In the medical device industry, design/manufacturing engineers typically begin with a preliminary vision of the design of a particular device or component. However, a lack of available resources, tools or the process and application know-how often puts limitations on these visions. In their quest to find an effective solution, engineers are often forced to change aspects of their original idea due to these restrictions. A particular challenge is creating components on a small enough scale that they can adhere to the increasingly minute requirements for medical devices used inside the body. A strong partnership with a components manufacturer, with the unique capability to create these miniature parts, can open the door to make a manufacturer's vision a reality.
Implantable medical devices fall into two categories: Those intended to be in the body for short periods of time, such as angioplasty probes; and those intended to be semi-permanent implants like heart stimulating systems. Both types of devices are subject to similar critical chemical reactivity and physical size issues. Here we address physical size and configuration issues effecting the development of 21st Century implantable medical devices.
The semiconductor industry has made great progress in building more complex semiconductors with more memory and logic, while at the same time reducing their physical size. Unfortunately, until recently, the same cannot be said about the flexible interconnect systems, such as traditional flex circuits (TFC), that interconnect IC chips and often become part of the packaged system. The overall size of the flexible interconnect system is a function of the number and size of chips, complexity of I/O connections, and minimum trace and space width and aspect ratios.
For decades, the generally accepted TFC minimum conductive trace and space has been 0.075 mm. This limitation can be further complicated by current carrying requirements, i.e. the greater the current requirements, the greater the trace cross-section required. Since TFC trace thicknesses are generally fixed, wider traces are required to meet cross section/current carrying needs.
TFC manufacturing is a mature technology with a long history of improving and optimizing materials, manufacturing equipment and processes for the medical device industry. It is widely used in hundreds of applications where flexibility, weight and reliability are critical requirements. Such applications include non-intrusive medical devices, space-based systems, cell phones and digital cameras, to mention a few.
However, the TFC process does not address the needs of the latest generation of intrusive medical device that require significantly smaller conductive traces and pitch dimensions.
A new technology
During the past few years a new technology has emerged, capable of forming trace and space widths routinely to 0.005 mm with aspect ratios as high as 5:1. These advances were accomplished by combining three mature technologies:
Semiconductor UV photolithography,
thin film metal processing (controlled sputter deposition and ion beam removal) of a variety of metals,
and electrochemical metal deposition.
The result of this work is a new category of flex circuitry called Extreme Resolution Micro Flex Circuits. The ERMF Circuit category does not replace TFC. This technology is intended to address the growing field of medical application devices that have unique material and dimensional requirements not possible with TFC. This allows designers to now consider devices not possible before ERMF.
For example, circuits used for angioplasty procedures must be small enough and sufficiently flexible to be rolled into a cylinder for insertion into a blood vessel. Other leading-edge applications include implantable RF coupling transmitting and receiving systems used for monitoring critical life signs, drug dispensing and retinal implant imaging data. In this last example, the RF coupling coil is integral to the ERMF Circuit.
The good and the bad
The specific capabilities/advantages of the ERMF technology that make it ideal for current and next-generation implantable medical devices are:
Extreme resolution, micron level trace and space capability,
trace conductivity to match device requirements, i.e. signal or power,
suitable implant material options,
and extreme flexibility.
These advantages allow the device engineer greater design freedom to build less intrusive devices with more functionality within a single package.
Disadvantages include cost, lead time and current fabrication techniques that limit extremely high-volume applications, but approaches are being devised to resolve this.
ERMF is still a relatively new process that holds far more potential than is currently being tapped. The precise and repeatable nature of the technology holds great potential because of the enabling effect it affords the designer of implantable medical devices and other applications that have similar size and material limitations.
Working closely with the manufacturing company can often result in improved overall design approaches that take better advantage of the properties of these processes. This technology, as applied to micro-sized medical devices, is quite new. As more medical device designers become aware of this capability, it will likely have an even more dramatic impact on the overall growth of the medical device industry.