APPLICATION OF DIGITAL TEXTILE PRINTING TECHNOLOGY TO INTEGRATE PHOTOVOLTAIC THIN FILM CELLS INTO WEARABLES

J. S. Hynek¹ , J. R. Campbell², and K. M. Bryden<sup>3

1</sup>Iowa State University, Virtual Reality Applications Center, 1620 Howe Hall, Ames, IA 50011-2274, jhynek@iastate.edu
²Iowa State University, Lebaron Hall, Ames, IA 50011-2274, jrcamp@iastate.edu
³Iowa State University, Virtual Reality Applications Center, 1620 Howe Hall, Ames, IA 50011-2274, kmbryden@iastate.edu

Consumer electronics are typically designed with a battery power source and are carried in light limited pockets or bags. The marriage of electronic devices and wearables provides the opportunity to utilize surfaces exposed to the sun to generate energy to power the electronic devices. Photovoltaic flexible thin film converts solar energy into electrical energy. This photovoltaic thin film has a similar thickness to paper and has material properties much like those of camera film. The photovoltaic flexible modules come in various sizes ranging from 2x4in to 8.5x11in sheets. They can be rolled into a three inch diameter without physical damage and continue to function if scratched or punctured. The durability and efficiency of these photovoltaic flexible thin film modules have improved to a point where they are a viable option for incorporation into wearables. Ultimately, these thin film photovoltaic cells can reduce the amount of battery storage engineered into electronic devices and eliminate maintenance related to replacing batteries. Devices such as the Burton Shield iPOD Jacket, Memswear Fall Sensing Shirt, and Shimadzu DataGlass 2/A are examples of smart garments where battery maintenance or battery weight could be reduced by using photovoltaic flexible thin film for charging.

The successful integration of photovoltaic thin film cells into wearables is tightly tied to the consumer concept of fashion. In the consumer market, smart clothing must remain visually attractive and complement or enhance the wearer's appearance; otherwise it will not be commercially successful. In order maximize energy collection, it is necessary to place the photovoltaic film in visibly prominent areas on the wearable. These solar cells are graphically strong and as a result need to be more visually integrated into the garment structure.

Figure 1. Plain jacket with thin film photovoltaic modules adhered to back and arms.	Figure 2. Jacket concept with digitally printed pattern to mask photovoltaic modules.

Digital textile printing enables the designer to incorporate unusual components into a design by printing fabric that matches the pattern of the component. Direct digital textile printing technologies typically employ the use of ink jet printing to allow the user to print designs directly from the computer to fabric. For this paper, “digital textile printing technologies” refers to a number of integrated software and hardware components that are typically used to create digitally printed fabrics. These include off-the-shelf software packages (such as Adobe Photoshop® and Illustrator®), industry specific design and patternmaking software (PAD® patternmaking system, Pointcarré®), wide-format ink jet printers for direct textile printing (Encad 1500TX, Colorspan Fabrijet), RIP software used for processing the image files into information that is useful for the printer, and other support hardware. The wide variety of wearables to which photovoltaic thin film cells can be applied requires wide variety of fabrics to make these garments which eliminates the possibility of utilizing a large order of fabric. Digital textile printing makes it possible to do small scale production on a variety of fabrics. (Ross, 2005)

Digital textile printing has been used previously by Campbell and Parsons (2001) to intersect two design problems to create multiple points of exploration in the development of a uniquely patterned jacket. When the garment design process is mixed with an engineering design process, there is a greater need for pattern matching to incorporate electronic components and circuitry into the wearable. Digital textile printing provides the capability to create unique patterns opens doors to a wide range of design activities and diverse disciplines, (Parsons 2004).

In this paper, the application of digital textile printing to minimize the visual impact of the photovoltaic thin film cells on a tie will be discussed.

METHOD

The power requirements of smart garments have a direct impact on the photovoltaic thin film module surface area needed. The voltage requirement of the circuit in the garment also dictates how the photovoltaic thin film module pattern will repeat. The tie was designed with the intent of charging a 3V lithium ion battery which is typically found in cell phones. Three Iowa Thin Film MPT3.6-75 solar panels were applied to the front area of tie. These panels are shunted to 3.6V and when connected in series produce approximately ~150mA in full sun. The flexible solar panels were coated with a laminate that protects the solar material. When energized, the solar panels act similar to batteries. For connecting the panel to a circuit, a connection bus made of electrically conductive tape is adhered to the plastic panel. This makes it possible to solder connections. A multi-filar wire, having insulation on each filament, was soldered to the solar panel buses to connect them in parallel to a female plug that was removed from a Nokia cell phone charger.

To integrate the solar panels into the design, an Iowa Thin Film MPT3.6-75 thin film module was digitally scanned into Adobe Photoshop and visually color matched. The solar panel image was then adjusted for color correctness and modified to become a perfect repeating design. Next, a cotton sateen fabric was inkjet printed with the solar panel imagery. Minor adjustments were made to create more contrast between dark and light areas to compensate for slight color changes that occur as the inks soak into the fabric structure during fabric printing. The ENCAD NovaJet 880 inkjet printer was loaded with fiber reactive dyes.

The tie was constructed from a commercially available tie pattern. A small elastic pocket was sewn onto the back of the tie to provide a holding place for the phone and the charging wire. There was adequate slack provided in the wire to compensate for any elasticity in the fabric.

RESULTS

The scale of the digitally printed image matched the solar panel making it easy to line up the designs and visually integrate the panel into the surface of the tie. The cotton sateen was intentionally chosen for its lustrous quality, as a means to simulate the reflective nature of the solar panels. The tie does not stretch during use so there is no distortion of the digitally printed pattern that would create an aberration. The solar panels were a close chromatic match to the inkjet printed material. The difference between the sheen of the printed material and the reflectivity of the solar panels was the most revealing trait. In the future, changing or modifying the laminate on the solar panel would help to match the reflective properties. Selection of fabric with more luster, or treatment of the fabric with a reflective coating could help eliminate differences in surface appearance between the fabric and the solar panel.

The solar panels were attached using ‘liquid stitch’ adhesive, which eliminated the concern of accounting for a fastener in the design of the digitally printed pattern. The dot of solder used to connect the circuitry to the solar panels was a close color match to the connection bus adhered to the solar panels and was not visually distracting.

Application of the solar panels to the tie made it difficult to tie the knot. In typical tie knots, a small loop is created through which the large end of the tie is passed. At this step in the process, the physical width of the solar panel forces the user to enlarge their tie loop to allow the solar panel to pass through. The solar panels are limited to bending around a three inch diameter and should not be creased. Usage of narrower solar panels was considered, but there would be sacrifices in power generation. Applying solar panels to clip-on ties would eliminate this problem.

The tie was tested by a user for performance in a typical business environment. The trial consisted of wear during five business days and storage in a closet. During the trials, there were no safety infringements with respect to electrical shock. A visual inspection of the insulated wire revealed no degradation of the insulation after the trials. The wires maintained their ability to conduct electricity without resistance.

Figure 3. Solar tie worn with solar jacket.	Figure 4. Close up of three solar panels integrated with digitally printed fabric. Red spots on silver electric bus are soldered wire connections that carry electricity to phone plug-in.
Figure 5. Back of tie shows pocket to hold cell phone and 3.6V plug-in to charge cell phone.	Figure 6. Phone in elastic pocket where phone is charged while tie is worn.

CONCLUSIONS

Smart garments present new challenges to the textiles and apparel industry. There is a balance between integrating technology so it adds value to a garment and maintaining visual appeal. Smart garment solutions appeal to niche consumers meaning small quantity manufacturing will be necessary. Digital textile printing can effectively meet these challenging demands by providing a visually appealing medium for integration of flexible photovoltaic thin film cells or other electronic components. The digital print allowed the solar panels to be placed on the visually prominent face of the tie and create an unbroken pattern that gave the tie a formal-wear appearance. The rigidity of the solar panels must be taken into account when designing accessories. This application of digital textile printing technology shows that it can play a significant role in the creation of visually appealing solar enabled wearables.

REFERENCES

Campbell, J.R., & Parsons, J., (2001). Intersecting Two Design Problems to Create Multiple Points of Exploration. Exploring the Visual Future: Art Design, Science and Technology. Selected Readings of the International Visual Literacy Association. 51-56.

Parsons, J. & Campbell, J.R. (2004) Digital Apparel Design Process: Placing a New Technology Into a Framework for the Creative Design Process . Clothing and Textiles Research Journal, Special Issue on Design. Volume 22, (1/2), 88 – 98.

Ross, Teri. Digital Printing Alchemy. Techexchange.com. Retrieved Jan. 13, 2005, from http://www.techexchange.com/thelibrary/alchemy.html

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