## 5 Lattice Structures in Medical Devices and Applications
### Design Criteria for Cellular Scaffolds in Spinal Implants  
For a spinal implant to successfully integrate with surrounding bone tissue and promote vascularization, it must closely mimic the properties of the extracellular matrix (ECM) found in natural tissue. This involves achieving a delicate balance between the implant's functional, morphological, and structural characteristics [88; 89]. The ECM provides a biological framework that supports cell attachment, differentiation, and tissue growth, all of which are crucial for osseointegration. To replicate the ECM, a spinal implant must first meet specific mechanical and structural requirements. It must offer sufficient strength and stability to support the spinal column under physiological loads, ensuring that the implant can bear weight without causing stress shielding [88,89]. At the same time, the implant should possess a porous microstructure that mimics the natural morphology of bone, facilitating the migration of bone cells (osteoblasts) into the scaffold, promoting tissue regeneration. Morphologically, the implant's surface texture and porosity play key roles in encouraging cell adhesion and proliferation. A porous structure not only enhances mechanical interlocking between the bone and the implant, but also allows for the infiltration of blood vessels, essential for delivering nutrients and removing waste products during the healing process [88,89,127]. Vascularization is a critical factor in bone regeneration, as it supports the formation of new bone and ensures the long-term stability of the implant. Moreover, the implant must also achieve a functional balance, wherein it supports both the mechanical load of the spine and the biological processes necessary for healing. This includes ensuring that the material used for the implant is biocompatible, promotes bone growth, and minimizes the risk of adverse reactions such as inflammation or infection. Advanced materials like PEEK and bioactive coatings such as HA have been explored to address these challenges, providing implants with both the mechanical strength required for spinal support and the biological compatibility needed for integration with bone tissue. The successful design of spinal implants requires a comprehensive approach replicating the ECM's functional, morphological, and structural roles. Achieving this balance is the key to promoting the effective osseointegration, vascularization, and long-term stability of the implant. Ongoing advancements in biomaterials and 3 DP technologies further enable the development of implants that better replicate the natural ECM, improving patient outcomes after spinal fusion and other orthopedic surgeries. The key characteristic in scaffold design is biocompatibility, which supports natural cell growth on the implant's surface. Biocompatible materials enable cells to adhere, proliferate, and differentiate, forming new tissue around the implant [127]. Equally important is the mechanical alignment of the implant with the surrounding bone, as the mechanical properties directly affect how the load is distributed [128]. If the implant is too stiff, it can lead to stress shielding, where the implant bears too much of the load, causing the nearby bone tissue to weaken and resorb. Conversely, if the implant is too flexible, it may not provide the structural support needed for healing. Thus, optimizing mechanical compatibility is essential not only for load transmission, but also for facilitating tissue regeneration [88,127,128]. Porosity is another critical factor for the long-term success of spinal implants. A suitable level of porosity allows for cell growth, creating channels that support the transportation of nutrients and waste [106,129,130]. This feature is vital for promoting vascularization and the overall health of the newly formed tissue. Additionally, the porosity of the implant enhances its osseointegration capabilities [129]. Implants with an open cellular structure provide more surface area for cells to attach, which encourages bone growth [130].