Medical Device Link.

Originally Published MEM Fall 2002

Lighting Technology

New polymer chemistries have revived an old lighting technology, making it a viable option for the stringent requirements of medical electronics.

D. G. Sime

The all-polymer constructions now available provide high-reliability, low-noise displays in a wide variety of styles. Displays are now available in multicolor designs and in a variety of thicknesses, including very thin constructions. Together with simple uses in display backlighting applications, it is now also possible to integrate membrane (and other) switch technology with EL. Integrating these technologies avoids the difficulties, inefficiencies, and mechanical disadvantages associated with the use of light-emitting diodes (LEDs) or other illumination schemes for control systems. In addition, EL components are made with very-high-speed, large-volume, highly automated production facilities.

This article looks at the properties of EL technology and discusses its viability for medical electronics applications. New chemistries and construction techniques have made it a more attractive option. The properties of EL match the requirements for illumination of a variety of medical devices. EL provides an efficient, uniform light source that is effective at modest brightness levels.

History and Background

Although the basic phenomenon of thick-film EL has been known for some time, the materials and requirements for a practical lamp have until recently remained expensive, difficult to work with, and challenging to package into a useful and practical format.

A primary advantage of EL is that light can be generated over a wide area rather than at a point source or along a line source, as in other forms of illumination. Because it can be generated over a wide area, a mechanical (and thermal) structure to house and otherwise accommodate the light source is no longer necessary. The applications for such an extended source of light were immediate, and EL technology has proven to be the best option for many of today's compact portable devices.

Benefits. The fundamental advantages of EL remain those demonstrated in the earliest applications. It provides a uniform illumination over an extended field, and a cool light with no heat to dissipate. EL can be manufactured in a thin structure, and it offers a highly efficient use of power. Each property is unique to EL, and each has driven the development of low-cost, practical ways to realize the benefits.

Challenges. There are challenges, however, to achieve each benefit. Early on, such challenges were difficult to meet, leading to a reticence in many industries to use EL. A problem that plagued early use of EL was its poor structural integrity. The thin structure was achieved by roll- or blade-coating a slurry onto the substrate; the available chemistries were principally cellulose-based gel systems that kept the phosphor particle in suspension. The result was a soft lamp that could not be worked into practically useful shapes.

Another major challenge has been addressing the vulnerability of EL phosphors to moisture damage. At one time, their low resistance to environmental stress limited their useful service life to a few tens of hours. The chemistries used in early lamp development provided little protection against penetration by moisture. As a result, long-lasting lamps had to be made either on solid, impermeable substrates or, eventually, made by using hard-to-handle fluoropolymer packaging materials such as Aclar. Both approaches complicated the manufacturing and handling of EL and added significantly to its expense.

The development of the modern EL display has been the story of the response to the challenges to produce a compact, lightweight, thin, reliable, and inexpensive lamp that displayed EL's advantages, but in a reliable, affordable, and versatile form. Although there have been many attempts to express the various benefits, the best and most lasting results have come from material and technique advances that have addressed several key concerns. As a result, the current EL lamp industry is based on several new approaches.

Polymer Chemistry

The introduction of practical polymer chemistry approaches to the structure of the chemical matrix that holds the phosphor has revolutionized the building of EL lamps. With the use of this chemistry, EL achieved a major improvement by providing a much higher level of structural integrity. This higher level of structural integrity is achieved in several ways. The chemical binders dry completely and polymerize, forming a solid structural element. The lamp no longer exists only as a series of soft layers stacked between two sheets. The resulting lamp structure is of a solid polymer nature in which separately formed layers are cross-linked to provide an effectively monolithic structure. With the right chemistry, this creates a mechanically flexible and very thin result, and it can demonstrate remarkable adhesion to the substrate on which the layers are deposited. This is true even if that substrate has very low surface energy (it is difficult to adhere to), as in the case of indium tin oxide (ITO).

Such a polymer structure provides remarkable mechanical strength, even on a flexible substrate. A modern lamp should typically be able to be worked over a mandrel of 0.25-in. radius without damage. But more importantly, and as with many of these new approaches to EL construction, such properties open the way to new applications. EL can now add flexibility to its list of essential features and benefits.

The use of polymer chemistry opens a host of benefits and a wide range of new applications, not the least of which is the practical application of multiple-color lamps. Application of areas of different-colored phosphors on a single substrate opens up a wide variety of uses. Although the more graphically intensive products such as point-of-purchase displays may be obvious, the color coding of functional information, instrument status, and control information has become very popular and very easy to achieve in a backlighting package made with EL.

With these polymer chemistries, the chemistry itself can introduce a degree of protection to the embedded phosphor against moisture penetration. Together with the developments in the phosphors, the modern chemistries allow significantly enhanced service lifetimes to be achieved for inexpensive EL panels. The move toward building EL lamps with modern polymer chemistry has brought additional environmental resistance.

Phosphor Protection. A major improvement in environmental resistance arose with development of protective coatings for the phosphor particles themselves. The coatings provide protection from the special rigors of heat and humidity. In addition, they also improve the general working life for EL in more-normal room-temperature environments. The drive came from the appeal of this technology to the automobile industry. The goal was to have the functional lifetime for EL approach that of an individual incandescent bulb, namely about 1000 hours. This is now easily achieved with quality lamps operating at the standard reference operating points of 110 V ac and 400 Hz.

The obvious, although not easy, answer to protecting phosphor particles from moisture is to cover the particles themselves with an impermeable material. This approach was taken initially by Sylvania Corp. and led to the early availability of so-called treated phosphor, which essentially has a glass encapsulant entirely covering each crystal. The intrusion of coating, of course, brings a (brightness) penalty in the optical and electrical properties of the phosphor and an increase in cost, but it greatly extends the life of the products. Essentially, for all but the most rigorous environments, the need for Aclar and similar products has been removed.

Substrates. The widespread availability of substrates on which transparent conductive materials can be deposited is a parallel development of significance to the production of EL displays with high structural integrity. ITO can now be routinely sputtered in thin films onto a variety of substrates, including optically clear substrates.

Although sputter-coated glass has been available for a long time, it is possible now to generate mass-production quantities of ITO on flexible substrates sputter-coated onto flexible films. Although there are few restrictions intrinsic on the film, it is generally polyethylene terephthalate (PET) film that is coated because it can be provided with the precision, clarity, stability, and low cost necessary for the EL market. Sputter-coated polyester is therefore the most widespread substrate used for printed EL lamps. Of significant interest is its growing availability in varying thicknesses. Most lamps to date have been made on 7 mil (0.19 mm) thick PET, but the trend is toward thinner lamps in response to the growing demand for compactness in the displays for consumer devices.

Results

The outcome of the past several years of development has been an EL product that maintains and improves on the basic properties of EL and improves the practical manifestations in which it can be produced in the mass quantities needed for the consumer market. Table I shows a summary of typical modern EL display specifications. A modern lamp should feature the following:

• A rugged, mechanically flexible, and reliable structure that does not delaminate.
• Long service life (to be measured in thousands of hours at 110 V ac, at 400 Hz.
• High brightness.
• No thermal dissipation.
• Absence of mechanical noise.
• High resistance to the effects of adverse environments (high temperature, high humidity).
• Stable electrical properties.
• Multiple colors.
• Arbitrary shapes.
• Thin construction.

Electrical Characteristics. Much of the discussion in this article addresses the mechanical and optical properties of modern EL lamps and their suitability for use in handheld devices. The electrical properties are also important to consider. Although they have not perhaps been a driving force in the development of a modern lamp, they have in fact benefited from the development of the new materials.

The electrical characteristics of an EL lamp are those of a lossy capacitor. The effect of modern polymer chemistries has been to increase the effective dielectric strength and dielectric constant in the lamp. The result shows both in the resistance to shorts as well as in the generally higher efficiency of the lamp. With the resulting higher impedance, smaller currents are needed, and as a result, smaller driving circuits are required. This tendency toward better efficiency (at least in terms of light output per unit power) is enhanced by the use of better and more optically clear substrates.

A basic property of thick-film EL is the need to supply ac power. This, in turn, leads to the need, in most handheld or portable devices, for an inverter circuit to change the dc power supplied from the battery pack to the ac required by the lamp. Historically, these circuits were bulky, heavy, and electrically noisy. Indeed, their use often defeated the advantages of the light, compact, and efficient light source. However, the ability to print (and thus the ability to make the lamps as small as possible), the use of polymer chemistry, and the availability of high-voltage silicon switches in integrated circuit (IC) form have all led to improved efficiencies. In addition, the improved intrinsic efficiency of lamps and phosphors has allowed a new generation of inexpensive and compact IC-based, relatively noise-free EL lamp drivers to be developed.

A modern lamp typically shows resistance to breakdown at up to 1200 V and a stable and high dc effective resistance. The load is well modeled with a capacitance of 2.5–3.0 nF/sq in. (0.39–0.46 nF/cm2), with a resulting power dissipation of around 15 mW/sq in. at 110 V ac, 400 Hz. This represents a remarkable efficiency for the range of 15–20 fL brightness (luminance) and indicates that typical lamps themselves require only a few hundreds of microamps in operation.

The result of this efficiency is that the power budget for the application (an important aspect of the design) is likely to be dominated not by the current draw of the lamp, but by the efficiency of the inverter. With typical modern IC-based circuits, this is likely to be better than 50%, so a significant lamp area can be lit easily and brightly with a current draw at 3 V of less than 20 mA. This competes favorably with other approaches, and especially with LEDs, for which the typical current draw is more than this for displays of greater than about 1.5 sq in.

Resulting New Applications

The use of EL in backlighting liquid-crystal displays (LCDs) and other display types is a familiar one, and one that benefits from the intrinsic properties of EL: thin construction, absence of heat, uniformity of illumination, and in the case of, for example, handheld or body-borne monitors, low power use. A handheld monitor is an obvious application that benefits from the design flexibility and high reliability now available with the polymer chemistry in wide use.

Figure 1. Conventional electroluminescent panels for use in handheld remote control devices showing multiple phosphor colors. Also shown (second from bottom) is a panel that has no perforations, but has the top layer of the membrane switch printed on the underside of the lamp.

More generally, the existence of an efficient, thin and flexible, selectively deposited light source allows the introduction of light into applications that do not lend themselves to the more conventional forms of lighting. Handheld remote control devices are another strong application field for EL (see Figure 1). Here, the intrinsically thin structure allows light to be introduced without affecting the design or structure of the case. EL typically replaces 20 or more LEDs in such applications, and thus it offers a significant reduction in power requirements. The simplicity of assembly for EL, particularly with some of the more innovative new approaches, brings additional savings to the assembly and testing of the devices.

Both the reliability and the mechanical flexibility now available in EL enable the use of EL in membrane switches. A machine interface can be made that can be lit yet remain thin, compact, and sealed against the environment. The thin EL lamp (0.19 mm or less) can actually be made as one of the layers of the membrane switch construction. The illumination of membrane (and other) switches is therefore possible in a way not accessible before. Specifically, the EL can be made as a layer of illuminating material built right into the construction of the switch itself. EL significantly increases the functional value and convenience of man-machine interfaces.

A hybrid form of switch construction is now available by extending the use of EL in switch devices such as remote controls that use elastomer rubber keypads or similar pads. Beyond the mere insertion of an illuminating device into switch mechanisms, it is possible with EL (if properly made with the appropriate polymer chemistry) to integrate the switch shunt layer with the EL itself. This extends and enhances the functional and economic value of EL as a design element compared with other approaches such as LEDs.

The integration of switch and illumination functions can be extended to other applications, in particular to the development of self-contained, stand-alone switch packages for handheld devices that are no longer required to be mounted on printed circuit boards. Together with the economies offered by the use of EL and membrane switch techniques, the availability of packages integrating these other functions offers tremendous potential. In addition, use of the low-cost processes of printing also extends the potential for effective and user-friendly interface and display components in the next generation of handheld or vehicle-mounted devices (see Figure 2).

Medical Device Applications

The properties of EL are well suited to the illumination requirements for medical devices over a broad range of functions. Whether on a hospital ward, in transit, or in the home sickroom, an efficient, uniform light source that is effective at modest brightness levels has many advantages. The absence of hot spots in the illuminated field of an EL display means that less ophthalmic accommodation is needed and information is more readily viewed. The efficiency means that displays can remain lit for longer periods with less power drain. The design versatility provides a host of uses including illuminated control knobs, backlit graduated scales, and backlit sample holders.

Figure 2. A stand-alone package with a completely self-contained membrane switch and electroluminescent lamp for display and keypad, shown from both sides (DEC phone application).

With the modern thin, flexible EL, the same properties are now available to switch and control panels, which means that uniform illumination is possible for interface panels without affecting the flatness or compactness of the design. Indeed, designing with membrane switches that include the integration of illuminated elements, whether areas of EL material or surface-mounted LEDs, allows an inexpensive fabrication approach to be used in the development of cost-effective, user-friendly control panels.

Uniform, efficient illumination can be patterned and applied without the use of bulky, heavy, and expensive reflectors, diffusers, or light pipes. EL adds functionality to many displays and controls without the penalties that stem from conventional approaches. With selective deposition, lighting goes only to the required areas, such as the indicia on a dial, the set point on a scale, or the user instruction to be followed at any given time.

The efficiency of EL as a light source, together with the freedom from special mechanical requirements, means that the integration of EL has little impact on the packaging or weight of the final assembly. The intrinsic low mass of the illuminating elements and the low power requirements allow a designer to include the full functionality of an illuminated, user-friendly device, while maintaining compactness and lightweight designs required for handheld or portable use.

Finally, a point that must not be neglected in these applications is that modern all-polymer EL is an environmentally friendly material. Designers can avoid the use of hazardous materials typically found in conventional display lighting systems.

Conclusion

Thick-film EL lighting technology has greatly improved in the past decade, so that it is now available as a low-cost, highly reliable, and efficient source of illumination for portable and handheld control and monitoring devices. The use of polymer chemistry with printing techniques for manufacture also means that the technology provides a low-cost, high-reliability manufacturing process and expands the innovation that can be applied to both the graphic and visual versatility of the displays. This versatility enables the use of multisegmented and multicolor displays, variable sizes and shapes, and the integration of flexible lamp technology with switch packages that include membrane switch systems.

Such properties have made EL highly appealing for premium and mass-produced personal communications systems, especially because manufacturers are also no longer constrained by conventional approaches to the construction of EL lamps. The properties and manufacturing techniques are well-suited to the requirements of medical electronics.

D. G. Sime, PhD, is vice president of research and development for MetroMark Inc. (Minnetonka, MN). He can be reached at d.sime@metromark.com.

Copyright © 2002 Medical Electronics Manufacturing