As the number of biocompatible metals available for injection molding increases, the process is becoming a practical option for manufacturers of implants and surgical instruments.
Injection molding can be used to form complex parts as easily as simple ones. The process is generally best suited to parts measuring less than 6 mm thick and weighing less than 100 g. Newer binder removal techniques, however, have enabled the processing of cross sections above 12.5 mm and up to 400 g. For all size ranges the process can usually achieve tolerances within 0.3 to 0.5%. Higher tolerances are best met by machining critical dimensions after sintering.
Metal injection molding is used increasingly within the medical device industry to produce a variety of components. The technology has matured to the point where quality and delivery can be assured through the ISO 9002 and QS-9000 certification of metal injection molding suppliers. These companies can make components from many of the alloys used for medical devices with properties comparable to those of wrought and cast materials. Surgical instruments and implants are two types of medical devices for which this process is particularly well suited. As will be discussed below, there are now a large number of materials with a variety of desirable properties for these applications that can be used in metal injection molding.
MEDICAL INSTRUMENTS
Examples of the types of stainless steels used for various cutting and noncutting instruments are given in Table 1.1 Many of these components are traditionally produced in high volume by machining them from wrought material. Indeed, sulfur is often added to stainless steel to improve its machinability for high-volume production. Such high-volume, machined stainless-steel components are excellent candidates for metal injection molding.
Instrument | Type of Stainless Steel |
Cutting | |
Chisels | 302, 303, 410, 416, 420, 440 |
Curettes | 302, 303, 410, 416, 420 |
Cutters, bone-cutting forceps, skin punches, conchotomes | 420 |
Dissectors | 410, 416, 420 |
Knives | 302, 303, 420, 440 |
Osteotomes | 410, 440 |
Reamers | 410, 630 |
Rongeurs | 410, 420 |
Scalpels | 420, 440 |
Scissors | 410, 420, XM-16 |
Noncutting | |
Cannulae, needle vents | 302, 303, 304 |
Forceps | 302, 303, 304, 410 |
Retractors | 302, 303, 304, 410, 416, 420, 431, 440 |
Specula | 302, 303, 304, 316 |
Spreaders | 302, 303, 304, 410, 416, 440 |
Clamps | 303, 304, 410, 416, 420 |
Drills | 303, 440, XM-16 |
Handles | 303, 304 |
Hammers, mallets, rulers, screws, tunnelers | 303 |
Punches | 303, 410, 416, 420 |
Skin hooks | 303, 410, 416, 420 |
Suction tubes | 303, 304 |
Probes, tongs | 303, 440 |
Holders | 304, 410 |
Clip applicators, dilators | 410 |
Elevators | 410, 420 |
Burrs | 420F |
Orthopedic instruments | 430 |
Needles | 420, XM-16 |
Table I. Types of stainless steels used for medical instruments.1 |
New applications for metal-injection-molded components are trending toward smaller, more-complex devices for minimally invasive surgery, especially laparoscopic instruments for grasping tissue, cutting, and suturing.2 Such devices are being designed for greater freedom of movement, which has increased the numbers of metal parts used in the assembly. Metal injection molding has provided the design freedom to be able to produce such parts cost-effectively. A new area of exploration for the process is the production of microsized parts, which should help meet future medical needs as parts continue to shrink for minimally invasive surgery.
Mechanical Properties. Various grades of stainless steel are commonly available for metal injection molding, with lower costs for the more common grades. Generally, austenitic alloys such as 304 and 316 are only used in their low-carbon forms, i.e., 304L and 316L. The reason for this is that the injection molding process works best with minimal carbon, which also gives reduced susceptibility to sensitization and improved corrosion properties. Still, for martensitic alloys that require carbon for high hardness, such as 420 and 440C, carbon levels can be precisely controlled.
Mechanical properties of commonly available stainless steels for injection molding are well established and are very competitive with wrought materials, as shown in Table II 3–5. These properties are sufficient to meet the requirements for medical instruments as given in ASTM F899-95. Mechanical properties can be further modified through additional heat treatments or atmosphere changes during thermal processing.
Stainless Steel | Type | Tensile Strength | Yield Strength | Elongation | Hardness | ||
MPa | ksi | MPa | ksi | % | |||
304L austenitic | MIM3 Wrought4 | 620 >480 | 90 >70 | 310 >170 | 45 >25 | 30 >30 | 65 HRB <92 HRB |
316L austenitic | MIM3 Wrought4 | 515 >480 | 75 >70 | 170 >170 | 25 >25 | 50 >35 | 65 HRB <95 HRB |
420 martensitica | MIM5 Wrought4 | 1750 >1720 | 254 >250 | 1560 1480 | 226 215 | 2 8 | 52 HRC 52 HRC |
430 ferritic | MIM3 Wrought4 | 415 >415 | 60 >60 | 240 >205 | 35 >30 | 25 >20 | 65 HRB <88 HRB |
440C martensitica | MIM3 Wrought4 | 1670 1970 | 242 285 | 1615 1900 | 235 275 | 2 2 | 55 HRC 57 HRC |
630 precipitation hardeninga | MIM3 Wrought4 | 1185 >1170 | 172 >170 | 1090 >1070 | 158 >155 | 6 >10 | 33 HRB <35 HRB |
aHeat treated | |||||||
Table II. Typical mechanical properties of commonly available stainless steels for metal injection molding (MIM) in comparison with wrought materials.3-5 |
Corrosion Properties. Metal-injection-molded parts made of stainless steel have been subjected to numerous tests of general, pitting, and intergranular corrosion. Many standards exist for testing corrosion properties in various media, but even within these standards some variables are left up to the tester. Thus, comparing corrosion properties among reported values is difficult unless test conditions are identical. Still, several conclusions from previous studies can be summarized.6–10
Corrosion resistance is largely a function of composition. It can be affected by trace elements, so even within the compositional specification for a given stainless steel, corrosion resistance can vary. Injection-molded stainless steels generally perform as well as wrought materials in general corrosion tests. Pitting corrosion, however, is related to surface roughness; as-sintered injection-molded stainless steels often show reduced pitting corrosion resistance in comparison with wrought materials, if not sintered to a closed surface porosity (a density above 7.6 g/cm3). The surface finish for as-sintered stainless steel is typically 0.8 µm Ra. This can be improved by mechanical polishing or electropolishing. Polishing removes surface defects that serve as pit sites. Pitting corrosion resistance can be further improved by passivating the surface. Passivation involves subjecting the steel to nitric acid for 30 minutes to produce a protective film.
Medical instruments must meet ASTM F1089-87 corrosion requirements. One part of this test involves boiling the instruments in water and letting them air dry to see if they rust. The second part involves submerging the instruments in a copper sulfate solution, rinsing them, and inspecting them for copper plating. Tests on injection-molded stainless steels confirm their ability to meet ASTM F1089-87 criteria without additional polishing or passivation treatments.11
In addition to providing corrosion properties comparable to those of wrought materials, metal-injection-molded components can significantly reduce corrosion in cases in which wrought parts are brazed or welded together. Often, metal injection molding can produce the desired component as a single part, thus eliminating the need for brazing or welding and the consequent reduction in corrosion resistance at the weld.
IMPLANTS
Metal injection molding is also suitable for the production of components for medical implants; however, these components are subject to much more stringent standards than medical instruments due to the more severe in vivo environment. Type 316L stainless steel has been widely used for many implants, but is now generally restricted to temporary implants, owing to problems with pitting and fretting corrosion.
Materials that are more biocompatible, such as cobalt chromium or titanium alloys, are used for permanent implants. Tantalum also has both excellent corrosion properties and biocompatibility, but its mechanical properties have limited its use. These implant materials all have individual ASTM specifications for chemical, mechanical, and metallurgical requirements.
A list of materials used for medical implants is given in Table III, along with example applications 12. These alloys are less widely available for metal injection molding than are stainless steels. The Co-28Cr-6Mo alloy has been successfully injection molded, but its use to date has been limited. Titanium for metal injection molding is commercially available but is generally used for moderate- to low-stress applications such as surgical tools, golf club putters, and watch cases and bands. There are no published reports of tantalum being injection molded, even though tantalum powders are widely used to make components in the electronics industry. Tantalum could be injection molded for medical applications using those powders. Biomaterials for metal injection molding continue to be the subject of active research, and much progress has been made in meeting many of the standards for cast and wrought implants.
Material | Implant Applications |
316L | Bones, plates, screws, staples, pins, and nails Stents |
Co-28Cr-6Mo | Prosthetic replacements of hips, knees, elbows, shoulders, ankles, and fingers Bone plates, screws, staples, and rods Heart valves |
Unalloyed Ti | Bone plates, screws, rods, and staples Heart valves and pacemaker casings |
Ti-6Al-4V | Prosthetic hips, knees, elbows, shoulders, ankles, and fingers |
Unalloyed Ta | Wire, foils, sheets, clips, staples, and meshes Electrodes |
Table III. Materials used for medical implants12 |
Material, ASTM Specification | Type | Tensile Strength | Yield Strength | Elongation | Reduction in Area | ||
MPa | ksi | MPa | ksi | % | % | ||
316L F745 F138 | MIM3 Cast1 Wrought1 | 515 >480 >490 | 75 >70 >71 | 170 >205 >190 | 25 >30 >28 | 50 >30 >40 | Not reported 50 — |
Co-28Cr-6Mo F75 F1537 | MIM13 Cast1 Wrought1 | 1000 >655 >900 | 145 >95 >130 | 520 >450 >520 | 75 >65 >75 | 40 >8 >20 | 515 >480 >490 |
Unalloyed Ti F67 grade 4 | MIM5,14,15 Wrought1 | 575 >550 | 84 >80 | 500 >480 | 73 >70 | 16 >15 | Not reported >25 |
Ti-6AL-4V F1108 F1472 | MIM16 Cast3 Wrought1 | 910 >860 >930 | 132 >125 >135 | 800 >760 >860 | 116 >110 >125 | 11 >8 >10 | Not reported >14 >25 |
Unalloyed Ta F560 | MIM Wrought1 | — 210 | — 30 | — 140 | — 20 | — 25 | — — |
Table IV. Mechanical properties of metal injection molding materials in c omparison with ASTM specifications for cast and wrought metallic implants materials. 1, 3, 5, 13-16 |
Carbon and oxygen control are critical to achieving sufficient ductility for metal injection molding Ti-6Al-4V. The lowest reported oxygen content for sintered injection-molded Ti-6Al-4V is 0.27%.15 This is still slightly above the ASTM F1108 chemical requirements, but low enough to give suitable mechanical properties.
In addition to the property requirements summarized in Table IV, many implants must meet component-specific requirements. For example, femoral hip prostheses must meet specific fatigue property requirements as described in ASTM F2068-00. Since fatigue properties are highly sensitive to porosity, metal-injection-molded components must often be hot isostatically pressed to eliminate any remaining porosity to meet these requirements. Potential particulate inclusions may also reduce a material's fatigue resistance, especially for titanium and titanium alloys.
At the same time, metal injection molding offers opportunities for unique design solutions. For example, an implant with a porous coating for bone in-growth can be manufactured by metal injection molding as a functionally graded device with controlled surface porosity surrounding a fully dense core. The process can also produce composites of titanium and hydroxyapatite; such materials are advancing to animal studies.
Corrosion Properties. The corrosion requirements for metallic implants are much more stringent than for medical instruments. Limited testing of metal-injection-molded parts has been reported under conditions that simulate the salty, 37°C environment of the human body. ASTM F746-87 establishes a procedure for determining pitting or crevice corrosion of metallic surgical implants. Since pitting corrosion is unacceptable for metallic implants, additional polishing or passivation treatments should be expected for 316L stainless-steel implants. Injection-molded titanium and Ti-6Al-4V have been shown to resist pitting and crevice corrosion in various media, including artificial saliva, artificial sea water, 22% NaCl, and 6% FeCl3.14 The performance of metal injection molded Co-28Cr-6Mo alloy is expected to be similar.
Although metal-injection-molded biomaterials can meet corrosion requirements, additional qualification barriers must also be overcome. These include the effort of confirming biocompatibility and conducting clinical trials. Additional success in these areas can help metal-injection-molded implants gain acceptance by the medical industry.
CONCLUSION
Metal injection molding is fully capable of meeting the dimensional and material property requirements of medical instruments and has many demonstrated applications. It also shows the potential to produce implant materials; recent work has demonstrated its ability to meet most requirements for the chemical, mechanical, and corrosion properties required of such applications. More trials are needed for metal-injection-molded biomaterials to gain acceptance as implants. Besides making manufacturing of current medical devices more affordable, metal injection molding can enable the cost-effective production of novel designs, including microsized and functionally graded devices. Such developments may enable new solutions to current healthcare problems.
It is good to know that now a day’s metal injection molding companies provide a higher part in producing metals parts for the medical usages.
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