An In-Depth Analysis of the Flexible Endoscope's Structure

A flexible endoscope is a precision medical device that integrates optical, mechanical, and electronic technologies and is widely used for the examination and treatment of natural body cavities, such as the digestive tract and respiratory tract. The following provides an in-depth analysis of the internal structural components and their functions, including: insertion tube, distal end, bending section, external operating section, internal operating section (front), internal operating section (back), light guide, light guide head, auxiliary functions, and optical vs. electronic endoscopes—a total of ten sections.

1. Insertion Tube

The insertion tube contains four steel wires in four directions, each protected by a spiral tube. Inside, there are also CCD lines, water and air tubes, forceps tubes, and a light beam.

The insertion tube structure: Composed of a multi-layer composite structure, from the outside to the inside, it includes:
Outer layer: Medical-grade polyurethane (PU) or silicone material, with a smooth, corrosion-resistant surface that reduces friction during insertion and prevents penetration of body fluids. Braid: Woven with metal wires (such as stainless steel), it provides radial strength and kink resistance, ensuring the insertion site can bend flexibly without collapsing.
Inner Liner: Made of polytetrafluoroethylene (PTFE) or polyethylene (PE), it forms a smooth channel to protect internal optical fibers, guidewires, and instrument channels.
Function of the Insertion Tube: Serves as the main "channel" of the endoscope, carrying the distal end for penetration into the human body. Its flexibility and diameter (e.g., approximately 9-13 mm for a gastroscope and 11-13 mm for a colonoscope) determine the comfort and accessibility of the examination.

2. Distal End

Visible Front End: The optical eye, CCD, nozzle, and forceps opening.
Imaging System: Traditional optical endoscopes transmit light via optical fibers, transmitting an image of the observed area to the human eye or an imaging device.
Electronic endoscopes utilize a multi-purpose CCD or CMOS image sensor. The lens at the front of the end projects an image of the object onto the sensor, which converts the optical signal into an electrical or digital signal. This signal is then processed by an image processor and transmitted to a monitor. Light Guide: The light guide transmits light from a cold light source to the line segment for illumination. Some high-end models integrate an LED light at the tip to reduce fiber loss.
Nozzle: The nozzle's opening faces the CCD, and the other end is connected to an external water and air source for both water and air delivery. Controlled by buttons on the operating panel, this function can be used to flush the field of view (to remove mucus or blood) or to inflate and dilate cavities (such as during gastrointestinal examinations). It can also be used to clear fluid from the CCD surface that obscures the field of view. Forceps Tube: The working channel is used to insert instruments such as biopsy forceps, snares, and laser fibers for tissue sampling or treatment.
Suction Port: Some flexible endoscopes have a suction port at the tip, connected to a suction pump, to remove secretions, blood, or residual fluid, maintaining a clear field of view.

3. Curved Section

The curved section consists of a snake, steel mesh, rubber, and steel wire.
Structure: The innermost layer, known in the industry as the "snake," is composed of multiple hinged metal joints. The outer layer is steel mesh, and the outermost layer, a visible elastic rubber, ensures a tight seal and flexibility. Internally, the control knobs of the operating unit are connected by traction wires (usually 2-4).
Function: Pulling the wires through the operating knobs allows active bending of the bending section (e.g., 180°-210° vertical bending, 100°-160° horizontal bending), adapting to the physiological curvature of the human cavity (e.g., esophageal peristalsis and intestinal folds). This allows precise control of the tip's direction, avoiding blind spots and improving examination efficiency.

4. Operating Unit Exterior

Visible parts of the operating unit include the knob, button receptacle, instrument access port, and buttons. Accessories include a water vapor button and a suction button.
Angle Control Knob: The large knob controls vertical bending, while the small knob controls horizontal bending. These control the bending wires via a gear or wire rope drive system.
Water/Air Button: Blocking the button control while not pressing it allows air delivery; continuously pressing it allows water delivery. Suction Button: Connects to the suction valve. Pressing it opens the suction channel to remove fluid or tissue debris from the cavity. Instrument Access Port: A removable sealed interface for inserting instruments such as biopsy forceps. It features a built-in rubber seal to prevent air and fluid leakage.
Buttons: The buttons enable functions such as white balance, freeze, and photo and video recording.

5. Internal Control Unit (Front)

The front of the operating handle primarily houses the angle control components. These include the sprocket, chain, angle limiter, chain-wire connector, wire, and spiral tubing. The gear connects to the external knob. Turning the knob drives the internal gear, which in turn drives the chain. The end of the chain connects to the wire, pulling the wire and bending the tip. The angle limiter prevents excessive angles from damaging other components.

6. Internal Control Unit (Back)

The back of the operating handle houses numerous piping systems. Key components include the water and gas button jack, the water and gas pipe connector, and the suction tube. This area also houses some of the button cables, providing the primary access for button repairs.

7. Light Guide

Similarly, the interior of the light guide is relatively simple compared to the insertion tube and will not be elaborated on in detail. A cross-section of the light guide reveals the suction tube, CCD cable, key cable, water and air tubes, and light beam.

8. Light Guide Head

Memory Chip: Used to store basic endoscope information, such as time.
Water Bottle Port: Connects to a water bottle.
Suction Port: Connects to an external negative pressure source.
Suction Tube Port and Water and Air Tube Port: Water vapor channels, serving as internal connections to the light guide, are invisible to the naked eye.
CCD Holder: Connects to the power supply.
Light Inlet and Air Inlet: Connect to the cold light source port in the same orientation.

9. Auxiliary Functions

Instrument Channel Structure: A hollow channel running through the insertion section, with a smooth interior to reduce friction.
Function:
Diagnosis: Inserts biopsy forceps (approximately 1-2 mm in diameter) to remove tissue or a cytology brush to collect exfoliated cells. Treatment: Insertion of hemostatic clips, electric snares, stent pushers, etc. to achieve hemostasis, polypectomy, and stricture dilation.

The larger the diameter (e.g., therapeutic endoscopes can reach up to 4.2 mm), the more complex the instruments supported.

Air/Water Supply System

Air and Water Source: External medical compressed air or carbon dioxide cylinders (to reduce abdominal distension and discomfort); the water source is sterile water. The air/water flow is switched via a button on the operating panel, and released through the nozzle at the distal end. The pressure can be adjusted by the main unit.

Suction System: Negative pressure source: An electric suction pump or a central vacuum system, typically with a negative pressure of -200 to -400 mmHg. Used to remove bleeding, mucus, foreign matter, or to aspirate tissue during treatment.

10. Optical Endoscope vs. Electronic Endoscope

Optical Endoscope (Traditional)

Image Guide Fiber Bundle: Consists of tens of thousands of micron-sized optical fibers (quartz glass or plastic), each fiber aligned at opposite ends, transmitting images through total internal reflection. Disadvantages: Fiber breakage can easily produce "black spots," and resolution is limited (approximately 3000-5000 pixels). A separate light source is used, independent of the image guide bundle, to transmit the light source and prevent heat transfer to the tip (cold light source technology).

Centralized in the tip of a modern electronic endoscope, the image sensor (CCD/CMOS) can reach millions of pixels (e.g., 1920×1080 pixels for high-definition endoscopes), offering a resolution far exceeding that of fiber optics. It directly converts optical signals into electrical signals, which are then transmitted via a cable to a host computer for digital processing. It supports image enhancement (such as NBI narrow-band imaging) and electronic magnification.

As can be seen from the above analysis, each component of a flexible endoscope is designed around the principles of "precise imaging, flexible control, and integrated functionality." Its complexity and precision directly determine the effectiveness of clinical diagnosis and treatment. In practice, physicians must master the operation of each component, taking into account the patient's anatomy and lesion characteristics, to achieve optimal examination results.