Building Better Phantoms
The Challenge: Matching highly specific medical device developmental needs with precisely the right materials.
Varian, Inc., a leader in Nuclear Magnetic Resonance (NMR) technology, produced the first commercial NMR spectrometer more than 50 years ago. Since then, NMR has become a standard technique for chemical, biological, medical and physical research. NMR imaging systems are recognised as significant tools for medical diagnostics.
NMR works because atomic nuclei behave like magnets when exposed to an external magnetic field. Under such conditions, the spinning nuclei create small magnetic fields that can be stimulated using specific frequencies of electro-magnetic energy. The result is that radio waves are emitted. Those waves can be measured to create a visual image of both the molecular structure being studied and its environment.
In developing NMR technology, it is essential to anticipate the performance of emerging devices. This is complicated by the fact that imaging systems may feature varying global standards. Therefore, it is usually not practical to use humans to test and verify performance. This challenge is resolved by using what is called a 'phantom'. Phantoms are objects that do not occur in nature, but can be created to replicate a human standard that can be imaged to test the performance of magnetic imaging systems. These phantoms are kept on site to verify system performance through quality assurance protocols.
Recently, Varian was commissioned to develop miniature spatial phantoms for use in testing and calibrating gradient systems in small and micro-bore magnetic resonance imaging systems. For clinical systems, these types of phantoms are most often constructed from machined plastic. For the smaller scale phantoms needed for micro-imaging, this is not feasible and we turned to stereolithography technology to achieve the needed detail. Portions of the inside of the phantom are removed to create a test pattern, after which the phantom is filled with an aqueous solution. When imaged, the NMR device displays the signal from the water in the water-filled sections of the plastic. It was therefore necessary to create a device that could withstand rigorous use with aqueous substances. To do so, we had to first identify a resin with characteristics encompassing low water absorbency coupled with superior transparency necessary for detecting and removing undesirable air bubbles.
In exploring resin opportunities, Varian were quickly able to identify several options that seemed to match our requirements. They ultimately selected a SL resin that simulated the appearance and physical characteristics of polycarbonate. At the surface level, this seemed appropriate because polycarbonate has low water absorbency and is the plastic of choice for larger phantoms. Moreover, polycarbonates feature high temperature resistance, high impact resistance and clear optical properties. Consequently, they are widely used in the medical and chemical industries.
The Wrong Approach: Improper material selection leads to prototyping failures While Varian's first choice did have many attributes similar to polycarbonate, it was not tolerant of aqueous environments. Within one week, the prototyped product created failed because the part was moisture sensitive. As a result, it expanded and distorted in an aqueous environment. The vendor did say that the resin was sensitive to humidity but they did not have any water absorbency data available.
Pictured: Miniature spatial phantoms manufactured from moisture resistant WaterShed XC 11122 stereolithography material.
The Right Approach: Testing SL resins demonstrates that proof is in the performance Varian's unfavorable experience with the initial SL resin caused us to seek input from Protogenic, Inc. (Varian's SL prototype supplier) who suggested several alternatives. This time, however, rather than proceeding to actual prototyping of the device, we established a resin evaluation procedure.
Varian subjected small tokens made from a number of resins to a long-term soak test in a relevant aqueous solution and made periodic weight and size measurements of those samples over the course of seven months.
Although time was obviously consumed by the procedure of testing SL resins, the result was that Varian saved time and backtracking by first determining the resin most appropriate to their needs. The resin that prevailed was WaterShed developed by DSM Somos.
Conclusion: Proper resin evaluation took time initially, but saved time in the end, by leading Varian to the best material option for rapid, direct manufacture of a medical device component.
WaterShed is an optically clear rapid SL prototyping resin developed by DSM Somos to provide ABS-like properties, clarity and excellent temperature resistance. WaterShed proved, in Varian's application, to produce clear, functional, accurate parts that simulate acrylic in appearance.
Most importantly, WaterShed XC 11122 provided the lowest available water absorption versus alternative resins. Varian successfully used two grades: WaterShed XC 11122 and XC 11112. Resistance to moisture resulted in excellent dimensional and physical property stability. This facilitated direct, rapid manufacture of the phantoms using the stereolithography process.
Varian, Inc. is a diversified, global technology leader helping customers innovate with confidence, enhance their competitiveness, and boost their efficiency. Varian's people, products, and solutions enable customers to meet increasingly complex challenges in life sciences and industry. Website: www.varianinc.com
Protogenic is a division of Spectrum Plastics Group, and a professional service bureau that specializes in Additive Manufacturing Technologies. Protogenic is a leading service provider in the Stereolithography market, using a wide variety of resins provided by DSM Somos. In addition to Stereolithography models, Protogenic also specializes in providing customers with high quality production cast polyurethanes, Objet Connex 3D printing, and direct manufacturing using Laser Sintered nylon.
DSM Somos is one of the world's leading material suppliers to the rapid prototyping industry, providing stereolithography resins used for the creation of three-dimensional models and prototypes directly from digital data. Patented SOMOS® ProtoFunctional® materials are used in a variety of industries, including automotive, aerospace, medical and telecommunications. SOMOS® resins for stereolithography provide advanced technology to respond to the changing needs of new product development and industrial design. These materials demonstrate a varying range of performance properties including: transparency, superior humidity and heat resistance, and mechanical properties which replicate those of many production grade plastics such as polypropylene, polyethylene, ABS and PBT. Technical data on all SOMOS® materials may be found at http://www.dsmsomos.com/
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