Interview published in May 2023 and produced in collaboration with Evonik. Marc Knebel, then Head of Medical Systems, presents the supplier’s perspective on implant polymers. Material properties alone do not establish a clinical benefit for a finished device.
Material selection contributes to medical implant design and durability. Outcomes also depend on device, indication and patient. Registry data need interpretation within their scope: the 2019 AJRR report concerned hip and knee procedures recorded between 2012 and 2018 in the United States, not a worldwide annual total.
As you may guess, off-the-shelf implants have made it easier for medical professionals to treat the most common patient needs quickly. As technology advances, one realizes that Additive Manufacturing is not just an alternative method for manufacturing hip cups and spine cages at scale; the technology helps to reshape what implants can do, and how patients can be treated. The problem is, very often the use of AM in medical applications is highlighted through the use of metals. Yet, they are not necessarily the ideal materials for every application.
This article aims to highlight the role of polymer AM materials in creating and improving the next generation of 3D printed implants. To do so, we will discuss:
- The use of polymer materials in AM vs the use of polymer materials in conventional manufacturing
- The metals vs polymers choice
Before we dive into this topic, it’s crucial to distinguish between biopolymers and biomaterials.
“Per definition, all materials used for long-term implant applications are biomaterials since they are in contact with body tissue, liquids and/or bone. One crucial requirement for biomaterials should be their excellent biocompatibility.
“Biopolymers” on the other hand are defined as a polymer of biological origin, e.g. microbial. It is not a general requirement for a polymer biomaterial to be a biopolymer”, Marc Knebel, Head of Medical Systems at Evonik emphasizes.
Biological origin, biodegradability and biocompatibility are distinct properties. A biobased polymer is not automatically resorbable or suitable for an implant; medical use requires assessment of the material and device.
Evonik offers the RESOMER® range of resorbable polymers and biostable VESTAKEEP® PEEK grades. Available forms and compatible processes vary by grade, including filament, powder and granules. A brand does not identify one material usable in every process.
Polymers: additive and conventional manufacturing
From a manufacturing standpoint, traditional subtractive manufacturing makes sense for manufacturers who want to produce ready-made solutions that can treat the most common patient needs. However, given the complexity of anatomical structures, such technology rapidly becomes limited in the geometries it can produce, as well as the number of pieces of equipment required to make an implant. Not to mention that the time required for machining can be significant, especially when working with harder metals.
First and foremost, “3D printing of polymer-based implants is not restricted to a particular type of implants. All current implants for which polymers are being used are possible. With the new possibilities of additive technologies, potentially even wider range of applications is achievable. One example shows the company BellaSeno [a developer of 3D printed absorbable scaffolds] uses 3D printed resorbable implants in applications that use silicone (breast implants). The most important is that 3D printed implants fulfill the biocompatibility and mechanical requirements for an implant.
More generally – 3D printing has substantial advantages in two main fields: (1) Patient-specific implants that has an n=1, that is unique to the patient and that conventionally would result in a large amount of material loss, e.g. from machining, easier logistics and less working capital. (2) New designs and functionalities, not achievable with conventional technologies. This includes hollow or porous structures of particular surface properties,” Knebel explains.
The device must meet requirements for its intended use regardless of process. This does not mean mechanical properties are automatically identical after machining, moulding or printing: orientation, structure and post-processing can change them. Knebel emphasizes validation of the manufacturing chain:
“The same quality and documentation for the material are required as it is for any long-term body contact (implant) application. Same as for other processing technologies, all processing steps have to be validated and qualified (IQ/OQ/PQ) and it must be assured that a safe medical device is produced.
The difference, e.g. to injection molding, is that 3D printing offers substantially more variables to adjust in the process and all these parameters have to be checked and validated.”
Needless to say that decisions across the material, the ideal 3D printing process, and design strategies are made in a nonlinear way that requires careful consideration and knowledge of their relation to a given application. Choosing a polymer material, therefore, requires considering a wide range of measurable properties that we cannot assess here. However, for the manufacturing of 3D printed implants, Evonik recommends focusing on “Smart Biomaterials”, a type of material that can combine multiple features.
“One example of this is the VESTAKEEP® Fusion Filament for FFF printing. In this product, we have integrated a bioactive additive that combines the excellent biocompatibility and mechanical properties of PEEK with bioactive properties for faster bone healing and therefore a faster patient recovery”, Knebel explains.
Improved healing is a property claimed by Evonik. Patient recovery time needs assessment for the device and indication concerned; it cannot be inferred from a bioactive additive alone.
Porous structures can support fixation or tissue integration in some designs. Their presence guarantees neither biological acceptance nor freedom from complications. Geometry, surface, material and manufacturing need assessment together.
Choosing between metals and polymers
Choosing a metal or polymer depends on loading, implant function and available evidence. Neither material group is superior for every indication.
A stiffness mismatch between implant and bone can change load sharing. Stress shielding describes reduced mechanical loading of the bone, which can promote bone resorption. Its significance depends on material as well as design and fixation.
Knebel highlights PEEK’s lower stiffness compared with many metals and its imaging properties. These can benefit some designs without guaranteeing freedom from loosening or artifacts for every implant. X-ray imaging and MRI are distinct modalities; MRI does not use X-rays.
“As a metal-free solution, polymers can be the material of choice for allergy patients.
With respect to the 3D printing of metal implants, the SLS technology is well established. The major obstacle here is the powder handling and subsequent powder removal from the final implant. This very work and time-consuming powder handling and removal limit the use of this technology to a low number of centralized facilities.
The FFF-technology for processing of polymers offers the advantage of decentralized point-of-care implant production, e.g. directly in the hospital. Machines are smaller, filament handling is safe and easy, and post-treatment much easier. Implant production can be directly integrated into the fully digital workflow in the hospital for a faster patient treatment,” he continues.
For metal implants, the process discussed generally belongs to powder bed fusion, rather than the narrower term “SLS”. Hospital FFF production remains subject to the device’s applicable qualification, sterilization and regulatory requirements; machine size alone does not satisfy them.
From a healing standpoint, “healing is generally also well understood and positive for metal materials. However, allergies occur more frequently vs. no allergic reaction is reported for PEEK.
Healing of the implant can be further increased by 3D printing with structure and surface design, e.g., the roughness of porous structures. Also, fast healing materials such as VESTAKEEP Fusion for faster bone growth accelerate the patient recovery”.
The absence of allergic reactions suggested in the interview should not be read as a guarantee: PEEK hypersensitivity cases have been reported. Those observations alone do not establish frequency. Healing benefits still require evidence for each device.
Conclusion
The FDA classification depends on device type and the controls required: not every implant automatically falls within Class III. Additive manufacturing requires control and validation; it does not inherently reduce risk. The review by Shan and colleagues cited in the initial version studies mid-term quality of life after hip replacement. It does not establish survival beyond 25 years for 80% of all implants or a longevity benefit from 3D printing. Knebel concludes by stressing collaboration between suppliers, manufacturers and clinicians: “a close connection to key opinion leaders (e.g., surgeons) and OEMs to understand unmet needs; trust in the market and reliable producer of safe materials, as well as a material expertise and backward integration to understand polymer well and having the capabilities and know-how to modify polymer design as needed.”
This content has been created in collaboration with Evonik.
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