1. Introduction

　　With the rapid iteration of modern medical technology, minimally invasive treatment, wearable monitoring and implantable medical devices have become the core development directions of the medical industry. Traditional rigid printed circuit boards are limited by poor flexibility, large volume and low biocompatibility, which cannot adapt to the complex curved structure of human body and the flexible deformation requirements of medical equipment. Medical biocompatible flexible PCB circuit, as a customized flexible electronic carrier for medical scenarios, is manufactured with high-purity biocompatible raw materials and sophisticated precision processing technology. It has outstanding advantages such as bendability, torsion resistance, ultra-thin thickness and biological safety. It can be closely fitted with human tissues and organs, and steadily transmit electronic signals. At present, it has become an indispensable core electronic component in high-end medical equipment such as implantable devices, surgical minimally invasive instruments and vital sign monitors.

　　In the medical electronic industry, product safety and biological compatibility are the primary evaluation indicators. Different from industrial-grade flexible PCB, medical biocompatible flexible PCB circuit needs to pass strict medical certification standards. It must maintain stable physical and chemical properties in human body fluid environment, avoid toxic precipitation, tissue irritation and immune rejection reaction, and meet the dual requirements of long-term service stability and human body adaptation. Industry data shows that about 30% of medical electronic equipment failures are caused by improper selection of flexible PCB materials. Therefore, standardized material selection, optimized production process and strict compliance testing are the key guarantees for the performance of medical-grade flexible PCB.

　　2. Core Characteristics of Medical Biocompatible Flexible PCB Circuit

　　2.1 Excellent Biocompatibility

　　Biocompatibility is the most essential attribute that distinguishes medical flexible PCB from ordinary flexible circuit boards. All raw materials including substrate, covering film, adhesive and surface coating need to comply with ISO 10993 international medical biological evaluation standard. High-quality medical flexible PCB adopts medical-grade polyimide (PI) represented by DuPont Kapton HN. The material has a cytotoxicity rating of grade 0, with no harmful substances precipitated after being soaked in body fluid for a long time. It will not cause inflammatory reaction, allergic reaction and cell damage to human tissues. In addition, the material has excellent corrosion resistance to human body fluid such as blood and lymph, which can effectively resist biochemical erosion and ensure the biological safety of long-term contact with human body.

　　2.2 Superior Mechanical Flexibility and Durability

　　Medical scenarios put forward high requirements on the deformation resistance of circuit boards. Medical biocompatible flexible PCB circuit has ultra-high flexibility, which can realize arbitrary bending, twisting and folding. Its bending life can reach more than 100,000 times, and the circuit structure will not break or fail during repeated deformation. The thickness of the ultra-thin customized product can be controlled within 20μm, which is 90% thinner than the traditional rigid circuit board. It can adapt to the narrow internal space of minimally invasive medical instruments and the curved surface fitting requirements of wearable medical devices. Meanwhile, the tensile strength of the medical-grade PI substrate is more than 200MPa, with stable mechanical properties, which can resist the extrusion and stretching of human body movement and avoid circuit damage.

　　2.3 Extreme Environmental Adaptability

　　Medical equipment needs to adapt to complex temperature and humidity environments as well as various sterilization conditions. The medical biocompatible flexible PCB circuit has a wide temperature resistance range of -269°C to 400°C, with stable dielectric properties and no performance degradation under high and low temperature alternating conditions. It can withstand common medical sterilization methods such as high-temperature high-pressure steam, ethylene oxide and ultraviolet sterilization. After repeated sterilization cycles, the insulation performance and circuit conductivity remain unchanged, which effectively solves the aging failure problem of circuit boards caused by sterilization in medical work.

　　2.4 High Precision and Stable Signal Transmission

　　Diagnostic and treatment medical equipment has extremely strict requirements on signal transmission accuracy. Medical flexible PCB adopts modified semi-additive process (mSAP), which can realize ultra-fine circuit wiring with line width and spacing ≤20μm. The internal circuit layout is compact and orderly, which reduces signal interference and loss. Its signal recording accuracy can reach 99.9%, which is suitable for high-precision signal collection scenarios such as nerve signal monitoring and vital sign detection. In addition, the thermal expansion coefficient of the substrate is only 15-20ppm, which avoids circuit displacement and short circuit caused by thermal expansion and cold contraction, ensuring long-term stable signal output.

　　3. Core Material Selection and Production Process

　　3.1 Medical-grade Core Materials

　　Material selection determines the biocompatibility and service life of flexible PCB. The mainstream substrate of medical biocompatible flexible PCB is high-purity medical-grade polyimide. Compared with industrial-grade PI with cytotoxicity grade 3, medical-grade PI has higher chemical purity, fewer impurity residues and better biological inertness. For implantable medical devices, degradable medical PI and nano-coating materials are also used to further reduce tissue rejection. The conductive layer adopts high-purity oxygen-free copper foil or precious metal conductive layer, with low resistivity and strong oxidation resistance. The surface is coated with medical biocompatible insulating coating to prevent ion migration and ensure electrical safety in human body environment.

　　3.2 Standardized Production Process

　　The production of medical flexible PCB must be carried out in a dust-free purification workshop that meets medical-grade standards to avoid dust, bacteria and chemical impurity contamination. The production process includes raw material purification treatment, precise cutting, circuit etching, lamination pressing, surface passivation treatment and biocompatibility detection. In the production process, harmful chemical adhesives are prohibited, and medical environment-friendly adhesive materials are used to ensure no harmful substance precipitation. After production, all products need to pass cell toxicity test, skin irritation test, hemolysis test and corrosion resistance test to meet ISO 13485 medical quality management system certification requirements.

　　4. Main Application Fields in Medical Industry

　　4.1 Implantable Medical Devices

　　Implantable devices have the highest requirements for PCB biocompatibility. Medical flexible PCB can be implanted into human body for a long time, and is widely used in cardiac pacemakers, implantable nerve electrodes, intracranial signal detectors and other equipment. The flexible electrode PCB developed by advanced technology has a thickness of only 20μm, which reduces the tissue reaction rate by 92% compared with traditional electrodes, and the continuous signal recording time can reach 5 years. It has been applied in neuroscience research such as China Brain Project, providing accurate data support for the diagnosis and treatment of neurological diseases.

　　4.2 Minimally Invasive Surgical Instruments

　　Endoscopes, minimally invasive cutting instruments and interventional catheters have narrow internal space and complex bending angles. Ultra-thin flexible PCB can be embedded in the slender tube body of minimally invasive instruments to realize real-time transmission of image signals and control signals. It has the characteristics of small occupied space and strong bending resistance, which will not affect the flexible rotation of surgical instruments. At the same time, it can withstand high-temperature sterilization, meeting the aseptic operation standard of minimally invasive surgery, and reducing the surgical infection rate.

　　4.3 Wearable Medical Monitoring Equipment

　　Wearable medical devices such as intelligent ECG patches, blood oxygen monitors and body temperature sensors need to be closely attached to human skin. Medical biocompatible flexible PCB has soft texture and skin-friendly performance, which will not cause skin irritation after long-term wearing. It can fit the skin curves of human chest, wrist and other parts, accurately collect vital sign data such as heartbeat, blood oxygen and body temperature. Its stable electrical performance can avoid data distortion caused by human body movement, which is suitable for daily health monitoring and postoperative rehabilitation monitoring of patients.

　　4.4 Auxiliary Medical Diagnostic Equipment

　　In medical diagnostic equipment such as medical monitors and ultrasonic detectors, flexible PCB is used for internal circuit connection of curved sensing components. It replaces the traditional hard wiring mode, optimizes the internal space structure of the equipment, reduces the equipment volume, and improves the portability of medical diagnostic equipment. In addition, it has anti-electromagnetic interference performance, which can eliminate the interference of human body electromagnetic signals and external noise, and improve the accuracy of medical diagnosis data.

　　5. Industry Compliance Standards and Quality Control

　　The medical flexible PCB industry has extremely strict compliance supervision. All products must comply with international and domestic medical industry standards, including ISO 10993 biological evaluation standard, ISO 13485 medical device quality management system, and meet NMPA medical device registration requirements. In terms of quality control, manufacturers need to establish a full-process traceability system, record raw material batches, production parameters and test data. Before leaving the factory, each batch of products shall undergo random inspection on biocompatibility, mechanical performance and electrical stability to eliminate unqualified products. For implantable flexible PCB, long-term aging simulation test in human body fluid environment is also required to verify its service life and safety.

　　6. Industry Development Trends and Technical Prospects

　　With the continuous upgrading of medical intelligent technology, medical biocompatible flexible PCB is developing towards ultra-thin miniaturization, biodegradation, intelligent integration and high flexibility. In terms of materials, biodegradable medical polymer materials will gradually replace traditional PI materials, realizing harmless degradation of post-abandoned implantable circuits and reducing secondary surgical trauma to patients. In terms of technology, flexible PCB will be integrated with sensing chips, micro-batteries and wireless transmission modules to form integrated flexible electronic medical components, realizing real-time remote monitoring of human physiological signals.

　　In addition, with the breakthrough of stretchable circuit technology, the new generation of medical flexible PCB will have tensile deformation capability, which can adapt to the violent stretching of human muscle movement. It is widely used in bionic electronic skin and rehabilitation medical robots. At the same time, under the background of the expansion of the global medical device market, the iteration speed of medical flexible PCB technology is accelerating, and the product cost is gradually optimized. It will be more widely used in primary medical treatment, home health monitoring and other scenarios, bringing more convenient and safe medical services to patients.