Post written by: Netaniah Pinto
1. Introduction
Figure 1. Medical PMMA by Novus
PMMA, or polymethyl methacrylate, is a polymer synthesized from methyl methacrylate monomers9. PMMA is a highly favored thermoplastic material for manufacturing and medical applications due to its exceptional impact resistance, lightweight nature and ease of processing23. Furthermore, PMMA is a thermoplastic material with an amorphous structure and exhibits a significant degree of biocompatibility. Consequently, PMMA has found extensive application in the medical field for a range of procedures such as intraocular implants, artificial teeth, and cranioplasty implants1. This article will examine the material and mechanical properties of PMMA, as well as its medicinal uses, in order to provide a deeper understanding of this filament.
2. Material and Mechanical Property
Figure 2. Chemical structure of PMMA (Image retrieved from Ramanathan et al., 2024)
PMMA is a transparent, colorless polymer that belongs to the acrylate family and has an amorphous structure. It has a molecular structure of (C5O2H8)n and chemical structure as seen in Figure 2. We can infer from its fundamental chemistry that PMMA can exist in isotactic, syndiotactic, or atactic forms. PMMA's glass transition temperature (Tg) and its solubility in water can be influenced by several factors such as its monomers, secondary and tertiary structures, and bond stereochemistry. Additionally, these factors can also affect PMMA's capacity to crystallise1.
PMMA exhibits a glass transition temperature range spanning from 100°C to 130°C and has a density of 1.20 g/cm3 at standard ambient temperature. It exhibits little change in UV radiation, rendering it highly resistant to UV rays1. Furthermore, it demonstrates exceptional thermal stability, withstanding temperatures ranging from -70°C to 100°C. It also possesses a refractive index of 1.490, which indicates exceptional optical quality and compatibility with human flesh1.
In terms of mechanics, PMMA is a highly durable thermoplastic that exhibits a high Young's modulus and a low elongation at break1. However, compared to other high-performance polymers such as polycarbonate (PC), PMMA is quite brittle and cracks easily26. While PMMA may showcase chemical resistance towards most laboratory chemicals’ aqueous solutions, it lacks a significant resistance to esters, ketones, chlorinated compounds and aromatic hydrocarbons and is susceptible to dissolution and swelling. This is mostly due to the presence of easily hydrolysed ester groups in PMMA6.
PMMA is very suitable for medical applications due to its combination of strength and lightweight properties. Moreover, it possesses exceptional durability and exhibits good impact resistance, surpassing that of polystyrene and glass24. In addition to a good impact strength, PMMA has a compressive strength of 85 to 110 MPa and a tensile strength of 30 to 50 MPa. Despite PMMA’s high levels of scratch and impact resistance, when subjected to a load or impact forces, it exhibits brittle behavior20. Table 1 summarises all of the major characteristics of PMMA.
Summary of the Major Characteristics of PMMA | |
Glass Transition Temperature | 102 °C (range 100-130 °C ) |
Density (room temperature) | 1.20 g/cm3 |
Melting Point | 130 °C |
Compressive strengths | 85 to 110 MPa |
Tensile Strength | 30 to 50 MPa |
Young’s Modulus | 2.4 to 3.3 GPa |
Refractive Index | 1.490 |
Water Absorptivity | 0.3% |
Table 1. Major characteristics of PMMA
It is worth mentioning that applications that prioritize properties like flexural strength, transparency, tensile strength, and UV tolerance over impact strength, heat resistance, and chemical resistance are better suited for PMMA2. PMMA is also a good substitute for polycarbonate due to its cost efficiency3. Moreover, PMMA is particularly well-suited for medical applications because of its absence of bisphenol-A (BPA), a compound present in polycarbonate. A study by Hafezi & Abdel-Rahman demonstrated that BPA is hazardous and can disrupt endocrine function, thus making the BPA free PMMA a more appropriate choice for medical use10. Lastly, PMMA has a light transmission rate of 92-93%, which is better than polycarbonate’s light transmission rate of 86-89%, making PMMA the best transparent material for surgery as it allows for better visibility13.
3. Clinical Applications of PMMA Filaments
Figure 3. Biomedical Applications of PMMA (Image retrieved from Ramanathan et al., 2024)
As illustrated in Figure 3, PMMA has been widely employed in a diverse range of biomedical applications, spanning from ophthalmology to dentistry. PMMA has demonstrated exceptional biocompatibility due to its high tolerance to the temperature stresses and chemical reactions that occur within human tissue environments27. Owing to its exceptional bio- and hemocompatibility, as well as its ease of manipulation, PMMA has found widespread utilization in various medical devices, such as blood pumps and dialyzers17. Furthermore, the material's favourable optical characteristics render it suitable for use in implantable ocular lenses and rigid contact lenses. In addition, PMMA's advantageous physical properties and colouring capabilities have also proven beneficial for denture production17.
Dental Applications
Figure 4. and 5. PMMA Prosthodontics (Retrived from Subadental, 2021)
Due to its excellent durability and aesthetic qualities, PMMA filaments are commonly used for dental prostheses and orthodontic appliances such as denture bases, orthodontic retainers, artificial teeth and provisional crowns (Figure 4 and 5)28. For instance, some dentists will add a rubber-based copolymer to PMMA to create artificial teeth with increased strength (Hassan et al., 2019).
Additionally, the surface chemistry of biomaterials plays a crucial role in the healing process surrounding dental appliances. A recent study by Zhu et al. using high-resolution X-ray photoelectron spectroscopy (XPS) revealed the presence of nitrogen on the surfaces of teeth. Since PMMA does not inherently contain nitrogen, it is likely that adding diazonium compounds to the PMMA surface could facilitate improved interaction between the biomaterial and gingival epithelial cells, which are important for the healing process29. A study by Abdallah et al. also discovered that amination changed the chemical makeup of PMMA's surface, which made it better for teeth because it is better at interacting with oral fibroblasts and gingival epithelial cells12. Furthermore, in comparison to metallic denture base materials, PMMA possesses advantageous characteristics including aesthetic appeal, reduced weight, convenient handling, and cost efficiency, making it an excellent biomaterial for dental use12.
Figure 6. 3D Printed PMMA Implant for Gingival Smile Treatment (De Castro et al., 2023).
PMMA has also been emerging as a popular choice for dental implants. In a recent case study, De Castro et al. utilised PMMA to create customised 3D printed implants for the treatment of excessive gingival display, commonly referred to as a "gummy smile"5. The researchers reported the case of a patient presenting with a high degree of gum exposure while smiling, a condition that required intervention5. To address this issue, the clinicians performed a vestibular incision subperiosteal tunnel access (VISTA) procedure, which provided access for the subsequent insertion of the custom-designed PMMA implant, as illustrated in Figure 65. Following the surgical intervention, a 24-month postoperative evaluation was conducted, which revealed a significant improvement in the patient's gingival display and overall smile aesthetics5. The successful integration and long-term functional outcomes of the PMMA implant demonstrate the material's suitability and versatility in the field of oral and maxillofacial surgery, highlighting the potential of 3D printed PMMA implants in addressing complex dental and gingival aesthetic concerns and contributing to the advancement of personalized treatment approaches in the dental healthcare domain5.
Medical Applications
Figure 7. Cranioplasty Implant Printed using Novus PMMA
Various medical applications extensively use PMMA filaments. PMMA filaments are suitable for 3-D printing of permanent implants and medical equipment, offering a precise and customizable option for different orthopedic and craniofacial treatments. In addition, as shown in Figure 7, PMMA can be used to create cranial implants for cranioplasties.
In a recent case study, Da Silva et al. documented the treatment of a patient who had sustained a craniofacial defect following a motor vehicle accident, which necessitated a decompressive craniectomy procedure4. To address the resulting osseous defect, the researchers employed computed tomography (CT) imaging to generate a 3D prototype of the patient's craniofacial anatomy4. This 3D model was then utilized to fabricate a customized PMMA implant, which was subsequently surgically implanted4. Post-operative analysis revealed an uneventful recovery process and good aesthetic outcomes for the patient. This case study highlights the great potential of PMMA in cranial and maxillofacial surgical interventions, as the material's exceptional biocompatibility facilitates favourable healing and recovery, while its tailorable properties enable the achievement of superior aesthetic results4.
Other than its cranial applications, PMMA also serves as a material that enables us to investigate a wide range of cancer or bacterial linked diseases. Recently, PMMA was utilized in a study to create dosimeters for the purpose of investigating and visualizing the distribution of radiation dosage in radiotherapy, showcasing its versatility in research use8. Moreover, PMMA's favorable optical characteristics and established biocompatibility make it well-suited for the manufacturing of intraocular lenses used in cataract surgery11. Beyond its application in ophthalmology, PMMA filaments are highly versatile and can be leveraged to create a diverse range of prosthetic devices, including artificial limbs11. As for PMMA’s use in 3D printing, the filaments' exceptional strength, resilience, and malleability enable it to be convenient for printing, rendering them an invaluable material for this purpose. Therefore, due to PMMA’s robustness, transparency and affordability, the filament has become a practical option accessible to a diverse spectrum of patients.
4. Sterilization Methods
PMMA may be sterilized using a wide range of processes, however there are certain methods that it is not compatible with. For instance, thermal sterilization methods involving dry heat or steam, such as autoclaving, should be avoided for the sterilization of PMMA. The high temperatures and pressurized steam used in autoclaving processes can exceed the glass transition temperature of PMMA22. Exposing PMMA to these conditions can result in material deformation or exfoliation due to the thermal stresses induced, therefore, the use of autoclaving for PMMA sterilization should be circumvented22.
Nevertheless, alternative sterilization methods such as ethylene oxide (EtO), hydrogen peroxide gas plasma (HPGP), and γ-irradiation are appropriate substitutes. A study by Münker et al revealed that HPGP and EtO had no substantial impact on the material properties of PMMA. However, it was unexpectedly observed that γ-irradiation enhanced the flexural strength of the materials16. The observed enhancement in flexural strength is believed to be associated with a modification in the material's wettability, rather than an increase in crosslinking. Previous studies indicate that γ-irradiation can reduce the molecular weight of PMMA by causing chain scission, which generally leads to a deterioration in mechanical properties16. The observed increase in flexural strength in the water-immersed samples after γ-irradiation can be attributed to a decrease in hydrophilic side-groups, resulting in a stronger material compared to the control. Hence, it is crucial to meticulously contemplate the choice of sterilization methods for PMMA-based medical equipment in order to guarantee the preservation of their intended characteristics and functionality16.
In addition, EtO sterilisation is a good candidate for PMMA sterilization as it works well for heat sensitive materials. It is important to note that EtO sterilisation requires users to follow ISO11135 and ISO10993-7 standards for safe usage25. Moreover, EtO sterilisation chambers come in a variety of sizes, hence, each company can simply find one that suits its manufacturing needs25. Lastly, Hydrogen peroxide is another good sterilization method as it is non-toxic and exhibits strong microbicidal properties against a wide range of bacteria. Hydrogen peroxide plasma generates large quantities of free radicals for sterilization while allowing PMMA to remain biocompatible 16.
5. A Brief Introduction to PMMA’s Printing Process
The recommended temperature range for 3D printing with PMMA filaments is 230°C to 250°C. The optimal temperature for the bed of the 3D printer is approximately 60 °C7. Printing at lower temperatures can result in the end product warping and shrinking7.
Figure 8. 3D printed Cranioplasty Implants by Novus
It is crucial to acknowledge that the liquefied PMMA filament releases gases during the 3D printing procedure that can pose health risks if breathed in. In order to tackle this issue, it is imperative to guarantee proper ventilation in the 3D printing space, and the user should refrain from staying in close proximity to the 3D printer for prolonged periods while it is in operation. In addition, slow printing is the secret to success, as PMMA is not suitable for high-speed printing. The optimal printing velocity is 30 mm/sec or below, as it allows for enhanced fluidity and overall printing quality20. Implementing these measures can effectively mitigate any potential health issues related to the emissions produced during the PMMA 3D printing procedure.
3D Printing using Novus PMMA
Ossfila's PMMA is ISO 10993 and USP Class VI certified, indicating its suitability and biocompatibility for medical applications14. The ISO 10993 certification signifies that Ossfila's PMMA has undergone the required testing to demonstrate general biological safety and compatibility for use in medical devices, as per the specifications established by the international standard15.
A recent study by Obaeed and Hamdan investigated the use of Ossfila's PMMA in the production of a cranial implant for cranioplasty18. Typically, cranial implants are produced by first creating a polylactic acid (PLA) mould, into which the PMMA is then cast. However, this study explored a novel approach of directly 3D printing the PMMA implant without the need for a PLA mould18.
Figure 9. (a) PMMA specific cranial implant, (b) PLA side skull defect section, (c) assembly of the specific cranial implant and skull (Obaeed & Hamdan, 2024).
The researchers utilized a standard STL file format to obtain the necessary surface geometry, which was then uploaded to a QIDI X-MAX 3D printer. The PMMA implant was created using the fused filament fabrication (FFF) principle, whereby the thermoplastic material was deposited layer-by-layer to create the final product18. The study's findings demonstrated the successful reconstruction and manufacturing of a high-quality cranial implant using medical-grade PMMA as the primary material (Figure 9). Post-production, both quantitative and qualitative analyses were performed, confirming the perfect fit and attractive appearance of the implant18. This highlights the versatility and suitability of Ossfila's PMMA for cranial implant production, as it was able to create a high-quality transplant using this novel technique18.
Fused Deposition Modelling using Novus PMMA
A study by Obaeed and Hamdan examined the fused deposition modelling (FDM) printing parameters when utilizing Ossfila's PMMA19. The study found that decreasing the layer height during FDM significantly increased the flexural strength, a critical mechanical property. This indicates that layer thickness is a crucial FDM parameter, as it directly impacts interlayer bonding, anisotropy, and overall mechanical performance19. To manufacture 3D-printed PMMA products with excellent flexural strength, the researchers emphasized the need to optimize the balance of layer thickness, infill density, and raster angle parameters19.
6. Conclusion
In conclusion, PMMA emerges as a versatile and highly favoured thermoplastic material with a prominent foothold in the medical field. Its transparency, coupled with exceptional strength and lightweight properties, render PMMA a compelling candidate for diverse surgical applications1. Furthermore, PMMA's status as a more economical thermoplastic option compared to alternatives such as PC enhances its accessibility and affordability for a broader patient population. Presently, the primary applications of PMMA include cranioplasties and the fabrication of dental prostheses; however, the material's versatility has facilitated its expansion into novel research domains, such as the development of dosimetric devices. Consequently, medical professionals have come to highly value the utilization of PMMA filaments in 3D printing processes, owing to their customizability, durability, biocompatibility, and ease of manufacturing. Regarding sterilization methods, autoclaving should be avoided, whereas ethylene oxide (EtO) sterilization is a commonly utilized and effective approach. Overall, the findings underscore PMMA's potential as a reliable and efficient material within the realms of 3D printing and medical device manufacturing, paving the way for continued advancements in healthcare technology and patient care.
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