Plastics Processors Are Suddenly Systemically Important

What do employees in an isolation ward and those in cleanroom production have in common? Both groups work in full kit. And what are the differences?

The former wear their protective clothing to avoid becoming infected in a highly contagious environment, the others so as not to contaminate the sensitive and often life-saving products manufactured at their workplace. It is between these two poles that the world itself turns – or stands still.

We are living in an age in which the corona virus governs virtually every area of public, and even much of private, life. The wave of canceled events, blocked deliveries and orders that has been piling up for weeks is unparalleled. What began with the (temporary) postponement of Chinaplas from April to August, and now seems a far distant prospect, also set off a chain reaction over here.

The precautions taken to combat the rapid spread of the virus put an end to countless trade shows, congresses and business meetings. Production was brought to a standstill locally as companies whose employees tested positive were quarantined and supply chains were disrupted; meanwhile the stock exchanges virtually went into free fall. It is as though the corona virus has choked off the world’s coronary arteries. We are becoming painfully aware of on what a fragile foundation our existence rests.

For our sector, there remains one realization: plastics processors are suddenly systemically important. Since, without a constant supply of respiratory masks, infusion sets, diagnostics equipment and other plastics products for emergency medical use – including protective clothing, of course – patient care faces collapse. Let us hope that it does not come to that!

Editorial - Doriat englisch

Doriat, Clemens, Dr.

Online Tool for an Easier Adhesive Search

Searching for the right adhesive is a very complex process. The wide range of manufacturers and products makes it hard to keep track. Users can find a suitable adhesive more easily and quickly with the Substratec search engine. Further functions have now been added to this search engine.

These days, even in the business world, products are often searched for online. For complex problems, however, this search, frequently via Google, quickly reaches its limits. The range of suppliers and products is just too confusing. Since 2016, the search engine Substratec (<a href="http://www.substratec.com">www.substratec.com</a>) has been offering a more targeted way to find the right adhesive. This year, the website is to be relaunched with the introduction of various new functions.

At the heart of Substratec is the substrate navigator. This is an online tool that makes it possible to search for adhesives according to various categories. Users first specify the two substrates they want to join. Then they can choose further parameters such as heat resistance, curing mechanism, and processing. Over 100 different substrates and 26 parameters are available.

On the basis of this selection, an algorithm determines which adhesives can be considered and how suitable these are. On the results page, users can see the possible adhesives displayed together with an assessment of how far, in percentage terms, they meet requirements. The basic data for the algorithm come from the technical data sheets of the adhesives and the information provided by the manufacturers. “We carry out a substrate comparison together with the manufacturers, in which we discuss how well an adhesive works with different substrates,” explained the founder of Substratec, Andreas Reisenzahn.

For the algorithm, the various parameters were also assessed for their relevance to the adhesive bond. Heat resistance, for example is therefore a KO criterion. If an adhesive fails to meet the specified expectations in this respect, it is automatically rejected. With other parameters, on the contrary, it is only downgraded.

Until now, the relevance of the criteria has still depended on the assessment of the Substratec creators. After the relaunch, however, users themselves can determine how important certain parameters are to them and what might even be KO criteria. “In the new version of Substratec, users themselves can determine which adhesive properties are key to them. While we may not regard, for example, the color of the adhesive to be particularly important, it may play a crucial role for many users and even be a KO criterion. In future, we would like to allow users to make this judgment themselves,” explained Reisenzahn. Once website visitors have decided on an adhesive, they can see additional information displayed on an overview page and download the data sheet. They also have the option of contacting the manufacturer via a form.

So far, some 400 adhesives from 30 different manufacturers, including names such as Weiss Chemie and Rampf, can be found through the search engine. This number is set to increase, thinks Reisenzahn, especially since the website has been available in English as well. International companies, in particular are showing great interest. Merz+benteli AG and Kisling AG, two Swiss companies, for example, now are added. The search engine’s database does not actually include every available adhesive and manufacturer. The Substratec business model is based on companies paying an annual fee for mention on the website. The fee is determined by how many products are listed. At present, companies can list a maximum of 20. This is intended to prevent a manufacturer from dominating the search engine.

Besides searching via substrates, it will also be possible to see suitable adhesives displayed for different application sectors such as automotive, medical technology or microelectronics. This application navigator is at present still in the beta phase but should be fully available from May. In addition, visitors to the website can see companies displayed by competencies and areas of application served. As well as these navigators already available on the website, further ones will be introduced during 2020. For example, one is planned for application technologies and machinery. With this navigator, users will be able to search for fully and semi-automated adhesive application machines and manual dispensing equipment. “At this point, crosslinking between the search engines begins. Website visitors searching for a specific adhesive will also receive suggestions as to how it can be applied. And vice versa, the application machinery display will show the adhesives for which the equipment is suitable. This gives users a comprehensive insight into the whole subject of adhesives,” said Reisenzahn, summing up the concept.

By the end of the year, a search engine for coatings is also planned. With this tool, users will be able to see coatings displayed for various substrates. Here again, it will be possible to refine the search using different parameters such as corrosion classes, and abrasion and chemical resistance. In 2021, a navigator for 3D printing will be added. This will include both materials and software tools as well as the actual printers. Reisenzahn believes the individual navigators complement each other, which therefore offers users of Substratec an extra benefit: “Where companies carry out adhesive bonding, they are frequently also involved in coating application, and vice versa. In 3D printing, too, post-processing is very important, often with coatings. There are very many synergies between these two sectors.”

The adhesives market is complex. The Substratec search engine promises a quick overview of possible products

Substratec - Hauptartikel

Beschichten von Kunststoffen

Oberflächentechnik

Chemisches Bearbeiten (Auftragen)

Kunststoffe

Photonik

Optik

Streifinger, Florian

Beschichtungen

Additive Fertigung

Klebstoffe

Verbindungstechnik Verfahren

Chemisches Fügeverfahren

Housings not only protect the inner workings of medical equipment, but also protect patients from electric shocks, for example. This function is very important for the increasingly widespread medical wearables, which are often in constant contact with the patient. When designing such devices, it is therefore important to select the appropriate housing material. It must not only be insulating and at the same time well accepted, but also allow an attractive and user-friendly design.

Housings not only protect the inner workings of medical equipment, but also protect patients from electric shocks, for example. This function is very important for the increasingly widespread medical wearables, which are often in constant contact with the patient. When designing such devices, it is therefore important to select the appropriate housing material.

Several trends in the modern healthcare industry are increasing the demand for connected devices that are in direct contact with the patient. Aging populations as well as an increase in chronic diseases represent important trends in healthcare that have brought added pressure to reduce costs and find new solutions.

Shifting therapies from clinics or hospitals to the home by using connected therapy systems is an attractive option. For such applications it is necessary to monitor both the patients and their medication remotely. The increased trend of patient monitoring is further exemplified by preventive patient monitoring with connected devices to screen body signals such as ECG, EEG or blood sugar.

The requirements for materials used for wearable devices depend on the function of the components. In less critical cases, such as fitness or wellness trackers, mechanical properties or freedom of design are important. For more critical cases such as for the injection of pharmaceuticals, dosage accuracy and consistency, sterilization and biocompatibility of the materials are additional critical considerations.

As such, it is not possible to realize wearable devices with just one technology or one material. In this article the author will summarize important considerations for selecting a housing material and how these can be linked up. In particular, the focus of the article is rigid housings made from polycarbonate (PC), a well-established material in both electronic as well as healthcare industries and therefore a material suitable for the combination of both areas. For all housings  in these applications, the following demands have to be met:

<ul><li>Protect the inner parts of the wearable (i. e. electronics or pharmaceuticals). This includes resisting the outside environment (moisture or chemicals) and the user of the device (e. g. impact resistance),</li><li>protection of the user from direct contact with electronic parts,</li><li>communication between the device and user. The housing can have an active and direct role such as implementing light signals, displays or an indirect role such as signal transmission to other devices like a smartphone,</li><li>provide an attractive and user-intuitive design to the device.</li></ul>

These aspects will be briefly discussed in terms of property requirements and polycarbonate capabilities in the sections which follow.

One of the most common materials from Covestro for electronic housings is Bayblend FR3010. The properties of this flame-retardant PC+ABS blends (polycarbonate + acrylonitrile butadiene styrene) are balanced for performance and productivity and suitable for many electronic applications. It has, for example, a tensile modulus of 2700 MPa, a yield stress of 60 MPa and an Izod impact strength of “no break” at 23 °C. But these numbers about mechanical properties are not enough to specify a good housing material, because protection of the inner parts is more than mechanical stability. The selection should be based on several aspects as mechanical properties, dimension and thermic stability, flame retardancy (i. e. UL listing requirements) and other properties such as processability during the injection molding and assembling.

One good starting point of the material selection process for wearable devices is the definition of the dimensional requirements. How thick can or should be the wall thickness be and how big are the different parts? This, in turn, can be important to selecting a material which is moldable at the chosen thickness. The trend for wearables is to reduce the size and weight to improve the wearing comfort and the user acceptance of devices. In addition to smarter battery concepts, lighter housings with thinner walls are used to reach this requirement. Wall thicknesses for small devices can even be below 0.5 mm, which mean small devices would favor ductile and rigid housing materials that can also fill extremely thin walls during molding.

The rigidity and toughness of the housing material is a special challenge since the first impression is that thick walls can withstand mechanical stresses better than thin walls. The standard DIN EN 60601–1 is specialized for medical equipment with power supplies, but the mentioned tests methods for mechanical properties can give a roughly guiding value for the mechanical requirements, which are needed for medical devices. Standard DIN EN 60601–1 mentions three different tests that assembled devices should be able to meet without significant damage.

First, an impact test with a continuous force of 250 N for 5 s; second, an impact test with a 0.5 kg steel ball dropped from at least 1.3 m on the housing and, third, a drop test of the housing itself from at least 1 m height. One material property which is related to the mechanical stability of the device is the notched impact strength at different temperatures. Materials such as Makrolon 2458 or Bayblend FR3010 do not break at 20 °C during this test and would be expected to satisfy this requirement at ambient conditions. Even more stringent impact requirements such as low-temperature multi-axial impact performance are more likely to require the impact performance offered by PC.

As mentioned earlier, designs with thin walls can also present a challenge from the manufacturing perspective. To fill a device with a wall thickness of below 1mm, it is necessary to use polymers with low melt viscosity. For example, the flow path of Makrolon 2458 polycarbonate for walls with 0.5 mm is only a third of the flow path for walls with 1 mm thickness. Although polycarbonates with low viscosity, and therefore a low molecular weight, could easily fill injection molding tools with low stress in thin-walled devices, there is a trade-off in terms of mechanical performance. The relationship between molecular weight (and therefore viscosity) and mechanical properties of polycarbonate is shown in Figure 1. A decrease in the viscosity will lead to a decrease of the mechanical properties, as represented with the Izod notched strength. For low molecular weight materials, increasing the wall thickness would be necessary to compensate for the lower performance. Therefore it is important to find the right balance between flow and robustness of the material and to combine this with adjusted injection molding process parameters.

Furthermore, the chemical properties of polycarbonate are also dependent on the molecular weight. Polycarbonates with longer polymer chains are more resistant against chemical attack. Simultaneously, internal stress (i. e. molded-in stress) has a significant influence on the chemical resistance of polycarbonate. Lower overall molded-in stress levels of the materials in the devices favor better chemical resistance. This means that the viscosity of a polycarbonate should be chosen wisely to get suitable mechanical properties without increasing the internal stress.

Wearables can often be exposed to a huge variation of chemicals such as skin lotions or disinfectants for cleaning. In addition to the nature of the chemical exposure, contact time and temperature are important. Long contact times, either due to a long product life or intensive exposure, or high temperatures can be challenging conditions for housing materials. But depending on polycarbonate grade, a low stress level and circumstances (contact time, composition, etc.) most typical disinfectants as alcohols, bleach, hypochlorite, peroxides and aldehydes should work.

For optimum resistance a low stress level (either applied or molded-in) helps ensure resistance to most liquids encountered by wearables, as e. g. sweat or lotions. However, many are complex formulations with changing compositions and therefore every individual liquid/polycarbonate material might behave different. Critical mediums for condensation polymers such as polycarbonates are mediums with high pH value (>10) and/or amines/ammonium salts. For applications that require better resistance to chemicals than PC or PC+ABS, combination of polycarbonate with semicrystalline resins (e. g. PC/polyester blend Makroblend) offer high resistance to oils and most cleaners.

Another parameter which is influenced by the wall thickness is the flame retardency. Standard DIN EN 60601–1 mentioned a necessary flame protection of V2 or better (UL94 V rating) for the thinnest wall thickness, but this standard is for medical devices with a permanent connection to a power supply. A new standard for wearables (DIN IEC 63203–101–1) is at the moment in preparation, but currently it is still unknown if any flame-retardancy requirements will be mentioned in this standard. If flame protection is necessary, it will depend on the power supply and the risk of damages for patients during a failure of the device. Fortunately, polycarbonate already has some inherent self-extinguishing characteristics (e. g. most grades can easily meet a V2 UL listing).

Further important material properties are the thermal stability and the dimensional stability. Thermal stability of a material can be important for production processes as well as for conditions of use of the device. Is it for example possible that the device could be forgotten inside a car at a hot summer day where temperatures as high as 80 °C can be reached. But also for solder reflow, often used to join electronic components on a circuit board (e. g. plastic camera lenses), can involve heating entire assemblies in ovens where temperatures as high as 150 °C can be reached. A thermally stable material can prevent warping or deformation during the solder reflow process. Makrolon polycarbonates are suitable up to 100 °C for permanent use and 121 °C for short time use and can easily meet such requirements. A material with high dimensional stability is important to maintain precise tolerances and stay together during the use of the device as temperatures fluctuate. Generally, a materials’ CLTE behavior is used to calculate potential dimensional changes and Bayblend and Makrolon have a low CLTEs of 0.75 x 10–4/K and 0.65 x 10–4/K, respectively. The high heat resistance of polycarbonate means that Makrolon’s low CLTE is maintained up to almost the glass transition temperature of 145 °C.

Thus, is highly advisable to select the material for the wearable housing at an early stage of the design process and to incorporate all aspects of the application, including manufacturing to help choose the housing material.

As implied by the word “wearables” these devices are in contact with the user and their skin. Therefore the task of the housing is not limited to protecting the inner components of the device, but should also protect the wearer from substances such as colorants, residual catalysts, leachable substances or other additives that could migrate from the housing. One of the relevant standards for the material selection for materials used in medical devices is ISO standard 10993. This standard describes the assessment of the biocompatibility of materials and divides the devices in different groups depending on the type and duration of contact with tissue.

For medical devices which contact only unharmed skin, it is necessary to test the cytotoxicity, which determines whether materials are toxic to biological cells (ISO 10993–5). The sensitization is also evaluated to detect the allergenic potential of a test article and if the material could generate any skin irritation (ISO 10993–10). Although these tests have to be done with the final device, it is useful to use materials which are known to already fulfill these biocompatibility requirements. Even wearables which might be classified as “wellness devices” and are not intended to be considered as medical devices are best designed with materials that minimize potential risk to the wearer (e. g. avoid potential allergic reactions). Thus, resins that are able to meet biocompatibility requirements of ISO 10993–5 and –10, such as Makrolon 2458 are advisable when designing new wearable devices.

Wearables are often connected with other devices as smartphones or special diagnostic devices. For this connection it is necessary to transmit signals through the housing, with minimal loss in signal. The quality of signal transmission depends on the material which is located between signal transmitter and signal receiver and the frequency of the signal. To describe the transmission quality through a material it is necessary to know the dielectric constant (Dk) and dissipation factor (Df) of a material at a certain frequency, which is given for several materials at 5.0 GHz in Table 1. The quality of signal transmission generally improves with low values of Dk and Df. Polycarbonate has low Dk and Df values and is therefore a good choice to maintain high signal transmission. This benefit is especially the case for higher frequencies (over 30 GHz) that are foreseen for big data transmission. As mentioned previously, weight reduction of wearables to improve customer satisfaction is highly desirable. Because low frequencies need less power but are also suitable for transmission of a low data volume, they are typically used for wearables. The frequency used will determine the reachable distance for signal transmission. For signal transmissions over several kilometers (e. g. for a tracker), frequencies below 1 GHz are possible. In contrast to that is Bluetooth at a frequency of 2.4 GHz is only able to achieve near-field communication.

Signal transmission with low frequencies and low power consumption need long antennas, which can be a challenge for small wearable devices. One option for these antennas are PCB or Chip antennas, which are placed inside the device. Both technologies need sufficient ground clearance, keep-out areas and no enclosure on-top, which is clearly impractical for compact consumer electronics devices.

In addition, internal structure features will occupy available space for antenna and complicate the pattern design. An innovative solution to this challenge is to integrate the antenna in the housing and to increase the size of the antenna pattern as well as save space for battery or other functions. One state-of-the-art technology for electronic integration is laser direct structuring (LDS) technology. For LDS, the device is molded out of a specially formulated resin, where the areas for the electric elements are integrated and activated by a laser on the housing surface. These activated parts can be afterward selectively coated with different metal layers.

An alternative technology is injection molded structural electronics (IMSE). For this technology the electronic circuits are first printed on a thin film. In a second step these films are reshaped and over-molded with the housing material to produce circuits embedded in the housing. A benefit of polycarbonate for IMSE is its high thermal stability, where connecting electronic components as chips or camera lenses with solder reflow can mean processes temperatures of over 150 °C.

For the design of wearable devices, including those with active interaction with the user, are many options possible with polycarbonate. The most basic elements are colors and surface structures on the housings. These basic elements have a huge influence on the first impression of the device and can even be critical in understanding how it should be used. Polycarbonate has a high gloss surface, which can help create a premium look. Furthermore polycarbonate allows for easy incorporation of additional surface features besides high gloss. Figure 2 shows examples of surfaces which are producible with polycarbonate. Structured surfaces can be useful to create intuitive design with tactile features for patients with impared vision, or to simply create an attractive appearance.

With modern technology several other design and communication elements are possible to integrate in device housings. For example, integration of LED technology can permit direct communication between the device and the user. For LED covers, it is important to have a high light transmission to make the LED light as visible as possible. At the same time, however, it is necessary to have a high degree of diffusion to minimize hot spots and keep the single LES lamps invisible (Fig. 3). Polymers with high transparency, such as polycarbonate, blended with special colors and light diffusers can achieve this effect. With these kinds of special colors it is furthermore feasible to create materials which are transparent at certain wavelengths. Such materials appear black, yet are transparent to infrared light and allow designers to conceal a sensor without limiting its usability. Other color effects, such as fluorescence, are furthermore possible and shown in Figure 4.

To cover elements with a smooth surface until they are used is one further potential of transparent polymers. Beside LEDs and sensors it is also possible to cover displays as a uniform black panel. This means that the display remains a uniform black surface until the display element is switched on. For these sophisticated displays, a combination of materials and processing know-how is necessary. The freedom of design with large uniform surfaces, however, is huge, and enables the creation of 3D formed displays and integration of functions such as touch panels into the housing. With these kinds of technologies it is possible to convert a simple housing in an active interface with the customer. This technology is particularly appealing for wearables and medical devices, because smooth surface without edges mean a device that is easier to clean and to disinfect.

A housing material for a wearable device is more than a simple plastic cover. With modern technologies such as integrated electronics or displays it is possible to create housings which represent  the surface of the device and the interface to the user. The foundation to this result is the correct material selection which fulfills the basic requirements as biocompatibility, mechanical properties and chemical resistance and maximizes the possibilities of different designs. To select the correct material based on the many requirements is a challenge, which has to be implemented in the development process of the device in an early stage and where experts are needed.

Fig. 1. Mechanical properties such as notched impact strength also depend on the viscosity of a polycarbonate (PC). PC with a lower viscosity also has a lower notched impact strength

Fig. 2. With medical wearables, it is not only the function of the housing that plays an important role, but increasingly also its appearance

Fig. 3. LED covers should be translucent, but also have a certain degree of diffusion so that the individual LEDs are not visible

Fig. 4. Colors in housings are not only used  for purely visual design, but can also take on additional functions 

Housings play an important role in medical wearables. They protect their inner workings, but at the same time protect the users of the devices

Table 1. Dielectric constant and dissipation factor for low frequencies of several polymers

Covestro - Hauptartikel

Ulrike Lützow works at Covestro Germany AG in Market Development Healthcare in the Polycarbonates segment.

Gehäuse aus Kunststoff

Elektromechanik

Kühlkörper

Medizintechnik

Polycarbonat

Kunststoff Werkstoffe

Thermoplaste

Medizinprodukte

New Ways for Production of Plastic Implants by 3D Printing

In no other field the possibilities and advantages of 3D printing are more pronounced than for medical devices. With the right high-performance materials, complex patient-customized implants can be produced that medical doctors are using to push the boundaries of the possible – for the benefit of patients.

It is said that those who know metals choose plastics. The dictum originated in mechanical engineering, where engineers meant that plastics often offer a more convincing alternative to metals. This paradigm change is now also emerging in medical field: innovative specialty materials and new technical possibilities in additive manufacturing are taking high-performance polymers to new heights.

Polymers have a long tradition in implants: ultra-crosslinked polyethylene is routinely used in artificial knees and hips as a tribological partner with metal or ceramics. The high-performance polymer PEEK (polyether ether ketone) has been supplementing titanium as a material for implants since the 1980s. Bioresorbable polylactide-based polymers have been used for more than 30 years in orthopedics as they are degraded naturally by the body, leaving no foreign matter behind.

Until today some challenges in production have been unresolved: standard processes are injection molding and – particularly for patient-specific implants – milling from semi-finished products, in which material losses of up to 80 % may occur.

Additive manufacturing now opens new ways for individualized production. And for implants the advantages are more pronounced than in any other area. A customized product can be produced for every individual patient. The breakthrough for additive manufacturing of plastics is now imminent, with three aspects coming in place:

<ul><li>Suitable materials of assured quality and with the corresponding documentation are available for use as implants.</li><li>Printing technology has improved to the point that high-quality parts can be reliably produced.</li><li>Appropriate for processes for production are set up in line with the quality management standards in the medical sector.</li></ul>

Now, since for the first time the boundary conditions are fulfilled, an alliance between raw material producers, printer manufacturers and medical device companies can bring this technology to a break through.

Evonik is one of the first companies in the world to offer 3D-printable medical-grade filaments for long-term implants. These consist either of the high-performance polymer PEEK, offered under the brand name Vestakeep, or of resorbable polymers of the Resomer brand family, which may contain polylactide as a constituent.

In order to materialize the potential of 3D printing in the medical field the collaboration of all players in the value chain is essential. Over the last few years, for example, Evonik has been working in conjunction with various printer manufacturers, who have succeeded in further improving their equipment for Fused Filament Fabrication (FFF).

An important finding from this collaboration is that printing strategy decisively affects the result (Title figure). Moreover, the bond strength between the individual print layers is crucial to the performance of the implant. This means that users will obtain good results from 3D printing only if producers of materials and equipment work closely together and share information.

Routine clinical practice must also be taken into account, as a collaboration between Evonik and Meditool illustrates. The Shanghai-based company, in which Evonik’s venture capital unit has made a direct investment, uses data from imaging techniques such as computer tomography to print PEEK implants. Its founding team has many years of experience in both medicine and industry; as a result, the joint development work can directly take into consideration requirements in the operating room as well as current academic and clinical research.

In Europe Evonik is collaborating closely with companies such as Apium Additive Technologies GmbH in Karlsruhe, Germany, and Kumovis GmbH in Munich, Germany. In addition to working in the area of implants, Apium is focusing on a possible application of additive manufacturing in dental medicine. Both companies offer dedicated printers for medical technology. Kumovis has developed a novel temperature and cleanliness management system that significantly improves the quality of 3D-printed components particularly in the area of high-performance plastics [1].

In conjunction with its partners, Evonik has been able to show that parts produced by additive manufacturing can now attain equal mechanical values with those of injection molded parts. A thorough understanding of the printing process and the specific features of the part to be printed is necessary to optimally exploit the diverse parameters of 3D printing. Customer feedback also indicates that – apart from the excellent printability – the interlayer bond strength is better for Vestakeep than for other PEEK polymers. Although PEEK is often regarded as a very rigid material, amazingly flexible structures can be produced by 3D printing, such as meshes that may open up new applications, for example in the cardiovascular area (Fig. 1).

In developing Resomer, Evonik has taken care to design the portfolio of bioresorbable molecules to be as versatile as possible. Polylactide (PLA), poly(lactide-co-glycolide) (PLGA), polycaprolactone, and polydioxanone serve as starting materials. In vivo resorption times ranging from a few months to many years can be attained this way. The filament portfolio also includes materials with the mechanical strength necessary for orthopedic applications such as PLA and PLGA (see Fig. 2 for a sample implant) as well as flexible plastics (such as polydioxanone and polycaprolactone) that can be used in regenerative medicine and as soft-tissue substitutes. An overview of the filaments available is given in Table 1.

3D-printed Resomer tensile bars hardly differ in their mechanical properties from their injection molded counterparts. Figure 3 shows some sample results and Figure 4 scanning electron micrographs of the surface. The properties depend decisively on the printing strategy and the equipment used. With a suitable choice of parameters, 3D printed parts achieve strengths similar to conventionally produced parts. Only in February 2020 Evonik launched PrintPowder, a bioresorbable Resomer that is the world’s first powder specifically for SLS (selective laser sintering) to be produced commercially under GMP conditions. This allows laser sintering systems to be used for the first time to produce complex bioresorbable implants for clinical use with precisely tuned mechanical properties.

Additive manufacturing, with its unique possibilities, supplements current conventional production methods. It consumes less material and opens the door to entirely new design possibilities that would not be accessible otherwise. This is evident, for example, in Evonik’s collaboration with the US company FossiLabs, which has developed special software to allow printing of specific porous structures. This produces 3D-printed implants (Fig. 5) on which bone tissue can grow and that therefore accelerate the growth of bone tissue into the implant after surgery, as is desired for example with intervertebral disk replacements in spine surgery or joint replacements.

Research Confirms Advantages of 3D-Printed Implants

An interesting phenomenon has been observed by researchers at the University of Basel, Switzerland [2]. In in vitro tests on parts printed by the FFF technique, it was found that bone-forming cells (osteoblasts) grew significantly better on the natural, rough surface after printing: after only five days, cellular activity on the printed parts was twice as high as on polished parts and three times higher than on sandblasted parts. This suggests that the healing process in the body can be accelerated by selectively modifying the surface properties of implants.

With resorbable materials, biocompatibility presents particular challenges for the 3D printing process because any embedded foreign particles would be released during the degradation of the material in the body. Work done jointly with the University of Hanover, Germany [3] shows that printed Resomer filaments are not cytotoxic and promote cell adhesion to the surface (Fig. 6).

Entirely new possibilities arise in 3D printing by combining different materials, for example by teaming Vestakeep, a permanent material with good mechanical properties, with the naturally bioresorbable Resomer (Fig. 7). Collaboration between raw material producers and printer producers and medical device companies is key here. 3D-printed polymer-based implants are already at the verge of clinical use. The CAMed (Clinical Additive Manufacturing for Medical Applications) research project coordinated by the Medizinische Universität Graz, Austria, in which Evonik is also participating, is preparing the world’s first clinical study in Austria for insertion of 3D-printed PEEK implants in humans. The project, which is being funded until October 2022, is thus an important step on the path to routine use of patient-customized implants produced by additive manufacturing.

1 M. Haerst, St. Leonhardt, L. Strauss, M. Eblenkamp: Hot Printer – dividualized Implants Made from High Performance Polymers via Additive Manufacturing. Kunststoffe internationl 04/2018, pp. 28–31

2 X. Han et al.: An in vitro study of osteoplast response on fused-filemant fabrication 3D printed PEEK for dental and cranio-maxillofacial implants, J. Clin. Med. 2019, 8, 771; DOI: 10.3390/jcm8060771

3 L. Raddatz et al.: Comparison of different three dimensional-printed resorbable materials: In vitro biocompatibility, In vitro degradation rate, and cell differentiation support, J. Biomater Appl. 2018, 33(2), 281–294 DOI: 10.1177/0885328218787219

Sample implants of Vestakeep filaments, printed with a 3D printer from the Chinese startup Meditool

Fig. 1. Flexible lattice structure of Vestakeep, printed with a 3D printer from Apium Additive Technologies

Fig. 2. Sample implant of Resomer filament

Fig. 3. Mechanical properties of printed and injection-molded Resomer materials

Fig. 4. Scanning electron micrograph of printed Resomer filaments based on polycaprolactone

Fig. 5. Microporous structures in a 3D-printed workpiece that accelerate the growth of bone-building cells

Fig. 6. Cell adhesion on the surface of poly(L-lactide) and polycaprolactone [3]

Fig. 7. Combined spinal cage (cage for the intervertebral space) made from the materials Vestakeep (exterior) and Resomer (interior) on the Kumovis printer

Name - kk_Evonik

Dr. Philip Engel is Business Manager Medical Systems at Evonik Resource Efficiency GmbH, Marl, Germany; <a href="mailto:philip.engel@evonik.com">philip.engel@evonik.com</a>

Thomas Perl is Project & Portfolio Manager at Evonik Resource Efficiency GmbH, Darmstadt, Germany; <a href="mailto:thomas.perl@evonik.com">thomas.perl@evonik.com</a>

Dr.-Ing. Cecile Boudot is Head Application Technology Biomaterials Medical Devices at Evonik Nutrition & Care GmbH, Darmstadt, Germany; <a href="mailto:cecile.boudot@evonik.com">cecile.boudot@evonik.com</a>

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New Possibilities for Use  in the Human Body

Medical Product Manufacturers Are Processing Original Materials Using the Freeformer

Arburg Plastic Freeforming is predestined for the additive manufacturing of medtech components. The associated Freeformer-type additive manufacturing systems process the same plastic pellets that are used in injection molding. That makes the open systems very economical to use. They can also process, among other things, biocompatible, resorbable and sterilizable, as well as FDA-approved original materials. This also opens up new possibilities for applications in the human body.

Arburg Plastic Freeforming is predestined for the additive manufacturing of medtech components. The associated Freeformer-type additive manufacturing systems process the same plastic pellets that are used in injection molding. That makes the open systems very economical to use.

As has now been widely demonstrated in practice, the APF (Arburg Plastic Freeforming) process now permits even challenging medtech applications that are not feasible by any other process. Thus, implant plates of the resorbable, FDA-approved original material Resomer LR 706 – a composite of poly(L-lactide-co-D,L-lactide) and β-TCP (manufacturer: Evonik Nutrition & Care GmbH, Essen, Germany) – can be manufactured, which can be used directly in the body on bone fractures (Title figure). The polymer composite imitates human bone and contains 30 % ceramic additives based on β-TCP (tricalcium phosphate). The component is thereby more stable, and also releases calcium to promote bone growth. After a predetermined time, the implant dissolved completely. The example shows how such applications are increasingly approaching the individual requirements of human medicine.

The Freeformer is available in sizes 200–3X and 300–3X, and builds up components layer by layer from extremely small plastic droplets. Its users can process their own original materials and perform their own process control optimization. Alternatively, they can access the database of the machine manufacturer (Arburg GmbH + Co. KG, Lossburg, Germany), where they can select and process the certified plastic pellet stocks that are also used in injection molding. Such materials, such as ABS (acrylonitrile-butadiene-styrene copolymer), amorphous PA (polyamide) and PC (polycarbonate), elastic TPU (thermoplastic polyurethane) and semicrystalline PP (polypropylene), but also PLLA (poly-L-lactide) and other original materials especially for medical applications.

Functional MedTech Parts with Anti-Inflammatories

Resorbable cranial, cheek and finger bones of medical PLLA (type: Purasorb PL18, manufacturer: Corbion NV, Amsterdam, Netherlands, or Resomer LR 708, Evonik) have the advantage that they do not need to be surgically removed after healing. In addition, the plastic pellets can be loaded with, e. g., anti-inflammatories to minimize rejection problems. In order for the material to dissolve in the body at the right time, it is very important to select the correct material grade. In addition, permanent implants, e. g. of PCU (polycarbonate urethane) are also manufactured by the AKF process. They are suitable, e. g., for use in spinal surgery.

Additively manufactured functional components are also finding growing applications not only in the body. The process is also predestined for medtech equipment and accessories. In 2018, Arburg presented the first ever processing of a medically approved elastomer (SEBS, styrene-ethylene-butylene-styrene, type: Cawiton PR13576, manufacturer: Wittenburg BV., Zeewolde, Netherlands), with a hardness of 28 Shore A. The very soft material is leaktight and tear resistant, and is suitable, e. g., for manufacturing functioning bellows.

The Freeformer is the only additive manufacturing system available to date that can process the FDA-approved TPE Medalist MD 12130H (manufacturer: Teknor Apex; hardness 32 Shore A), for example for flexible hand orthoses (Fig. 1) or for customizable respirator masks, whose seal conforms to the patient’s face contour. The degree of filling of the component can be selectively varied while retaining constant parameters, and the mechanical properties of the parts thereby modified. In the case of TPE, this results in e. g. different Shore hardnesses. Different material densities can also be adjusted within one and the same part.

With minor adjustments, all Freeformers are suitable for use in cleanrooms. They feature low emissions and are almost dust-free, and their build chamber is generally made of stainless steel. An optional robot interface allows additive manufacturing to be automated and the machines to be integrated into IT-networked manufacturing lines. The process quality can be reliably documented and the parts clearly traced if necessary.

Aesculap: MedTech Products  “in the Blood”

Typical examples of a medtech accessory are sawing templates of PA, which are used as customized operation aids. Aesculap AG in Tuttlingen, Germany, a subsidiary of B. Braun Melsungen AG, has already been using additive part manufacturing in medtech since 1993. The company has been working with a Freeformer 200–3X since February 2018. The Freeformer offers us clear advantages: processing of medically approved, resorbable PLLA; high-quality parts thanks to extremely thin layers and the option of manufacturing hard/soft combinations,” says Hans Keller (Fig. 2), Director for R&D Productions at Aesculap.

There is ever greater demand for specially adapted bone components and implants for individual patient treatment. Not least for reasons of cost, ever more medical products, such as sawing templates or trial implants, are being made of plastic. According to Hans Keller, “While the special materials cost us up to EUR 200 per kilo, the Freeformer allows us to process commodity plastic pellet stock, at only a fraction of the costs.”

Samaplast AG, a manufacturer of medtech and technical products with headquarters in St. Margrethen, Switzerland, uses a Freeformer 200–3X to manufacture extremely small series down to batch size of one. It also produces combinations from a prototype project through to OEM manufacturing. With the AKF process, it manufactures implants of FDA-approved PCU and resorbable materials such as polylactide or components of two materials with special geometries.

Thomas Mösli, assistant CTO at Samaplast, describes their first experiences with the Freeformer as follows: “We manufactured prototypes for our customers on injection-molding machines in the past. We now offer additive manufacturing of components with the Freeformer in addition, particularly in the medical sector.” For example, a permanent implant for the spinal region (Fig. 3), which is used to stabilize a slipped disk, was made from an FDA-approved PCU (type: Bionate; manufacturer: DSM Biomedical B.V., Geleen, Netherlands) in the various Shore hardnesses 90A, 80A, 55D, 65D.

Other tests involved parts made from two materials. For example, a spiral sheathed with a second polymer, two rods of different hardnesses combined or a resorbable polylactide manufactured.

All the trials clearly highlighted the advantages of the Freeformer. According to Thomas Mösli, “We can test new materials as well as unusual geometries rapidly and at low cost, without needing to develop and construct expensive injection molds. There is also the fact that implant materials are expensive. With the Freeformer, we can manufacture test products with low material consumption from commercially available, qualified injection moldable plastics. That makes us much more flexible.”

Karl Leibinger: Bioresorbable Implants for the Facial Area

Karl Leibinger Medizintechnik GmbH & Co. KG of Mühlheim an der Donau, Germany, a member of the KLS Martin Group, is a leading innovator in medtech. Since December 2017, the company has worked with a Freeformer to manufacture custom parts and implants at short notice. Different polylactides and polymer-metal composites are processed, to manufacture, for example, tensioning rods, prototypes and functional components. With the Freeformer, they are able to selectively influence the part properties, process their own plastics and optimize the process control themselves. Arburg’s material database immediately provides the necessary key data.

This is where the specialists at Karl Leibinger Medizintechnik can also see the particular strengths of the industrial additive manufacturing system. Frank Reinauer (Fig. 4), Head of Innovation and Production Biomaterials, says, “The possibility of processing a wide range of original plastic pellet stocks and using their properties for medical products makes our production just as flexible as the Freeformer.”

One product area is bioresorbable implants for the mouth, jaw and facial areas. They are used for osteosynthesis (surgical fixture to bone) and for reconstruction, and they degrade in the body over a period of up to 24 months. Such complex geometries can also be economically manufactured in small quantities on the Freeformer. The quality of the parts is comprehensively assured by means of geometrical and chemical analysis. Since personalization is becoming increasingly important in health care for implants directly adapted to the patients, Karl Leibinger, with the Freeformer, sees itself particularly well positioned here.

Röchling: Center for Additive Manufacturing

The Röchling Direct Manufacturing Center (RDMC) in Waldachtal, Germany, focuses on industrial additive series manufacturing and services such as design for 3D-printing. “Additive manufacturing has long achieved an industrial standard. With the RDMC, we are expanding our 3D printing expertise for the entire Röchling Group“, notes Dr. Axel Höfter (Fig. 5), one of the two managing directors of Röchling Direct Manufacturing GmbH. They want to manufacture new, complex geometries additively and equip new products with advanced functions. The focus here will be on industrial additive manufacturing of fully functional prototypes in series, as well as on design that is appropriate to the production process.

The AKF process is particularly suitable for additive manufacturing in medtech. Geometrical freedom combined with the wide material selection opens up entirely new plastic applications, including for applications in the human body. A focus of the Freeformer manufacturing system: the additive manufacturing of custom orthoses, implants from biocompatible standard pellet stocks and auxiliary means for surgical preparation. The machine can also be integrated into fully automated manufacturing lines.

Fig. 1. The medical-grade TPE Medalist MD 12130H can be  used to manufacture flexible, individually tailored hand orthoses 

Fig. 2. Hans Keller, Director of R&D Productions at Aesculap, sees the potential of the Freeformers e. g. for additive manufacturing of prototypes, trial implants and drilling templates

Fig. 3. With the Freeformer, Samaplast has additively manufactured a permanent spinal implant e. g. from FDA-approved PCU, for stabilization in the event of a slipped disk 

Fig. 4. Frank Reinauer, Head of Innovation and Production Biomaterials at Karl Leibinger Medizintechnik, welcomes the new possibilities that the Freeformer offers for manufacturing implants 

Fig. 5. Jens Harmeling (left) and Dr. Axel Höfter head the new Röchling Direct Manufacturing Center. In the global additive manufacturing center, Röchling also employs a Freeformer 300–3X

In the AKF process, resorbable original materials such a plate-implants similar to bone, can be manufactured. They are gradually replaced by endogenous tissue

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Lukas Pawelczyk is Head of Freeformer sales at Arburg GmbH + Co KG, Lossburg, Germany.

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Temperature Control with Water up to 230 °C as a Substitute for Oil

Processing of High-Performance Plastics without Purity Problems

In order to minimize the risk of contamination in the production of medical products, Scholz HTIK (High Tech in Kunststoff) decided to change its temperature control processes from oil to water as a heat transfer medium. As practical experience has shown, the P230S pressurized-water temperature control unit developed by Regloplas meets all the requirements for utilizing the better physical temperature control properties of water in the temperature range up to 230 °C.

In order to minimize the risk of contamination in the production of medical products, Scholz HTIK (High Tech in Kunststoff) decided to change its temperature control processes from oil to water as a heat transfer medium.

Semi-crystalline high-performance plastics, such as polyetheretherketone (PEEK) and (polyphenylene sulfide) PPS, which are high temperature-resistant thermoplastics, are nowadays used in high-precision applications in the fields of industrial components, medical technology, diagnostics and the automotive industry. Thanks to the high strength and durability of the plastic, these plastics are increasingly used as metal substitutes in the manufacture of medical implants.

In injection molding the temperature control process for plastics with melting points above 300 °C and mold temperatures greater than 200 °C was, until now, determined by temperature control units with oil as the heat transfer medium. With the development of the high-precision P230S temperature control unit with a patented, integrated pressurizer (manufacturer: Regloplas AG, St. Gallen, Switzerland), it is now possible to carry out the temperature control of injection molding machines, on which high-quality plastics are injected, with water as the heat transfer medium.

Compared with oil, water has double the specific heat capacity (describes the property of how much heat energy a heat transfer medium can transport) (Table 1). Another advantage is the low viscosity over the entire range of application compared with oil. With a system pressure of approx. 30 bar, water has the feature of becoming less viscous.

As a heat transfer medium, water also offers many advantages in its application: alongside at least double the heat transfer coefficient, it also offers a reduction of oil-dependent contamination. There is no evaporation of oil, which means higher purity in the machine as well as a significantly minimized risk of injury to the machine personnel in the event of leaking heat transfer medium.

Lower Flow Temperature,  Shorter Heat-up Time

Weighing these arguments, Horst Scholz GmbH & Co. KG, Kronach, Germany, decided to change its temperature control processes in the manufacture of medical products from oil to water. The mold sizes in production at owner-managed family business, which has maintained a business relationship with temperature control unit manufacturer Regloplas for more than 20 years, move in a range of 50 to 250 kg. An oil temperature of 236 °C must be configured in the temperature control unit in the case of a mold temperature of 220 °C with an oil temperature control unit, whereas a flow temperature of just 224 °C is needed in the device with water as the heat medium. The temperature control process is thereby shortened, which is especially evident in the heat-up time that is almost halved.

High-performance plastics such as PEEK are attractive alternatives to metal. Scholz uses the material for a wide range of applications. In the field of medical technology, Scholz produces rotors made of PEEK material for use in heart pumps. Due to the purity in the production process, the P230S temperature control unit is used in this application, which works with the temperature control medium water in the temperature range up to 230 °C.

Rotor and Bearing Cage for a Cardiac Support System

Miniaturized components, which are directly used in the bloodstream, belong to the highest risk class. This includes the various types of ventricular assist devices (VAD). This is a mechanical pump that supports the heart when it is too weak due to illness. Extremely high demands are placed on the durability of the rotor (Fig. 1), whose blade profile with a wall thickness of 200 µm must be able to withstand rotations of 30,000 to 50,000 rpm in use [2]. PEEK has various advantages in this case, such as its very low weight, high chemical resistance, simple sterilization, biocompatibility and high specific strength.

The rotor made of PEEK is used in a heart pump (Fig. 2) manufactured by Abiomed. With an outside diameter of just 4 mm, this is one of the smallest support systems in the world according to the manufacturer. A ball bearing cage (Fig. 3) is also used in the Abiomed heart pump. This component is also made of PEEK – in a single tool with seven radially aligned sliders. With an outside diameter of 2.3 mm and a molded part weight of 0.8 mg, it is the lightest injection-molded part that Scholz HTIK currently produces. A small component in a high-performance application – the processor also uses the P230S pressurized-water temperature control unit for this manufacturing process.

Water Treatment is Mandatory at  High Water Temperatures

With this temperature control unit, Scholz HTIK can now process the material PEEK in tools that are tempered with the heat medium water. This makes it much easier to maintain the necessary purity in the processing procedure. In the worst case, temperature control with oil can repeatedly lead to oil deposits in the entire machine area and make additional cleaning steps necessary. The maintenance cycles on the machine are also reduced due to less contamination. Water as a tempering medium offers a further advantage. Thanks to the heat transfer, which is more than twice as high as that of oil temperature control, extremely precise temperature control is achieved, which contributes to a stable production process.

Extreme conditions of over 200 °C and more than 30 bar of system pressure prevail in the pressurized-water temperature control unit during operation. Under these conditions, salty water causes accelerated corrosion. Reactions in the water are accelerated by the high temperature. Chloride in the temperature control medium attacks the passive layer of the metal surface, which in turn leads to increased corrosion.

Water treatment is essential for applications at higher water temperatures. This is generally the case at water temperatures above 180 °C. At temperatures above 200 °C, the use of treated water is vital. Contamination and calcification of the mold and device result in poorer performance through the reduction of heat transfer between the mold/heater and the temperature control medium.

Regloplas offers for that purpose a mobile water treatment system (type: waterCare) that provides water suitable for systems. The water is automatically fully desalinated and then manually treated with the appropriate anti-corrosive agent. Through additional filters, dirt particles – such as those from the mold – are also efficiently filtered out of the system circuit. Thanks to the mobile water treatment system, the overall cost of maintenance and cleaning of the temperature control units is greatly reduced.

Temperature control with water opens up new possibilities in the processing of high-temperature and special plastics. Regloplas is continuously developing its range of pressurized-water temperature control units. As the newest member of the family, the Swiss company recently introduced the P250M model, which reaches its initial temperature (250 °C) at a pressure of 40 bar.