Basic principle : Skin Structure(Epidermis, Dermis, SMAS)
The epidermis is composed of the outermost layers of the skin. The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis provides tensile strength and elasticity to the skin through an extracellular matrix composed of collagen fibrils, micro fibrils, and elastic fibers, embedded in hyaluronic and proteoglycans.
The SMAS lies deep to the subcutaneous fat, envelops the muscles of facial expression, and extends superficially to connect with the dermis. The SMAS layer is composed of collagen and elastic fibers similar to the dermal layer of the skin; however, it has more durable holding property and less delayed relaxation after lifting procedures than skin alone.
Thus, the SMAS is a desirable target for noninvasive skin tightening procedures.
(MacGregor and Tanzi, 2013)
How does it work
Thermal stimulation due to RF temporarily changes the helical structure of the collagen within the skin dermis and induces its momentary thermos-coagulation. It also thermally stimulates dermal fibroblast and induces production of new collagen and elastin. Such scenario causes structural change in the dermis and induces skin tightening.
Reference: Weiss RA. Noninvasive radio frequency for skin tightening and body contouring. Semin Cutan Med Surg. 2013 Mar;32(1):9-17.
HIFU is a typical nonsurgical treatment that stimulates collagen within the SMAS and the dermis, and induces skin tightening. It can effectively induce the contraction of both without stimulation by focusing ultrasonic waves in a specific part of the dermal tissue.
Reference: MacGregor JL, Tanzi EL. Microfocused ultrasound for skin tightening. Semin Cutan Med Surg. 2013 Mar;32(1):18-25.