, has limitations on the ability to grow more (and thicker) metals on the surface. Nevertheless, the current process is highly beneficial for producing quality traces on thin films.

Figure 4: Mask pattern for the front and back of the COP substrate. The front and the back are sensitized sequentially, followed by a metallization of both sides and vias that occurs simultaneously.

Figure 3: A summary of the workflow process outlined in this publication is shown. UV treatment followed by surface pretreatment, surface catalyzation, and subsequent metal electroless plating(s) result in conductive surface traces.

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Figure 5: Cross sectional electron microscopy image of nanopores generated in surface of polymer after UV irradiation. The image is taken after plating and the metal has plated inside the nanopores. This layer is 30-40 nanometers thick. The scale bar is 10 nm.

Figure 6: Fine lines may be patterned on COP due to the highly smooth surfaces of the COP after surface treatment. These traces were patterned using a subtractive process.

Electroless nickel plating bath

NaH2PO2・H2O (NH4)2SO4 Glycine Temperature 45 °C

This approach provides ability to produce a two-sided

circuit in a single manufacturing process without the need to

perform special via filling steps. To demonstrate this, we fabricated a working circuit using the UV patterning and metallization processes described above.

Electroless copper plating bath

EDTA・4 H2O 2.2’-Bipyridine Polyethylene glycol - 1000 Ph-CHO Temperature 60 °C

Process parameters pH Adjustment for bath (titration) -- NaOH or H2SO4 UV lamp to sample distance -- 30 mm UV Intensity (@253.7 nm) -- 80.36 mW/cm2 UV Exposure Time -- 5min Sample time in plating bath -- 30 min

Table 2: Formulations for the electroless copper and nickel baths for metallization on COP post UV sensitization.

The circuit, a simple NOR gate oscillator device, was

fabricated for demonstration purposes only (Figure 7). The board was designed specifically to demonstrate the

ability to produce traces on the COP that can connect to a second layer through metalized vias and produce a working circuit. The process proceeded as described above. First via holes were drilled, followed by cleaning of the COP film, followed by exposure to UV light through the masks. After the plating process was complete, the film emerged from the baths with patterned copper traces and conductive coated vias. Surfaces were prepared for bonding by gently wiping mounting pads and component leads with isopropanol.

Following this, electrical components were assembled on the circuits and bonded using conductive epoxy (GC Electronics, Rockford, Il. CAT# 19-2092). The components were carefully aligned and brought in contact with the epoxy coated circuit board pads. Curing of epoxy to hardness for handling was achieved in about 1 hour in room temperature. Rapid curing to handing hardness can be accomplished by placing the uncured epoxy in a 70 degree C oven for 10 minutes. Full strength hardness was achieved in about 24 hours at room temperature. Faster processes for surface mounting components using conductive epoxy are readily available.

Characterization and Demonstration

The circuit described in (Figure 7) was realized using the mask shown in Figure 4. A plating time of 30 minutes in a Ni plating bath followed by 30 min in a copper plating bath resulted in ~1 micrometer total thickness traces of copper on nickel on COP. The formulations for the plating baths are detailed in Table 2. The final unpopulated board was washed with isopropanol followed by DI water, then placed in a 60 °C oven for 1 hour to dry residual water. Components were manually placed and bonded using conductive epoxy are previously described. Curing was accomplished using the

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accelerated method (at elevated temperatures) described above. The resulting circuit is shown in Figure 8a.

We used a NOR gate dual oscillator as a demonstration

circuit as a proof of concept for the ability of this plating process to be used as a PCB replacement (Figure 7). An alternating flashing between a pair of LEDs is used to indicate operation of the oscillator. All circuit operation was accomplished on the board, and the only input interface was a constant 5 volt DC power supply. Operation of the circuit was maintained even with vigorous flexing of the COP substrate. This indicated that the traces maintained conductivity before, during, and after the flexation of the board. Continued flextation eventually lead to failure. The quality of the board traces and vias were inspected via electron and optical microscopy methods. Destructive and non destructive inspection were used in characterization. In destructive inspection, the COP was carefully cut using a scalpel and viewed under light and electron microscopy. At ~20x, clear inner plating of the vias can be seen. Under a scanning electron microscope, the continuity of the copper can be confirmed (Figure 9a and 9b). When viewed from the top of the board, bottom traces appear black due to the nickel that is first applied to the PCB before the copper is applied. All traces showed good conductivity—the largest resistance was 3 ohms. Traces were approximately 200 microns wide. Qualitative assessment of the circuit durability was performed by manually flexing the device under operation. The flashing LEDs were observed while flexing the board by hand to a bend angle of 45 degrees. Testing continued to board failure. We flexed the board along the short and long side of the COP. After 27 flexations, the through-hole vias began to fail at the sharp edge of the through-hole. Reinforcement of the electrical conductivity for the through vias was accomplished by applying a thin layer of conductive epoxy to the top and bottom of the via holes. High angle flexing (bending approaching 180 degrees) create large forces between the traces (which conformed to the bending of the substrate) and the epoxy interface that links the components to the surface of the board, resulting in epoxy delamination. The separation generally occurred within the epoxy itself or right at the interface with little damage to the underlying trace itself. Repairs to these type of failures could be accomplished by stripping of residual epoxy and replacement of components.

Typically the epoxy separated from the traces and the traces themselves were not destroyed. Slight bending and unbending of the board during the early curing process seemed to help reduce failures from this mechanism. The quality and quantity of epoxy on the bond pads seemed to correlate with improved durability. Notably, ductility in the boding epoxy between the component and the trace seemed to improve the chances of the components remaining in good electrical contact during flexing.

Discussion This investigation demonstrated that two layer circuits,

with conductive thru vias can be fabricated in a biocompatible polymer film, Cyclo Olefin Polymer. Selective metallization of COP substrate has been demonstrated indicating a possible low cost, highly scalable manufacturing process for circuits that is compatible with common manufacturing infrastructure, such as panel-based lamination and roll-to-roll processes. COP is a solid candidate for conformal, bioflexible, and optically transparent PCBs which can be fabricated at low cost. These results show a promising direction for producing

Figures 8a and 8b: A populated PCB is shown (Figure 4a) using surface mount components and 2 sided construction. A close-up of a mounting pad set and a through hole via (Figure 8b) shows uniform plating across the traces.

Figure 7: Oscillator circuit used for the demonstration of the proposed metallization process to produce PCBs.

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such devices on COP, however, significantly more work is needed to quantitatively investigate the quality and reliability of the devices.

New research should be performed to produce useful and relevant bioelectronics devices in COP, to investigate the practical, large scale manufacture of integrated biodevices, and to produce quantitative assessment of the performance and reliability of such devices.

The use of microelectronics manufacturing to produce integrated bioflexible devices is an exciting new direction for the electronics manufacturing industry. Miniaturized integrated bioflexible devices for human use represents a potentially large emerging market, with applications in health monitoring, portable diagnostics, and therapeutics. However, current materials and processes are not well suited for biomedical applications or are too expensive to produce large scale disposable devices at low cost. New manufacturing methods and materials, such as those described in this investigation, will be needed for the microelectronics industry to adequately service this new market.

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Figures 9a and 9b: Inspection of the vias was accomplished through electrical conductivity initially followed by imaging using electron microscopy (Figure 9a) and optical microscopy (Figure 9b). Visual inspection of the vias with electron microscopy allowed visualization of the surface structure to confirm a true conformal coating of metallization was indeed what was producing electrical conductivity through the via.

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