Progress and Challenges in Joining for Precision Endoscope Fabrication

This review summarizes the base materials, joining methods, filler materials, and principal technical challenges in endoscope joining fabrication, and proposes practical strategies to improve joint reliability under clinical constraints. We conducted a comprehensive search in multiple databases, including Web of Science, Google Scholar, patent databases, Scopus databases, and Medline (via PubMed), for articles on the joining for precision endoscope fabrication, covering the period from 1950 to 2026. We employed the combinations of keywords, "endoscopy", "minimally invasive surgery", "welding", "joining", "sealing", "soldering", "bonding", and "brazing". Approximately 500 references were retrieved. After excluding duplicates and irrelevant studies, 158 publications met the inclusion criteria. Data on base materials, joining, processes, filler materials, and technical issues related to sterilization, corrosion, and microstructural evolution were extracted and analyzed. Endoscopes are multi-material systems, involving metallic biomaterials (stainless steels (SSs), titanium alloys, nickel-based alloys, etc.), optical functional materials (glass, sapphire, quartz, etc.), engineering plastics, ceramics, composite materials, and coatings. Joining, sealing, and functional integration have been achieved via adhesive bonding, laser soldering, laser brazing, wave soldering, reflow soldering, fusion welding, and other joining techniques. The main challenges include how to reliably join highly mismatched dissimilar materials, how to fabricate low-residual-stress joints, and how to increase the long-term resistance to sterilization-induced degradation and thermal aging over repeated 100-200 °C thermal cycles. Conventional joining techniques struggle to balance mechanical integrity, joint hermeticity, and long-term stability under such harsh cyclic conditions. The resulting joints may suffer surface yellowing, interfacial debonding, microcracking, delamination, or progressive property degradation during service. We propose the following three strategies to achieve reliable, low-residual-stress, and sterilization-resistant joining of dissimilar materials for endoscopes: (1) A synergistic design that combines thin-film engineering (including evaporation, sputtering, and electroplating) with silver anti-oxidation layers is proposed to reduce residual stresses and to enhance the joint hermeticity. (2) To develop principles for the selection of multi-joining processes to achieve the multi-material integration and functional assembly of dissimilar material components. (3) To develop the laser-based joining methods (fusion, brazing, or braze-welding) for precision control of heat input, bonding quality, and the least damage to the heat-sensitive components.