A woman's vagina is a muscular tube with a wide upper part and a narrow lower part, and the front and rear walls are close together, which has good extensibility (Figure 1- 1, left). Under normal circumstances, the upper width of the vaginal segment is arc-shaped and the lower narrow segment is "H"-shaped. During sexual excitement, the lower third of the vagina is further narrowed (the inner diameter is reduced by about half), and the upper two thirds of the vagina (including the quadrangular vault) is fully expanded due to the Fang Xiangsheng elevation of the uterus along the vaginal axis, thus forming a spherical cavity with a diameter of about 65mm in the deep part of the vagina, which is called "internal erection" (right 1-2). At this time, the cervix rises with the uterus, leaving the fully expanded vaginal vault, so that the bottom of the vagina passes through the plane of the cervix to form a concave basin-shaped expansion area, which is called "tent effect" (no response after the uterus). The sexual physiology of these vaginas was first discovered by masters, an American physiologist, and published in his masterpiece Human Sexual Response. In this respect, it is regarded as one of the landmark discoveries by contemporary medical circles.

In addition, a woman's vagina is a sexual organ with complex senses-both sensitive and dull. The lower segment is dominated by superficial senses (warmth, pain, touch, etc.). ), and the upper part is dominated by deep senses (movement, vibration, etc.). ) and visceral sensation, the types of nerve receptors also have corresponding distribution. Tactile receptors are mainly distributed in the superficial mucosa near the vaginal orifice, while the upper vaginal segment and related surrounding tissues are mostly pressure and tension receptors.

The so-called physiological essence of sexual feeling is a variation of touch. Therefore, the sexy area of women is mainly concentrated in the lower third of the vagina, which will become narrower when excited, especially the front wall is more sensitive, while the sexy (touch) of the upper part of the vagina is extremely dull. These special anatomical, physiological and functional settings are themselves the external manifestations of physical desires. Therefore, women's sexual desire is mainly manifested in the touch demand of the lower vagina and vulva, while the deep vagina is manifested in the filling demand. The theory of Yin and Yang in TCM grasps the basic laws of nature to a certain extent and summarizes the common phenomena of things. Among them, Yin He's relativity principle is verified by the ubiquitous objective facts. As far as the human body is concerned, the external genitalia of both sexes have more distinct relative characteristics of yin and yang: the male is a solid cylinder and the female is a hollow potential catheter. To design a barrier contraceptive device, we must first deeply understand the unity of opposites of Yin and Yang and the characteristics of bisexual organs.

Among all barrier contraceptives, male condom is the most widely used and successful one. Therefore, in order to design a successful barrier contraceptive device for women, it is necessary to find the symmetry (duality) of the effective characteristics of male condoms according to the principle of relativity of yin and yang, such as when male condoms pursue the characteristics of light, thin and small sleeve wall; Female users must be the opposite, to a certain extent, pay attention to thickness, weight and size. The principle of relativity of yin and yang applied in this study actually includes the reverse thinking mode. The function of barrier contraception is only to prevent bisexual germ cells from meeting, not to inhibit their production, so the side effects are minimal, but it often inevitably blocks the natural needs of both sexes and greatly reduces their acceptability. In order to change this shortcoming, it is a rational choice to design a new generation of barrier contraceptive devices and learn and imitate the function and structure of biological organs. Therefore, the bionic design of barrier devices placed between hermaphrodites should be able to imitate the strong penis of men for women and the tight vagina of women for men.