Many medical devices and applications require small metal components that demand very specific characteristics. These can include:

  • Ultra-high precision
  • Very thin material
  • Unique physical features
  • Unique surface textures
  • Identification features
  • Extreme consistency part-to-part
  • Ease of integration into the final device
  • Competitive cost

Fig. 1 – A patterned series of tapered holes in stainless steel for a filtration application. The holes are 150 micron in diameter on one side, and 35 micron diameter on the opposite side.
In addition, different medical applications require components made from a variety of different metals including stainless steel, copper, alloys, laminates, and clad materials. It is often a significant challenge to balance the combination of the above characteristics with the desired material, while still maintaining cost and production volume requirements.

Photochemical Etching Can Improve Production

While numerous manufacturing technologies exist to create precision metal components, one technology in particular offers the capability to incorporate all of these characteristics into one seamless manufacturing process. Photochemical etching, also referred to as photochemical machining or chemical milling, possesses the unique ability to meet all of the above demands when compared to other technologies such as stamping or laser machining. Photochemical etching also eliminates the need for certain post-processing, such as removal of burrs and thermal stresses, sharpening of blades, etc. The development of continuous “reel-to-reel” etching production has dramatically improved production capacity while reducing cost and improving consistency in quality.

Common examples of etched components are surgical blades, surgical needles and lancets, micro-separation filters, screens, implants, springs, electrical contacts, and many others. Parts can be manufactured in thicknesses as low as 0.025mm (0.001"), up to 0.500mm (.020").

Advancements in etching technology allow levels of precision, both in physical feature size and in dimensional tolerances, which were previously considered unachievable for volume-production components. Micron-level precision is common for many applications. (See Figure 1)

Typically, the precision of the location of a particular etched feature can be in the single-micron level, while the actual size limitations of features (for example, how small a hole can be) is dictated in part by the thickness of the material being etched. Essentially, the thinner the material, the smaller the individual features can be. A good ratio to work with is that a hole diameter can be as small as 0.8 of the thickness of the material.

How It Works

The etching process itself is straightforward. The base material (typically stainless steel, but many other materials can be etched) is cleaned, dried, and then coated with a special photoresist layer on both sides. The photoresist layer is then exposed using UV light and an ultra-precision glass photomask, leaving the desired shape and surface features of the component protected by the exposed photoresist. All unexposed portions of the material are then subsequently etched away in a highly controlled chemical bath, leaving the desired component as the finished result. Sounds simple, but it’s the level of process control and photomask tooling design that really bring out the advantages of etching technology.

Fig. 2 – Examples of the various hole shapes that can be produced using photochemical etching by varying the etching process on opposite sides of the material.
Because the material is processed from both sides, it is possible to utilize different photomask tools on either side to provide unique features and dimensions on different sides of the finished component. For example, a separation filter can have a conical or offset hole configuration allowing better filtration or ease of backflushing. (See Figure 2)

The smallest diameter of the hole dictates the maximum particle size that can pass through the filter, but the larger opening on the opposite side can improve the filter’s ability to trap a larger amount of material. In another example, a surgical blade or needle can have a varying texture on one side and an identifying mark (such as part number or company name) on the other. The process is so precise that the blade comes off the production line razor sharp with no need for additional grinding or sharpening. Partial-etch technology also allows the material to be very thin for functional areas while providing thicker, more robust material in zones where structural integrity is important, such as mounting.

This ability to partially etch the material also lends itself to processing of “clad” or multilayered materials. Clad materials such as stainless/copper laminates can be etched to expose the conductive copper layer in strategic locations while preserving the benefits of the stainless steel over the majority of the structure. Other unique materials include copper, nickel-silver, Invar, brass, and tungsten, which is frequently used in X-ray ID tags for pacemakers and other implantable devices.

Fig. 3a and b – Micro-textured surfaces can be patterned in either “positive” (as shown in Fig. 3a at left) or “negative” (Fig. 3b right) topographies, promoting grip, enhancing adhesion, or controlling the rate of liquid flow.
“Textured” surfaces can be beneficial in both functional and mechanical ways. (See Figures 3a and b) A textured surface can take various forms, from a randomized texture to improve grip or promote bonding of an adhesive, or a very structured pattern to dictate flow of a liquid. For instance, capillary plates can be produced with “half-etched” grooves on one side, then mated together in a “mirror image” to create a very precise capillary flow system. The process also helps facilitate integration and assembly, as “break-away” lines can be etched into one side of the material sheet or reel, allowing individual components to be broken off very precisely on an assembly line.

The precision in physical feature size comes from a combination of technology developments. The thickness (or rather thinness) of the photoresist layer is critical, as is the precision of its exposure pattern, which is dictated by the precision and placement of the photomask tool. These combined with extremely precise monitoring and control of the chemical etching rate allow features and precision that simply cannot be achieved in production with other manufacturing technologies.

Because the process is non-mechanical, there are no residual artifacts such as burrs or material stresses that must be removed. A non-mechanical process also ensures that there is no wear or degradation of tooling over time, providing the ultimate consistency from part to part. Production capacity from a single set of photomask tooling is essentially limitless. Also, because the process generates very little heat, there are no thermal stresses or localized hardening of the material, such as would result from a process like laser machining.

Fig. 4 – Several different types of filtration components produced on “endless reels”. These strips of finished components can be directly integrated into an automated assembly process.
The development of reel-to-reel photochemical etching has facilitated numerous benefits. By starting with a wide roll of the base material and processing the material along a continuous production line, virtually limitless production capacity is possible with extremely consistent quality. Monitoring systems can be placed at various stages in the production line to ensure quality and process control at every level. The process runs continuously in one setup, so millions of parts can be manufactured in one production run without risk of damage or error from moving “batches” from one production step to the next. Since the base material begins as a wide sheet, it is possible to produce a number of different components simultaneously that will later be separated into individual “strips” for packaging. (See Figure 4)

Re-winding the finished product onto smaller reels at the end of the process allows economical packaging that can easily be integrated into a manufacturing assembly line.

In addition, etched components can easily be post-processed if additional “forming” is required. They can be molded, stamped, and packaged to fit virtually any application. The ability to vary the thickness of the component through the etching process can actually make post processing much easier. For instance, the material can be made thinner in areas where it needs to bend or be formed, leaving thicker material in key structural or functional areas.

Even outside of component-level production, photochemical etching is providing advantages for medical devices. Because materials can be very thin and have very small features, etched materials are becoming popular for Electro- Magnetic Interference (EMI) shielding. From wide sheets to narrow strips, etched EMI filter material can be very easily added to or integrated into medical devices that are sensitive to this type of interference.

Summary

In summary, photochemical etching provides a compelling solution to many of the medical industry’s demands for high-precision metal components. Although not a new technology, it has evolved and improved dramatically in recent years. By offering a distinct combination of precision, unique physical features and surface textures, quality, and competitive cost, etching may be the ideal manufacturing technology for many applications. Product designers have new options for component designs that they may have considered to be impossible in the past. Device manufacturers have opportunities to reduce their component cost and simplify their assembly process while improving quality at the same time. The scalability of the photochemical etching process allows effective prototype manufacturing while also offering essentially limitless production capacity.

This article was written by Karl Martinson, North American Sales Director, Micrometal GmbH, Attleboro, MA, a member of the Wickeder Group. For more information, Click Here .


Medical Design Briefs Magazine

This article first appeared in the October, 2015 issue of Medical Design Briefs Magazine.

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