## 1 Introduction
### Implant Device Effect on Fracture Healing Process  
The duration and the outcome of the fracture healing process is significantly influenced by the implant device. [PERSON] [6] demonstrated that osteoblasts exhibit a differentiated phenotype when grown on grit-blasted titanium aluminum vanadium (Ti6 Al4V) alloys with micron-scale roughened surface than in case of smooth surface Ti6 Al4V or on tissue culture polystyrene. [PERSON] et al. [7] found that transcutaneous/transmuscular screws are beneficial to achieve primary fracture healing in closed fractures. [PERSON] et al. [8] investigated whether compressive or distractive displacements have a more pronounced effect on new bone formation. It was found that cyclic compressive displacements were superior over distractive displacement enhancing periosteal callus formation. The influence of the osteotomy gap size and interfragmentary motion on the healing success was investigated by [PERSON] et al. [9]. Increasing gap sizes delayed the healing process while increasing movement stimulated callus formation but not tissue quality. [PERSON] et al. [10] found that (i) differentiation of osteoblasts and the production of bone on the surface of the implant is a prerequisite of successful osseointegration and (ii) biological response can be modulated by the properties of the implant surface. [PERSON] et al. [11] found that perfectly rigid fixation with no micromotion can lead to suboptimal bone regeneration. The reasons for this phenomenon are not fully understood. Some amount of motion is required for bone regeneration but what is the optimal amount of motion is yet to be established.  
The implant material and surface condition are key factors influencing the dynamics and outcome of the healing process. [PERSON] et al. [12] modified titanium surfaces to produce smooth, micro, or nano-topographies. Experiments indicated that the nano-topography induced a stronger autophagic response, leading to degraded cytoplasmic YAP. With the lower levels of YAP, \(\beta\)-catenin transported and accumulated in the nucleus to activate TCF/LEF transcription factors, resulting in osteogenesis occurring at higher rate. [PERSON] et al. [13] investigated a bio-functional magnesium coating deposited on porous Ti6 Al4V scaffold. In vitro studies demonstrated that magnesium-coated Ti6 Al4V co-culture with MC3T3-E1 cells can improve cell proliferation, adhesion, extracellular matrix (ECM) mineralization and ALP activity compared with pure Ti6 Al4V cocultivation. A large number of studied (reviewed by [PERSON] et al. [14]) demonstrated that poly-dopamine and its derivatives can be used for the surface modification of orthopedic implants to modulate cellular responses, including cell spreading, migration, tissue proliferation and differentiation, and may thereby enhance the function of existing implants.  
Stress shielding occurs when Young moduli of the bone and implant differ significantly. Bone atrophy occurs in such cases resulting in a loose implant device and in the worst cases, refracturing the bone [15]. The higher the elastic admissible strain (defined as the strength-to-modulus ratio) is, the more suitable are the materials for such orthopedic implant applications [16].