Published by the IEEE CS n 1536-1268/10/$26.00 © 2010 IEEE PERVASIVE computing 59

SpotlightEditor: Brian Brannon n bbrannon@computer.org

B ridging the gap between the physical world and electronic

media is the main objective of perva-sive computing research. Application fields include industries ranging from automotive engineering to medical technology to consumer electronics. In the automotive industry, each part must be marked to fulfill legal require-ments. In medical technology, there’s a strong demand for labeling medical disposables.

There are many motivations for labeling items to link the real and the virtual world, including

• supply chain management (decreas-ing logistics costs);

• fabrication control process efficiency improvement (coordinating the flow of information and materials);

• complete product traceability (iden-tifying manufacturers in case of product recalls, often required by law);

• product liability (identifying coun-terfeit products);

• quality guarantees (quality stan-dards prescribe marking products); and

• recycling products correctly (indi-cating materials and substances).

Manufacturers use auto-ID (auto-

matic identification) technologies—the most common of which are bar codes and RFID—to label various products. Labeling technologies for industrial applications must meet sev-eral demands. First, they should inte-grate easily with the existing produc-tion process and the network. If legal requirements mandate labeling and it doesn’t add value, the process should

be as simple and cost efficient as pos-sible. This also applies to maintenance costs. Labeling shouldn’t deceler-ate production—that is, a minimum throughput according to the particu-lar process speed is required. Another important factor is reliable readability in the expected ambient conditions. Depending on the production pro-cess’s spatial conditions, the reading distance is also important. Labels and readers must withstand climatic influ-ences as well as mechanical and chemi-cal impacts during fabrication and use. In some cases, the code must still be

recognizable after surface modifica-tions or painting. Finally, the label-ing should be forgery proof. Labels’ required storage capacity strongly depends on the application and ranges from a few bits to kilobytes. Because modern production processes are mostly computer based, it’s often suf-ficient to attach a unique ID to the part and store the appropriate data in the network.

Labeling the world means labeling large numbers of objects. Typical lot sizes range from hundreds to millions of parts per year, requiring highly effi-cient mass production processes for tagging technologies and readers.

PRINTING PROCESSES AND PRINTED ELECTRONICSPrinted electronics technology has the potential to meet these demands. This technology experienced a devel-opment stimulus after Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shi-rakawa received the 2000 Nobel Prize in chemistry for the discovery and development of conductive polymers. Because these materials are soluble in various solvents, they can be pro-cessed from the liquid state as a solu-tion or dispersion. Hence, scientists can use these functional polymers to print electronic structures and simple

Labeling the World: Tagging Mass Products with Printing ProcessesKarin Weigelt, Mike Hambsch, Gábor Karacs, Tino Zillger, and Arved C. Hübler

If legal requirements mandate labeling and it doesn’t add value, the

process should be as simple and cost efficient as possible.

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electronic devices and circuits. Addi-tive printing can decrease the number of process steps because it deposits material only where it’s needed, com-pared to standard electronic manu-facturing processes that mostly use subtractive processes. Furthermore, printing can occur in ambient condi-tions and doesn’t require clean room conditions or vacuum processes.

Researchers have proposed various methods such as ink-jet and screen printing, but these techniques have limited throughput for electronics manufacturing. The layering require-ments in printed electronics are more stringent than in graphic printing. Homogeneity, purity, layer thick-ness, and resolution have consider-able influence on printed electronics’ functionality.1

Scientists have used ink-jet printing on a laboratory scale to fabricate vari-ous prototypes—for example, organic light-emitting devices (OLEDs). The wide range of substrates that ink-jet printing can handle is one advan-tage. The main constraint is that the ink must have low viscosity—that is, it must be a thin fluid. This conflicts with the aim of high conductivity, which requires a minimum layer thick-ness (approximately 0.5 to 5 µm). Only printing multiple layers can achieve such high performance, which makes the process very slow. Screen print-ing can achieve high layer thicknesses (more than 10 µm)—for example, it’s used to print antennas for RFID tags—but it’s comparatively slow.

The three major mass printing technologies—offset, gravure, and flexography—use printing forms with a permanent image carrier, enabling multiple image reproduction and high productivity compared to ink-jet print-ing and other structuring technologies. They’re distinguished by the principle of ink separation on the printing form. Arved Hübler’s research team uses all three technologies to print multilayer electronic devices because each results in different layer characteristics.1,2

Further development should explore printing machine precision and mate-rial compatibility and processability.

The most promising approach to individualizing codes is laser-cutting the connecting lines. A laboratory reel-to-reel machine successfully cut printed conductive paths on various paper and plastic substrates,3 enabling individual code production. For some applications, such as marking products with batch numbers, many products are labeled with the same ID (individu-alization isn’t necessary).

PRINTED IDENTIFICATION LABELSOffset printing is the most widespread graphic arts printing technology and therefore the first choice for printing simple ID codes on paper. Up to now, few inks have been available for offset printing, and processing was difficult.

The recently developed carbon-based conductive ink Printacarb alpha is applicable for any type of dry offset printing machine. This patented ink’s drying mechanism and cleaning don’t differ from standard inks. Because it can be overprinted and lacquered with-out deficiency, the conductive layer can be optically hidden and protected against environmental exposure. The sheet resistance is about 30 kΩ/sq, which is too high for producing RFID antennas, but Printacarb perfectly suits printing capacitive ID codes on paper substrates.

The ID codes’ reading technology is based on capacitive near-field cou-pling, which requires no direct contact. The printed memory structure’s layout

comprises at least one basic electrode and several memory electrodes, which are either connected or not connected to the basic electrode according to the stored data. The reading electrodes and printed electrodes represent two capacitors connected in series with the paper as dielectric material. The standard delivery form is the credit-card-sized ID card (54 × 86 mm). To protect the printed codes, a second layer of paper seals them via lamina-tion. ID cards can be color-printed on both sides and individualized accord-ing to customer needs. The current generation of ID cards has a storage capacity of 96 bits. Further application fields for capacitive ID codes on paper include smart packaging and product identification.

INTEGRATING ID CODES INTO PLASTIC DEVICESPermanently integrating printed elec-tronic labels with plastic parts opens up some interesting applications. We advanced in-mold labeling (IML) technology to integrate capacitive ID codes into plastic components.4 Teemu Alajoki and his colleagues reported on the integration of further electronic components—for example, LEDs—using the same technology.5 IML is usually used for decorative pur-poses; the printed film is preformed, if needed, and cut into single labels. A handling device inserts the single labels into an injection molding tool, and the film bonds to the plastic part during injection. Consequently, the decoration—or, in our case, the con-ductive structure—is located under the plastic part’s surface, protected against outside influences. The film serves as dielectric for the capacitive coupling.

Because injection molding is a true mass production technology that’s widely used for housing production, it lets us label a wide range of plastic parts. Because of the capacitive near-field coupling, this technology isn’t suitable to label parts with curved surfaces.

Offset printing is the most widespread graphic

arts printing technology and therefore the first

choice for printing simple ID codes on paper.

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The packaging industry could ben-efit from this technology in single-use and returnable systems—for example, containers for transporting and storing food, crates, pallets, and bottles. The labels withstand aggressive cleaning cycles and mechanical stress.

AUTO-ID TECHNOLOGY COMPARISONTable 1 compares printed ID codes to other auto-ID technologies. The inter-face technology defines the handling, the impact of position and direction on the reading process, and the reading distance, the latter being much higher for bar codes and (especially active) RFID tags compared to the printed ID code, magnetic stripe, and traditional smart-card technologies. There’s no optimum reading distance for all applications—for instance, small read-ing distances are desired for security applications, whereas long reading dis-tances enable a certain flexibility for logistic applications. Reading speed is considerably higher for RFID because bulk label reading is possible. Com-pared to bar codes, magnetic stripes, and smart cards, dirt and surface

scratches don’t influence labels with printed ID codes or RFID technology.

Bar codes and current printed ID codes are read-only memories, whereas magnetic stripes, smart cards, and some RFID tags are writable. Printed IDs’ storage capacity and density are similar to standard bar codes.

Bar codes, printed ID codes, and magnetic stripes mainly use printing and laminating processes, whereas RFID tags and smart cards require many steps, including antenna etch-ing/printing, chip production and placement, laminating/converting, and printing. This directly impacts label costs. Reading devices for printed ID codes containing standard electronic components cost much less than optical bar code readers or RFID readers.

Printed ID codes and RFID tags could be directly integrated into objects—for example, by in-mold labeling—to prevent forgery. Bar codes’ copy protection is poor because forgers can duplicate them with a photo copier. Magnetic stripes are similarly easy to manipulate.

The materials used to fabricate

bar codes are the substrate material (paper or plastic foil), inks, and—sometimes—adhesives. In addition, printed ID codes consist of a nontoxic and recyclable conductive ink, whereas magnetic stripe fabrication uses iron oxide. These tags are easily recycla-ble. In contrast, RFID tags and smart cards contain silicon and metals such as aluminum, copper, silver, gold, and nickel. Thus, RFID transponders have negative influences on plastic recycling and aren’t environmentally friendly.

Finally, standard bar codes, mag-netic stripes, and smart cards have high market acceptance, whereas RFID and 2D bar codes are emerging technolo-gies. Printed ID codes have recently undergone successful initial field tests.

FUTURE TRENDS FOR PRINTED ELECTRONIC LABELSMore complex printed electronic labels must be realized to open up future markets.

At present, printed electronic labels mainly use read-only or WORM (write-once, read-many) memories, which confine the range of pos-sible applications. Printed organic

TABLE 1 Comparison of auto-ID technologies.

Criterion Printed ID code 1D/2D bar code RFID Magnetic stripe Smart card (chip)

Interface technology

Capacitive coupling

Optical reading

Inductive coupling, electromagnetic coupling

Magnetic coupling

Ohmic contact

Handling Slip in, put on Intervisibility Put on or bring in proximity

Slip in Slip in, put on

Reading distance

0–1 mm 0–0.5 m 0–3 m (passive), < 20 m (active)

0–0.5 mm 0 mm

Writability Projected No Partial Yes Yes

Storage capacity

96 bits 40 bits (EAN13)/ 100 bits–3 Kbytes (2D)

1 bit–64 Kbytes ≈ 1,000 bits 50–100 Kbytes

Storage density

≈ 4 bits/cm2 ≈ 6 bits/cm2 (1D)/25–100 bits/cm2 (2D)

< 1.3 Kbytes/cm2 22 bits/cm2 1–2 Kbytes/cm2

Copy protection Medium Low Medium Low Medium

Reader costs 3–10€ 10–100€ 100–2,000€ 10–30€ (high service costs)

5–50€

Tag costs < 0.02€ ≈ 0.01€ > 0.10€ (passive), >10.00€ (active)

0.16–0.18€ ≈ 0.45–1.00€

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field-effect transistors (OFETs) could serve as storage cells instead of resis-tors or capacitors to enable pro-grammable memories that would extend label applications. Charging the transistor’s dielectric layer—for example, by a corona or direct contact—could change the transistor’s

threshold voltage.6 In such a memory, every transistor represents one bit. Research has proven the basic func-tionality of this approach, but devel-opments on selective charging and integrating the charging process into the printing machine are still ongoing.

Another approach based on active

electronic devices is integrating both addressing and evaluation logic into the label. These logic circuits become necessary if the memory’s complexity increases, and its readout therefore becomes too time-consuming. The main challenge is the single compo-nents’ reproducible fabrication.

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TABLE 2 Status quo of printed RFID labels.

Component Device Status

Antenna Realized by flexographic or gravure printing7

Rectifier Diode Realized for frequencies up to 10 MHz8 (frequency defined for smart tags: f = 13.56 MHz)

Capacitor State of the art in printed electronics

Resistor

Transponder Clock (ring oscillator) In terms of speed and supply voltage (fosc = 4 Hz for Vdd = -80 V), still insufficient for the

employment in RFID labels2

Addressing and evaluation logic

Partly realized by using organic materials;9 uniformity for printed devices insufficient

Memory

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RFIDPrinted electronics technology could decrease the cost of RFID labels. Table 2 lists different components’ states to show the printed RFID labels’ status quo.

There’s still a lot of work to do in the field of printed electronic labels. The ID labels we’ve presented can real-ize applications that require low costs rather than high memory capacities and reading distances. The industry hasn’t reached its final goal of pro-ducing reliable low-cost RFID labels, but researchers have made significant progress in developing these labels’ individual components. The Organic Electronics Association predicts that the first printed RFID labels for brand protection (1–4 bits, ROM) will be commercially available in 2011, and printed electronic product code labels for item-level tagging (HF) in 2018. To label the world efficiently, we must con-sider objects’ and labels’ production as a whole. Printing is a tantalizing tech-nology for integrating the two.

REFERENCES

1. U. Fügmann et al., “Printed Electronics Is Leaving the Laboratory,” MST News, no. 2, 2006, pp. 13–16.

2. A.C. Hübler et al., “Ring Oscillator Fabricated Completely by Means of Mass Printing Technologies,” Organic Electronics, vol. 8, no. 5, 2007, pp. 480–486.

3. T. Petsch et al., “Laser Machining of Thin Films on Top of Flexible Substrate Carriers,” Proc. Smart Systems Integra-tion 2008, T. Gessner, ed., VDE, 2008, pp. 259–264.

4. K. Weigelt and A.C. Hübler, “Printed Near Field Communication System,” Beherrschbare Systeme dank Informatik, LNI 133, vol. 1, 2008, pp. 301–306.

5. T. Alajoki et al., “In-Mold Integration of Electronics into Mechanics and Reli-ability of Overmolded Electronic and Optoelectronic Components,” Proc. European Microelectronics and Packaging Conf. (EMPC 09), Int’l Microelectronics and Packaging Soc., 2009.

6. K. Reuter et al., “Full-Swing Organic

Inverters Using a Charged Perfluori-nated Electret Fabricated by Means of Mass-Printing Technologies,” Organic Electronics, vol. 11, no. 1, 2010, pp. 95–99.

7. M. Fairley, “Printing Antenna with Conductive Inks,” RFID Smart Labels: A “How to” Guide to Manufacturing and Performance for the Label Converter, M. Fairley, ed., Tarsus Publishing, 2005, pp. 29–36.

8. K.E. Lilja et al., “Gravure Printed Organic Rectifying Diodes Operating at High Frequencies,” Organic Electronics, vol. 10, no. 5, 2009, pp. 1011–1014.

9. K. Myny et al., “Plastic Circuits and Tags for 13.56 MHz Radio-Fre-quency Communication,” Solid-State Electronics, vol. 53, 2009, pp. 1220–1226.

Karin Weigelt i s a

research assistant and PhD

student at the Institute for

Print and Media Technol-

ogy at Chemnitz University

of Technology. Her research

interests include printed

electronics and their integration into plastic

devices. Weigelt has a Diplom in micromechan-

ics and mechatronics from Chemnitz University

of Technology. Contact her at karin.weigelt@

mb.tu-chemnitz.de.

Mike Hambsch is a

research assistant at the

Institute for Print and Media

Technology at Chemnitz

University of Technol-

ogy. His research interests

include printed organic

field-effect transistors and circuitry. Hambsch

has a Diplom in electrical engineering from

Chemnitz University of Technology. Contact

him at mike.hambsch@mb.tu-chemnitz.de.

Gábor Karacs i s a

cofounder of Crosslink. His

research interests include

application development

for printed electronics.

Karacs has an MSc in

mechanical engineering

from the Technical University of Budapest.

Contact him at gabor.karacs@clnk.de.

Tino Zillger is a cofounder

of Crosslink. His research

interests include printing

of electronic structures,

application development,

and production line devel-

opment. Zillger has a Dip-

lom in micromechanics and mechatronics from

Chemnitz University of Technology. Contact

him at tino.zillger@clnk.de.

Arved C. Hübler is a

professor of print media

technology and director

of the Institute for Print

and Media Technology

at Chemnitz University of

Technology. His research

interests include printed electronics and their

applications. Hübler has a PhD in engineering

from the University of the Arts in Berlin. Con-

tact him at pmhuebler@mb.tu-chemnitz.de.

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