## 6 Current Challenges and Future Prospects
### Present Obstacles Faced by AM Technologies
#### 6.2.1 Bioprinting
The consumer products industry might see self-assembling products and programmable textiles, while robotics could benefit from smart systems that self-repair and adapt to new situations. Addressing material characteristics, process management, scalability, and cost through ongoing research and development is vital. With continued advancements, 4D/5D printing could transform industries, lead to new inventions, and create adaptive, responsive, and intelligent objects [46; 136; 137]. To fully capitalize on these innovative prospects, attention must now shift toward refining 4D printing technologies to ensure efficiency and sustainability in real-world applications. The concept of 4D printing, initially introduced by Prof. [PERSON] in 2013, incorporates smart materials into 3D-printed structures, allowing them to undergo controlled changes over time in response to environmental stimuli such as pH, temperature, or moisture. This dynamic evolution of shape, structure, or functionality sets 4D printing apart from conventional 3D printing. In tissue engineering, 4D bioprinting offers substantial advantages, particularly by enabling bio-structures to adapt their functional properties, a feature critical for applications such as bone tissue regeneration. [PERSON] and colleagues applied 4D printing techniques to develop bone tissue scaffolds that replicate the hierarchically porous structure of bone, enhancing both osteogenesis and angiogenesis. By utilizing type I collagen and hydroxyapatite (HA), they created a biocompatible scaffold that supported bone tissue regeneration. Additionally, 4D-printed polylactide (PLA) scaffolds infused with hydroxyapatite demonstrated self fitting properties due to their shape-memory capabilities, furthering their applicability in tissue engineering. In the context of spinal fusion, 4D-printed scaffolds have shown significant potential. A layered scaffold composed of type I collagen for flexibility and hydroxyapatite for bone stiffness was designed to match the curvature of the spine after hydration. This scaffold successfully supported the growth of bone marrow stem cells and the formation of a new bone matrix in vitro, and it promoted bone tissue formation in vivo, highlighting its potential as an effective approach for spinal fusion surgery. Building on the advancements in 4D printing for bone tissue engineering, a key area of focus is the development of materials for constructing scaffolds that support bone regeneration. Hydroxyapatite (HA), known for its bioactivity, is often combined with polymers to enhance both mechanical strength and biological functionality. The following section explores the materials used in 3D and 4D printing of HA-based scaffolds, highlighting innovations in creating biocompatible and mechanically robust structures for tissue engineering. Hydroxyapatite (HA) is a ceramic material recognized for its bioactivity, making it highly suitable for skeletal regeneration applications. However, due to its brittleness, HA is often combined with polymers to enhance its mechanical properties while maintaining bioactivity. These HA-based composites are extensively used in orthopedic and dental implants, promoting tissue growth through their excellent biocompatibility and osteoinductive properties. Incorporating polymers such as hydrogels with HA creates composite scaffolds that closely mimic the extracellular matrix, providing a conducive environment for cell proliferation and drug delivery. These scaffolds demonstrate potential in bone tissue engineering due to their structural and mechanical improvements. Natural polymers like collagen, chitosan, alginate, and hyaluronic acid have also been used in HA-based composites for bone regeneration. Collagen-HA composites enhance mechanical strength and promote osteoblast differentiation, while chitosan-HA scaffolds improve cell viability and osteogenic activity.