A close-up view of a fixture holding nitinol tubing illustrates the accurate features generated by an MDP patent-pending, threading process. This unique grinding technology often eliminates the need for subsequent operations. (Photos courtesy of TESCAN USA/OBERG INDUSTRIES)

Properties such as improved strength-to-weight ratios enable materials to have increased wear resistance but remain light for biomedical implantation. However, such alloys are difficult to conventionally machine or grind. Through friction, the manufacturing process produces unacceptable heat or mechanical stress. Exotic shape-memory alloys hold much promise for a growing number of medical applications but also present significant manufacturing challenges for many of the same reasons.

 Molecular Decomposition Process 

Magnified 6000 times, smears, tears, and folds are visible in titanium carbide when conventionally ground.

An MDP sample shows no smears, tears, or folds.

Finishing with a wire EDM machine reveals bumps and a sintered appearance.

 Research and development to applied MDP projects has been proven to eliminate the need for a secondary deburring or polishing operation. This preserves the integrity of the feature by not violating the specified geometry. For material removal with the applied MDP technology, finer grit sizes for the abrasive can be used to roughen and finish a product. A finer grit enables crisp geometry as related to the abrasive size of a proprietary-formulated grinding wheel.

 MDP was developed for removing or cutting material using an electrochemical action with an abrasive assist. The process uses an abrasive wheel combined with a steady supply of electrolyte solution and electric current. As current flows through the electrolyte between the positively charged workpiece and the negatively charged abrasive wheel, the material oxidizes at the point of contact, causing a decomposing action known as anodic dissolution. As the process repeats, the oxidized surface is then wiped away by the grinding wheel.

 Throughout its history, anodic dissolution machining has been used extensively to machine electrically conductive materials such as titanium, stainless steel, and other high-performance alloys, but repeatability was heavily dependent on operator expertise. Typically, the operator set all key parameters, including electrical current and feed rate, adjusting them as the cut progressed. Adjustments were usually based on visual inspection of the interface area, because the color and amount of sparking would indicate the reaction of the material to the removal rate and depth of cut.

MDP adds new levels of control that allow for greater dimensional accuracy, surface finish, and repeatability. Stringent sphericity and surface finish specifications with tolerances in the submicron level have been consistently achieved. The power supply is closely controlled with proprietary algorithms that maintain consistent power to the system, eliminating power spikes or brownouts. The lower levels of power required to operate the system make it more energy efficient. New developments in electrolytic solution further optimize current flow. Research in grinding wheel composition has helped to develop formulations that maximize conductivity for the material being shaped. Such formulations enable the removal of stock with only 10% of the abrasiveness created in conventional grinding, resulting in extended wheel life. Because the process produces no thermals, mechanical stresses, or burrs, MDP increases material choices for medical product designers while satisfying demands for precision and efficiency.

Medical Applications

MDP is an appropriate method for making profiles as sharp as possible without burrs. Traditionally, sharps—needles, trocars, and biopsy products—have required a series of manufacturing steps such as grinding, deburring, and polishing. MDP can produce well-defined edges in a single pass with no need for subsequent processing.

 Nitinol. Shape-memory alloys, such as nitinol (nickel titanium), are highly biocompatible and have numerous promising applications in implants and other medical products. Nitinol is used for devices that demand extraordinary flexibility and torque ability. This material can absorb large amounts of strain energy and release it as the applied strain is removed. The elasticity of nitinol is approximately 10 times that of steel. Add to this the material’s torque ability and kink resistance and these distinctive characteristics make nitinol an appropriate choice for medical guidewires.

 Other examples of superelastic devices include vascular, esophageal, and biliary stents; medical guide pins; surgical localization hooks; flexible, steerable, and hingeless laparoscopic surgical instruments; remote suturing and stapling devices; and bone suture anchors.

MDP is suitable for grinding memory alloys because the process introduces no heat or mechanical stress to the work piece. One patent-pending process has yielded threaded nitinol wire (0.04 in. diam.) with a thread pitch of 101 threads per inch. MDP enables the threads to be produced in a single cut at full depth, producing burr-free threads without any tearing or smearing of the nitinol material. The gentle process allows for the production of these fine features measuring 0.0035 × 0.003 in. deep. Examining the elemental profile of the material surface illustrated no changes to the chemistry of the product.

A 0.04-in. nitinol wire is ground using MDP. A grinding wheel with applied MDP technology helps maintain geometry and a burr-free condition.

Conventional grinding, however, can introduce a smearing action at the work surface. This not only makes achieving the best possible surface finish difficult, but it also transfers titanium particulate to the diamond wheel face.

In conventional grinding tests on a titanium carbide component using a 600-grit diamond wheel, grooving and smearing of material occurred at the work surface due to the abrasives within the conventional grinding wheel. In comparison, MDP produced a uniform surface with a similar 600-grit wheel due to the MDP deplating and wiping process without any material deformation.

The ease of equipment operation, tied to the mathematical algorithms, adds to the repeatability and dependability of MDP technology. From the early stages of development to current details regarding inputs and outcomes, separate lab experiments are fully recorded in which process parameters are developed inclusive of the necessary perishables required for the production of such fine details.

Keeping It Green

An MDP threading process enables fine feature development into exotic and conventional materials. A grinding wheel with proprietary MDP formulation is used to process fine features smaller than the burrs generated by conventional machining or grinding.

 Additional focus includes the control of conductivity within the electrolyte, fluid levels, and delivery to the interface area of product and equipment. The electrolyte formulations that are used mainly consist of simple salts and deionized water. A complete MDP system uses this electrolyte management system interfaced with the system controller to maintain electrolyte levels, conductivity, and cleanliness. The design allows for ease of operation, as well as extended electrolyte life to six months of manufacturing (compared with the industry standard of 40 hours) without the generation of heavy metals. These advances make the process safer for the environment while enabling small, very accurate feature production during the anodic dissolution process.

Achieving Finer Features

Generating and maintaining fine features during stock removal is a common challenge for manufacturers in general. Unique formulations are required to enable the material to be removed while attempting to gain as much tool life as possible. These problems grow exponentially when applied to shape-memory alloys. The advantages that these alloys present become the negatives in material removal, especially in the areas of fine features, as illustrated in the thread grinding of nitinol. Applying MDP technology to this part production enables the product to be manufactured with many benefits associated to the mechanical and elemental conditions of the subjected material (in this case nitinol). Extending or maintaining fine features during stock removal is still a focus point, because as with any manufacturing system, the longer the tool life, the more products are produced with less review of the system in general. This extends capability by requiring fewer tool changes, or dresses of the tool, during the required production.

MDP production of threaded 0.04-in. nitinol wire is produced with a single-pass grind and no secondary operations.

New technologies such as MDP can help medical device manufacturers machine materials more effiiciently. They eliminate the impediments of many promising new alloys for medical applications, both in implants and instruments. As a result, the future of promising new alloys achieving new standards of strength and endurance is very possible because of the innovative manufacturing techniques that shape and form them into products that improve the quality of life.

Joe DeAngelo is director of technical development at Oberg Industries (Freeport, PA).