Miniaturized Electronics: Driving Medical Innovation

Originally Published MDDI March 2003

COVER STORY: DESIGN AND DEVELOPMENT

Electronic assemblies keep shrinking, and the medical device industry is finding myriad uses for them.

by Gary Pinkerton

Medical devices incorporating miniaturized electronics are provoking a radical rethinking of chronic-illness and disability treatment. Electronic appliances combining digital and analog circuitry, sophisticated power sources, high reliability, and extremely compact dimensions can—or soon will—perform a variety of medical tasks. These tasks range from the mundane timing of medication doses to the complex restoration of basic senses. These miniaturization processes, such as those described in Figure 1, are responsible for many medical advancements. Because so much of the body's operation is electrical in nature, nervous response, sensory input, and muscle control are all likely areas for treatment, enhancement, or replacement using miniaturized electronics.

One example of the progress and expectations associated with medical mechanisms and electronics is the development of an artificial heart. The first artificial heart, the Jarvik-7, was an implantable device; however, it required that the patient be hooked up to an external air compressor. Physicians implanted the Jarvik-7 for temporary use, until the patient received a human heart transplant. It was, for all practical purposes, completely nonportable.

In 2001, a new artificial heart design, the AbioCor, made its debut. The AbioCor is an electromechanical device powered by an internal battery. The battery is charged wirelessly via induction from an external power pack and provides 30 minutes of operation if the charger is removed. 

Weighing approximately 3 lb, the pump portion of the AbioCor is completely implantable. The external power pack is also small enough to be worn or carried, which affords patients reasonably normal mobility. 

The AbioCor heart possesses the ability to sense different activity levels and adjust pumping action automatically. Criteria for its ultimate success include the patient's being able to interact with family and return to a productive lifestyle. The first implantation of an AbioCor heart was performed in the summer of 2001.

This type of development illustrates the constant rethinking of what is practical and achievable versus what is merely possible. Typically, devices and treatments evolve from the sensational to the forgettable—meaning they become so routine, reliable, and maintenance-free, they can be used (after some period of acclimation) without much patient concern or attention.

Other Treatments Using Miniaturized Electronics

Many new treatments and health aids now under development take advantage of implantable, miniaturized electronics. The diversity of these technologies is reflected in the results of a simple search on the Internet for terms such as "artificial vision" and "implant." Each returns hundreds of matches that outline technologies and therapies available now, or in trial stages.

Artificial Vision. Several teams of researchers are working on methods to restore sight electronically. There are two general approaches; the best method for each patient will depend on the cause of blindness. The simplest treatment involves implanting of a small (~2 mm diam) semiconductor wafer in the back of the eye, directly over the nerve ganglia behind the retina. This wafer functions as an artificial retina; light hitting the wafer causes it to generate electrical stimuli that are applied to the optic nerve and restore some degree of vision. The wafer is powered by the light striking it, or by an external light or laser source worn on a set of eyeglass frames, trained in the direction of the wafer. The design details depend on the type of artificial retina being used. This treatment is most useful when the loss of sight is caused by such degenerative diseases as retinitis pigmentosa or macular degeneration.

For cases in which the eye has suffered severe structural or nerve damage, researchers are developing more-elaborate systems that use solid-state cameras and electronics to send image information directly to the brain's visual cortex. Although these systems are electronically and functionally complex, they can be miniaturized sufficiently so that one portion can be implanted and the remainder worn or carried easily in a pocket.

Neuromuscular Stimulation. Another concept that relies on miniaturized electronics is the bioelectronic neuromuscular implant. Such devices are developed and used to modify or restore muscular activity through the application of electrical impulses to strategic locations in affected muscles. These implants have broad implications in cardiac assistance, neural control, and improved, more-natural prosthesis operation.

An example of such a device is the bionic neuron (BION) system developed jointly by Queen's University (Kingston, ON, Canada), the Alfred E. Mann Foundation (Valencia, CA), and the Illinois Institute of Technology (Chicago), and licensed to Advanced Bionics Corp. (Sylmar, CA). A BION is a leadless, single-channel electrical stimulator (approximately 16 ¥ 2 mm in diam) that can be injected into muscle tissue using a simple clinical procedure. It is encapsulated in glass or ceramic. Internal components include electronics, a tiny antenna, and electrodes that contact surrounding muscle tissue.

The BION receives power and command signals from outside the body via inductive coupling, from an easily concealed coil unit worn over the implant. The control unit drives the coil, which induces power and signals the BION. The controller can store various muscle stimulation programs to simultaneously power and control up to 256 BIONs independently of each other. Multiple BIONs can be coordinated to trigger and control complex muscular activities in patients who have neural damage.

Hearing Aids. A hearing aid is another type of implant that takes advantage of advanced miniaturized electronics. The photograph on the previous page shows a small, completely in-ear hearing aid.

In cases for which traditional amplified hearing-aid technology is ineffective, the cochlear implant is gaining popularity as a prosthetic treatment. Cochlear implants operate by delivering electrical stimulation directly to auditory nerves in the cochlea (inner ear) in response to external sounds. A portion of the equipment is implanted behind the ear and wired to electrodes inserted into the cochlea. The other equipment includes a miniaturized microphone and audio processor that amplifies, filters, and digitizes sound waves. These enhanced sound waves are then transmitted to the implant and electrodes electromagnetically or through a wired connection.

Miniaturization Tools and Techniques

Implantable medical devices are made possible, in part, through the miniaturization of increasingly sophisticated electronic circuits. Many components are now miniaturized to the size of sand grains and require robotic equipment for their assembly.

The majority of circuit miniaturization is achieved through a combination of integrated circuits (ICs), surface-mount devices (SMDs), and specialized printed circuit board (PCB) design. Except for SMD assemblies, ICs used in miniaturized devices are sliced directly from semiconductor wafers and used unpackaged. (Surface-mount ICs employ leadless packages that are considerably smaller than conventional ICs of the same type.) Traditional plastic or ceramic packaging used for off-the-shelf ICs wastes space, and the finished PCB will most likely be encapsulated anyway. Depending on the miniaturization method, the IC chip might be fabricated on the wafer with additional metallization or pad redistribution layers to make the chip compatible with construction processes.

Miniaturization improves power efficiency and circuit operating speed by significantly reducing the resistive and reactive effects of traditional wiring and IC lead systems. These characteristics help extend battery life, which is extremely important in most portable medical electronic applications.

Surface-Mount Devices. Introduced around 1980, SMD technology is one of the oldest miniaturization methods. It is used to produce leadless components (mostly resistors, capacitors, and ICs) that can be attached directly to pads on a PC board with reflow soldering or adhesives. Although not at the cutting edge of miniaturization, SMDs do usually require robotic systems for placement and assembly. 

Although resistors and capacitors can be built onto semiconductor wafers, SMDs remain an important part of miniaturization. They provide larger component values than can be fabricated on silicon. SMDs themselves continue to be miniaturized; recently, “0201” (20 ¥ 10-mil) components began being phased in to replace “0402” (40 ¥ 20-mil) components. 

Chip-on-Board and Chip-on-Chip. The chip-on-board (COB) process achieves a higher degree of miniaturization than an SMD alone. For COB, unpackaged IC chips are mounted directly to a circuit board with contact pads facing away from the board. Fine wires are then bonded between the die pads and PCB pads to configure the desired circuit (see Figure 2). 

A further improvement in space savings can be achieved with the chip-on-chip (COC) process, in which a smaller IC die is glued, pads-up, to the top of the COB chip. As with COB, wires are bonded between chip and PCB pads (see Figure 2). SMDs can be included on the PC board for COB and COC as well. The completed assembly is usually encapsulated to provide structural rigidity and protection against the environment.

COB and COC can save up to 50% of the space required to make the same circuit with conventional components, but there is still some potential for improvement. For example, the bonded wire interconnects around the chips limit the chips' placement proximity. In addition, COC requires the top chip be smaller than the bottom chip, as shown in Figure 2. The flip-chip process addresses both of these limitations.

Flip Chip. The flip-chip process provides for more aggressive miniaturization than SMD, COB, or COC by eliminating the bonded wire leads between the PCB and chips altogether. IBM introduced the flip chip in the 1960s as the solder-based Controlled Collapse Chip Connection process (C4). The term flip chip now refers to several methods for attaching unpackaged ICs, face down, to circuit board substrates. In addition to solder, flip-chip bonding methods now include conductive and nonconductive adhesives (see Figure 3).

Solder and adhesive attachment systems all involve some preparation of IC pads, in the form of metallization or metal bumping (also seen in Figure 3). Both the stage at which this preparation is performed and the complexity of the process affect the suitability of a given flip-chip technique to small, medium, or large production runs. Chips intended for solder and conductive adhesives require preparation that can best be performed during wafer fabrication. A flip chip using nonconductive adhesive requires comparatively simpler metal bumping of IC contact pads. This technique can be performed on either whole wafers or individual ICs. Thus, solder and conductive adhesive techniques are better suited to larger production runs, while nonconductive adhesive techniques work well for prototyping and small-to-medium production runs.

The miniaturization potential of flip chip is due, in part, to the fact that contact pads can be distributed over the surface of the die, rather than just around the edge. For this reason, flip chip is very efficient in accommodating the largest possible number of input/output contacts per unit of chip area.

Flip-chip technology is compatible with dies ranging from less than 1 mm2 to more than 2 cm2. No peripheral area is required on the PCB for pads and interconnect wires, so very close (<0.5-mm) placement of chips is possible. High interconnect densities (>800 connections per die) and contact pitches as fine as 100 µm are possible with a gold-on-gold AU flip chip. Finer contact pitches are limited by the quality of the PC board.

The elimination of extraneous interconnect wiring with flip chip reduces such undesirable circuit effects as resistance and inductance. As a result, circuits are able to operate at higher speeds than circuit assemblies using traditional packaged ICs and printed wiring. Additionally, heat dissipation is improved by the close physical and electrical contact of chip and substrate. Heat can be drawn off through both the contact or bonding system and the backside of the die. If necessary, a heat sink can be applied directly to the die. The cumulative effect of reduced circuit connections, shorter wire leads, lower power consumption, and lower operating temperatures enhances product reliability.

Most concerns about flip chip center on thermal issues, either during chip mounting or in elevated-temperature environments once a product is in service. ICs can be damaged by temperatures above 400°C. But at slightly lower temperatures, a physical mechanism that results in progressive electrochemical degradation and chronic failure can activate. Such damage is more likely in soldered die attachment than in lower-temperature adhesive-based processes. With solder-based processes, dissimilar coefficients of thermal expansion (TCE) between dies and PCB materials can lead to thermal stresses that subject joints to fatigue and possible fracture. This effect becomes more pronounced as TCE differences and die sizes increase. Adhesive-based attachment provides some give between the die and PCB, making thermal stress less of a problem. 

Multichip Modules. A multichip module (also known as an MCM, ball-grid array, or BGA) is a hybrid approach to miniaturization. In MCM, a relatively small but complex modular assembly is created for use in a larger circuit. The underside of the MCM is covered with a grid of solder ball contacts that have a relatively coarse spacing of 2–3 mm and can be attached to a circuit board using standard SMD or solder reflow techniques. Ostensibly, an MCM may not be at the cutting edge of miniaturization, but internally, it can be fabricated with SMD, COB, COC, or flip-chip processes. Thus, MCMs offer a means of confining complex, expensive circuits to an easily handled, custom-component module, while the remainder of the product uses less-expensive assembly techniques.

A variation on MCM is the micro ball grid (MBG). It is a somewhat miniaturized MCM, in which the outside dimensions are no more than 10 to 20% larger than the total surface area of the devices contained within the module.

3D-CSP. Another miniaturization technique is 3-D chip-scale packaging, or 3D-CSP. This technique achieves small, compact assemblies. It is often combined with generic, industry-standard processes to end up with a finished miniaturized module. 

In the 3D-CSP process, flip-chip circuits and SMDs are first assembled on flexible PCBs. The circuit board is subsequently folded or rolled to achieve a 3-D package (see Figure 4). For successful 3D-CSP, an engineer must design the boards with careful consideration of bend radii, chip spacing, trace routing, power distribution, and other parameters. The result is a 75 to 80% size reduction, compared with traditional assemblies.

Power Sources. A key requirement for any long-term electronic implant is the elimination of wires running from the outside world into the device. This requirement minimizes the possibility of infection. Medical electronic hardware is usually battery powered, but batteries ultimately become drained and can also present risks from corrosion or leakage. In the past, this problem had been overcome by balancing power consumption and battery design. For example, cardiac pacemakers use sealed lithium batteries that last 5 to 10 years. Fortunately, the surgical procedure used to replace the batteries is relatively minor. Additionally, the patient and a physician can monitor the pacemaker to detect failing batteries months before they become a serious problem.

Not all implants have power sources that are easy to replace. Engineers have had to develop other methods of keeping implants powered. The solution lies in a combination of reducing the voltage and current requirements of the electronics and using more-sophisticated battery technology and recharging techniques. Families of low-power analog and digital ICs now exist that operate on as little as 1.8–2.0 V. These provide more latitude for power supplies in terms of the size or number of cells used, and allow a battery to reach a lower voltage before requiring a recharge. 

Increasingly, implantable electronic devices are making use of rechargeable power systems that couple power to the implant through induction or radio-frequency waves. While not as effective as a wired connection, this technique can operate without any physical connection between the power source and the implant.

Other Issues

Implantable electronic devices used for various types of therapy—and even whole-organ replacement—must be made increasingly reliable, energy-efficient, portable, and impervious to such environmental conditions as magnetic and electrical fields. However, the success of this endeavor depends on many other parts of the healthcare equation—especially cost and the public's acceptance level.

Unlike consumer electronic products, which become less and less expensive as manufacturing techniques mature, miniaturization of medical electronic devices serves as a means of increasing market penetration of new treatments. Total treatment costs typically include physicians' office visits, tests, prescriptions, surgeries, and other expenses in addition to the medical device technology. Therefore, the total cost of treatment is usually considerable, even when the cost of the electronics is relatively small.

In addition, different philosophies exist concerning the combination of humans and machines in order to prolong life. Nevertheless, there is every indication that progress in electronic miniaturization and manufacturing technology will continue to fuel growth in the development of effective medical implants. 

Copyright ©2003 Medical Device & Diagnostic Industry