While in first part of the post While 3D printing in dentistry has been briefly described in general, this article takes a closer look at the printer materials and follow-up.
Marcel Reymus, Annett Kieschnick, Alexander Prechtel, Bogna Stawarczyk
PRINTER MATERIALS
The neuralgic point in 3D printing is the dental materials. Resins, thermoplastics or metal powders (SLM) are generally used.
Resins
Resins for polymerizing structures are liquid materials that are predominantly based on UDMA monomers (dimethacrylates) or oligomers, which are provided with initiator systems and other additives. Some manufacturers sometimes add ceramic fillers to a small extent. These resins are to be viewed as chemically critical with regard to their use in the oral environment. The many exclamation marks on the bottle alone warn of allergic reactions and skin and eye irritation when unpolymerized. Some resins are even declared to be environmentally hazardous and corrosive when unpolymerized.
It's worth taking a quick look at their safety data sheets and the classes of the Medical Devices Act (MPG). Materials that are used intraorally are classified according to I, IIa, IIb and III (Directive 93/42/EEC). An important feature is, among other things, the type and duration of application in the mouth. The majority of the printer materials on offer are assigned to MPG class 1 - continuous use in the mouth is limited to 29 days. Dentures must meet the requirement of “long-term use” and the materials are therefore assigned to MPG Class IIa (uninterrupted use in the mouth over a period of more than 30 days).
thermoplastics
Thermoplastics, which are processed using filament technologies, are monomer-free and therefore harmless. For permanent dentures, it is necessary to pay attention to the quality of the material – “medical grade” (requirement for plastics as a medical product). Thermoplastics include all polyacrylic ether ketone materials, such as: B. PEEK or PEKK. PPSU (polyphenylsulfone) and PC (polycarbonate) can also be thermally processed to create a restoration. Temperatures in the printing nozzle of up to 400° C are sometimes necessary here. By melting the material, the mechanical properties can be changed - compared to millable blanks. For this purpose, further tempering processes are often carried out directly during printing. The exact parameters are carefully worked out by the manufacturers and are usually kept secret. However, these materials and printing techniques are only just beginning to emerge in the dental industry.
3D printing: post-processing
In most cases, additively manufactured objects require some post-processing. Support structures must be removed and the surface polished if necessary. For objects made of photopolymers that have been reproduced using SLA or DLP technology, correct post-exposure is crucial. This is the only way to ensure biocompatibility and mechanical properties. The polymerization chambers offered on the market for this purpose are mostly known from plastic veneers.
The devices differ in the light emitters used. The NextDent company recommends using the company's own LC-3DPrint Box for its materials. This works with several UV lamps that cover wavelengths from 315 to 550 nm, with peaks around 360 and 435 nm. The company GC is represented with the Labolight DUO. This polymerization chamber is based on LED and covers wavelengths from 380 to 510 m with peaks around 395 nm and 475 nm. The Otoflash G171 from NK Optik (Baierbrunn, Germany) works with high-energy flash lights. The special thing about this device is that it can work under a protective gas atmosphere (nitrogen), which is intended to prevent the formation of an oxygen inhibition layer. This device covers wavelengths from 300 to 700 nm, with peaks around 480 and 530 nm. The initial investigations clearly show an influence of post-polymerization on the mechanical properties.
Good prospects
We will encounter interesting developments in the coming years, particularly in the dental sector. At the Additive Material Exhibition 2018, the Chinese technology company Prodways presented a ceramic printer that, according to a press release, could be of particular interest to the medical and aviation industries. The printer can print objects using ceramic materials such as aluminum oxide, zirconia and hydroxyapatite. Other companies are already present on the market with ceramic printers. It seems a matter of time that e.g. B. ceramics or zirconium oxide can also be printed for permanent dentures.