## 5 Lattice Structures in Medical Devices and Applications
### Lattice Structures Using PAEK/PAEK Apatite Composites  
PAEK composite lattice structures play a key role in advancing spinal and other orthopedic implants by combining mechanical strength, biocompatibility, and design flexibility, all of which are crucial for effective function and recovery [115]. The porous lattice design, often enhanced with bioactive fillers like HA, closely resembles natural trabecular bone, improving osseointegration and tissue integration. The integration of HA into PAEK matrices to enhance osseointegration is a challenging, yet promising area of research. Achieving the uniform distribution of HA within these polymer matrices is critical for optimizing their performance in biomedical applications, particularly in implants where bone integration is essential [41; 136]. The traditional methods, such as injection molding, have been considered for this purpose, but they present limitations in ensuring homogeneity and stability of the composite material. Currently, there are insufficient data to pinpoint specific formulations or techniques that consistently deliver optimal results. The initial insights from the work of others suggest potential pathways for improvement, but these findings are preliminary and require further validation through systematic experimentation [28; 137]. Additionally, understanding the influence of HA content, particle size, and distribution on the mechanical properties and biological performance of PAEK and PAEK/HA composites will be vital to achieving reliable and reproducible outcomes [128]. The ability to precisely control the pore size, shape, and interconnectivity within the lattice allows for structures that mimic the bone's microarchitecture, facilitating efficient bone growth and enhancing implant integration with the surrounding tissue [138]. This is particularly critical in spinal implants, as it improves implant stability and accelerates healing, significantly reducing the risk of implant failure. PAEK's mechanical properties, when reinforced with agents like HA or carbon fiber, can be finely tuned to replicate the stiffness and flexibility of natural bone [41; 136]. The lattice structure optimizes these properties by better distributing mechanical loads, reducing the risk of stress shielding [139]. Optimizing the scaffold microstructure to balance bioactivity and mechanical strength without compromising either property is expected to involve a targeted series of experiments designed to evaluate these attributes. Given the lack of existing literature on this specific issue, employing Design of Experiment (DOE) methodologies or artificial intelligence (AI) could provide valuable guidance in identifying and refining an effective experimental approach [140; 141; 142; 143; 144]. These techniques may help determine optimal formulations or lattice structures to achieve the desired balance. The bone-like mechanical response of the lattice helps to prevent this, supporting natural bone remodeling and promoting long-term implant success [5; 145]. Surface modifications, such as coating or texturing, can significantly enhance the bioactivity and mechanical interaction of implants with the surrounding tissue by improving cell adhesion, promoting osseointegration, and enhancing mechanical stability at the implant-tissue interface [146]. These techniques are particularly effective in line-of-sight applications,where the uniform application of coatings or textures to the implant surface is feasible. However, in more complex structures such as lattice designs, the challenge of uniformly modifying internal surfaces arises [147]. In such cases, dip coating is one of the few viable methods, as it enables surface modifications to be applied uniformly across both the external and internal surfaces of the lattice [148]. This capability is critical for ensuring that the bioactivity and mechanical properties are consistently enhanced throughout the structure, particularly in areas where other methods may fail to reach or provide adequate coverage.