Official Achievement Announcement

We have successfully developed composite‑material bidirectional articulated shafts fabricated from medical‑grade stainless steel and nickel‑titanium (NiTi) alloy, achieving an optimal balance between mechanical performance and biocompatibility. Through innovative material formulation and heat‑treatment processes, the product retains the super‑elasticity of NiTi alloy (8% recoverable strain) while raising the yield strength of stainless steel to 1200 MPa. Tests verify that the composite articulated shaft achieves a fatigue life of 800 000 bending cycles and passes corrosion resistance testing per ASTM F2129, delivering a reliable material solution for long‑term implantation applications.

R&D Background & Pain Points

Conventional single‑material articulated shafts suffer from inherent material performance limitations. Medical‑grade 316L stainless steel features high strength yet limited elasticity, with a maximum recoverable strain of only 0.5%, prone to plastic deformation under repeated bending. NiTi alloy exhibits super‑elasticity but relatively low strength (yield strength: 500–800 MPa), which may cause excessive bending in complex anatomical pathways. Differences in thermal expansion coefficients between the two materials induce interfacial stress concentration in composite structures and shorten service life.

Clinical studies show that the surface oxide layer of pure NiTi articulated shafts begins to peel after more than 300 000 cycles, potentially releasing nickel ions and triggering allergic reactions. Stainless steel articulated shafts develop permanent deformation with a 15% decline in deflection angle after only 50 000 cycles. Material selection has become a critical bottleneck restricting the performance of articulated shafts.

Core Technological Innovations

1. Gradient Composite Material TechnologyStainless‑steel‑NiTi alloy gradient composite tubes are manufactured via powder metallurgy and hot isostatic pressing to realize continuous material transition. From the inner to outer layer, NiTi content decreases gradiently from 100% to 0%, while stainless steel content increases from 0% to 100%. The transition layer thickness is precisely controlled at 50–100 μm to avoid interfacial stress concentration. After special heat treatment, the interfacial bonding strength reaches 450 MPa.
2. Nanocrystalline Structure Regulation ProcessA combined process of high‑pressure torsion and low‑temperature annealing refines stainless steel grain sizes to below 50 nm. The nanocrystalline structure raises the material yield strength to 1200 MPa while maintaining an elongation of over 15%. For NiTi alloy, aging treatment regulates the size and distribution of precipitated phases, confining phase‑transformation hysteresis within 5 °C and improving super‑elasticity stability.
3. Surface Functional Modification TechnologyA multilayer titanium‑nitrogen‑oxygen composite coating is developed, forming a 2–3 μm functional layer on the surface via physical vapor deposition (PVD). The coating achieves a hardness of HV 2500 and a friction coefficient of 0.15, with excellent biocompatibility. Trace silver ions (0.5–1.5 at%) are doped into the coating to deliver sustained‑release antibacterial performance, attaining a bacteriostatic rate of over 99% against Staphylococcus aureus.

Working Mechanism

The advantages of composite articulated shafts stem from multi‑scale synergistic effects. At the microscale, nanocrystalline stainless steel is strengthened via the Hall‑Petch effect, with dislocation motion hindered to enhance strength and fatigue resistance; reversible martensitic transformation of NiTi alloy under stress provides super‑elasticity. At the mesoscale, the gradient transition layer enables smooth variation of elastic modulus (40–60 GPa at the NiTi end, 190 GPa at the stainless steel end), matching biomechanical properties of different tissues. At the macroscale, the composite structure delivers a mechanical response integrating rigidity and flexibility: stainless steel provides axial pushing force and torsional rigidity, while NiTi alloy offers radial compliance and shape‑recovery capability. The functional coating reduces tissue adhesion by lowering surface energy, while sustained release of silver ions forms an antibacterial microenvironment.

Performance Validation

Material performance tests yield remarkable results. In super‑elasticity tests, the composite fully recovers under 8% strain, with a 30% smaller hysteresis loop area and reduced energy dissipation compared with pure NiTi. In fatigue tests under ±90° bending at 3 Hz, performance retention exceeds 95% after 800 000 cycles. In corrosion tests, after 90‑day immersion in simulated body fluid, the nickel ion release rate is less than 0.1 μg/cm²·day, far below the ISO 10993‑12 limit of 1 μg/cm²·day.

Animal experiments show mild inflammatory responses in surrounding tissues and a fibrous capsule thickness of only 50–80 μm (120–150 μm for the stainless steel control group) 6 months post‑implantation. In clinical trials of ureteroscopic surgeries using composite articulated shafts, the success rate of instrument crossing ureteral strictures rises from 78% to 94%. In complex cardiac arrhythmia ablation surgeries, catheters maintain stable performance during 4 hours of continuous intracardiac operation, whereas conventional products suffer a 12% decline in deflection angle after only 2 hours.

R&D Strategy & Philosophy

We uphold the R&D philosophy: Performance is defined by materials, functions are realized by structures, and establish the MIPS innovation system (Material‑Interface‑Performance‑System). At the material level, we build the world's first medical articulated shaft material database containing 368 performance parameters of 127 alloys. At the interface level, we study atomic‑scale bonding mechanisms and optimize interfacial design through first‑principles calculations. At the performance level, we develop multi‑scale simulation models to predict mechanical behaviors from nanoscale to macroscale. At the system level, we precisely match material properties with clinical requirements.

We have built joint laboratories with the Institute of Metal Research, Chinese Academy of Sciences, and Beihang University, focusing on fundamental research of shape‑memory alloys. Meanwhile, we implement material genome engineering to accelerate new‑material development through high‑throughput computation and experiments, shortening the R&D cycle from the traditional 5–8 years to 2–3 years.

Future Outlook

Medical materials will evolve toward intelligence, functionality and biomimicry. We are developing stimulus‑responsive smart materials whose mechanical properties adjust with body temperature, pH value or electric fields. Self‑healing composite materials are being developed to automatically release repair agents upon detecting microcracks. Bioabsorbable materials are explored for safe degradation within 6–12 months after completing device functions.

By 2027, we will launch tissue‑adaptive smart articulated shafts with surface‑modified extracellular matrix proteins to promote endothelial cell adhesion and reduce thrombosis risks. In the long run, 4D‑printed active materials will become reality. Such materials not only respond to external stimuli but also conduct biological signal communication with surrounding tissues to achieve true biological integration, pioneering new pathways for permanent implantable devices.