For manufacturing aligners with shape memory, techniques such as compression molding and the fusion and crystallization of polymers are required, which may involve bonding layers or components using heat and pressure. Below, I explain how these processes are applied specifically in the patent's manufacturing process, including the importance of glass transition temperature (Tg).
Compression Molding
Compression molding is used to shape shape memory polymers (SMP). This process ensures that the polymer retains the necessary mechanical and thermal properties for its final application. In devices like dental aligners or other medical products, sheets of SMPs are molded using compression at controlled temperatures.
Key Steps in the Compression Molding Process:
Preparation of the polymers: The base polymers, such as PBA (poly(4-butylene adipate)) or PCL (polycaprolactone), are heated until they reach their melting point. At this stage, the Tg plays a crucial role. PBA typically has a Tg of around -50°C, while PCL has a Tg of about -60°C. These low Tg values ensure that the polymers remain flexible at body temperature, which is important for the comfort and adaptability of dental aligners.
Compression in the mold: Once melted, the polymers are placed in a mold, and high pressure is applied to ensure that the polymer is evenly distributed and takes the desired shape, such as a sheet for a dental aligner. The Tg also determines when the polymer transitions from a hard, glassy state to a more rubbery and flexible state, crucial for the shape memory function.
Cooling and solidification: After molding, the material cools down, allowing it to crystallize and solidify, ensuring that it retains its final shape and required mechanical properties, such as elasticity and the ability to remember its shape. During this cooling phase, the temperature must remain below the Tg to ensure the polymer stays rigid until reheated for shape recovery.
Fusion and Crystallization of Polymers
After compression molding, the process of fusion and crystallization becomes critical for controlling the polymer’s shape memory properties. These steps ensure that the polymer can be deformed and later recover its original shape when heat is applied.
Key Steps in Fusion and Crystallization:
Polymer fusion: The material is heated above its melting temperature (usually between 37°C and 65°C, depending on the type of polymer) so that the polymer chains become mobile. In this state, the polymer is malleable and can be deformed or molded. However, care must be taken to avoid exceeding the Tg by too much, as this could lead to premature crystallization or unwanted deformation that could compromise the precision of the device.
Crystallization: During cooling, the polymer crystallizes, meaning the molecular chains rearrange into a more ordered and rigid structure. This step is essential for the polymer to maintain its original shape, which can later be recovered when reheated. The Tg plays a key role in controlling the cooling rate and ensuring that the polymer solidifies without remaining too flexible.
Control of crystallization temperature: The crystallization temperature can be adjusted within a specific range to ensure the device functions properly in the human body. By carefully controlling the temperature between the Tg and the melting point, the polymer’s mechanical properties can be optimized for orthodontic use.
Bonding of Layers or Components Using Heat or Pressure
During the manufacturing process, it is often necessary to use multiple layers of polymers or other components. These layers can be fused together using heat and pressure, ensuring the integrity and performance of the final product.
Polymer layers: Different sheets of polymers can be fused together during the compression molding process or via techniques like extrusion. The Tg is important here, as the layers must be heated above their glass transition temperature to ensure proper bonding without losing structural integrity.
Combined materials: In some cases, polymers with different properties (such as a more rigid polymer and a more flexible one) are combined to create a composite material that provides the desired characteristics. These materials can be processed together to create a device that offers both flexibility and strength.
Specific Application in Aligners
The use of shape memory polymers is especially beneficial for applications like dental aligners or medical devices that require the ability to change shape and then return to their original form. Compression molding and layer fusion using heat allow these devices to be manufactured with precision, enabling the following capabilities:
Maintaining their permanent shape (the original design).
Being temporarily deformed (when applied to the patient or device).
Recovering their original shape when heated to the appropriate temperature (using a device like the ClearX "booster" for reheating and reshaping).
The Tg ensures that the aligners remain rigid until reheated, allowing them to gradually adjust the position of teeth over time due to the shape memory properties of the material.
Composition of the Layers
The aligners are made using specific thermoplastic polymers and shape memory polymers (SMP) that are layered or combined to achieve the desired mechanical properties. These include:
Poly(4-butylene adipate) (PBA): This polyester is used for its shape memory properties, flexibility, and ability to crystallize during cooling, which maintains the shape memory effect. Its low Tg contributes to its flexibility at body temperature, making it ideal for aligners.
Polycaprolactone (PCL): A biodegradable polyester that is biocompatible and flexible. Its low Tg ensures flexibility during use but also maintains rigidity when needed for form retention.
Polyurethane (PU): Combined with the polyesters to form hard and soft segments, providing both structural stability and flexibility. The hard segments contribute to rigidity, while the soft segments allow for deformation and recovery.
Modifiers and nucleating agents: Fatty acids like dodecanoic acid or other agents can be added to adjust the crystallization and melting temperatures of the polymer layers, optimizing their performance for dental aligners or other medical devices.
Summary
In summary, the described process uses shape memory polymers that undergo compression molding and fusion to create flexible yet durable dental aligners. The Tg of the materials plays a critical role in ensuring that the polymers maintain flexibility during use and can retain their original shape when reheated. These processes are essential for creating medical devices like dental aligners, which can temporarily change shape and then recover their original form, making treatment more effective. This type of process can be adapted to devices such as those used by ClearX.
Bibliography
1 Elshazly, Tarek M., Ludger Keilig, Yasmine Alkabani, Ahmed Ghoneima, Moosa Abuzayda, Sameh Talaat, and Christoph P. Bourauel. 2021. "Primary Evaluation of Shape Recovery of Orthodontic Aligners Fabricated from Shape Memory Polymer (A Typodont Study)" Dentistry Journal 9, no. 3: 31. https://doi.org/10.3390/dj9030031
2 Weder, Christoph, Anuja Shirole, Carlo Perotto, Julia Lonsky, and Konstantin Silberzahn. Shape Memory Polymers. US Patent 11643550B2, issued May 9, 2023. United States Patent and Trademark Office. Accessed September 23, 2024. https://patents.google.com/patent/US11643550B2.
Comments