How Thermage FLX tightens skin: broad heat below a cooled surface

Anyone comparing skin tightening options soon runs into Ultherapy and Thermage, and Thermage is the older of the two. It was the first monopolar radiofrequency device for loose skin, and many newer devices follow its design. Our clinic uses Thermage FLX, the current generation. This article explains how it works: where the heat is made, why the surface stays cool, and what the treatment improves.

Thermage is still the benchmark

Thermage was developed in the United States by Thermage, Inc., now part of Solta Medical. The FDA cleared it for wrinkles around the eyes in 2002 and for the full face in 2004 (de Felipe 2007, PMID 17760693). A 2007 review called monopolar radiofrequency the "gold standard" for noninvasive skin tightening (Abraham 2007, PMID 17544932).

Nothing has displaced Thermage since. When a new monopolar device is tested, it is usually measured against Thermage. In one recent randomized trial, a newer monopolar device was compared with Thermage FLX side by side on the same faces (Roh 2025, PMID 41331720). Thermage FLX itself was cleared in 2017 (U.S. FDA 510(k) K170758).

The heat fills the deep dermis

Thermage is a monopolar device, which means the handpiece carries a single electrode. The other electrode is a return pad placed on your back or another part of the body.

The electrode in the tip is covered by a thin insulating membrane, so it does not touch the skin directly. When the tip is placed on the face, an electric field forms between the tip and the return pad. The field reverses direction 6.78 million times a second (6.78 MHz), and ions in the tissue shuttle back and forth with each reversal. The tissue's resistance to that movement produces the heat (U.S. FDA 510(k) K170758).

At the same time, the tip sprays coolant onto the skin. The whole dermis under the tip heats up, hottest in its deeper part, while the surface stays cool (Abraham 2007, PMID 17544932; Elsaie 2009, PMID 20161847). This pattern is called a reverse thermal gradient. It lets Thermage deliver intense heat below the surface without harming the surface itself (Elsaie 2009, PMID 20161847).

The heat also reaches the fibrous septa

The heat does not stop at the dermis. Below it, fibrous septa, thin strands of connective tissue, run down through the fat and anchor the skin to the layers beneath. In a computer model of 6.78 MHz heating, the heat concentrated along these septa (Ko 2025, PMID 41315066).

The heat tightens the septa as well as the dermis. That is one explanation for the change in facial contour seen right after treatment (Elsaie 2009, PMID 20161847).

Results build over months

Heated collagen contracts right away, and new collagen is produced as the tissue heals. In tissue treated with Thermage, researchers saw the collagen fibers contract immediately and the signal for making new type I collagen increase (Zelickson 2004, PMID 14967794).

The tightness you feel right after treatment comes from that contraction and from temporary swelling, so it fades within days. The lasting result builds over the following months as new collagen forms. In a multicenter study that followed 86 people for six months after a single Thermage session, improvement kept building as healing progressed (Fitzpatrick 2003, PMID 14571447).

It treats loose skin and fine lines

Thermage works on the skin itself: its firmness and its fine lines. Its FDA clearance covers wrinkles around the eyes, including the eyelids, and wrinkles elsewhere (U.S. FDA 510(k) K170758). Published reviews describe the right candidate as someone with mild to moderate laxity, skin that has loosened but not sagged heavily (Abraham 2007, PMID 17544932).

References

1. de Felipe I, et al. (2007). Adverse reactions after nonablative radiofrequency: follow-up of 290 patients. J Cosmet Dermatol. PMID 17760693 (opens in new window).
2. Abraham MT, Mashkevich G (2007). Monopolar radiofrequency skin tightening. Facial Plast Surg Clin North Am. PMID 17544932 (opens in new window).
3. Roh H, et al. (2025). Comparative analysis on the efficacy of monopolar radiofrequency with continuous water cooling and conventional cryogen spray cooling in facial rejuvenation. Ann Dermatol. PMID 41331720 (opens in new window).
4. U.S. Food and Drug Administration (2017). 510(k) Summary K170758: Thermage FLX System. accessdata.fda.gov.
5. Elsaie ML (2009). Cutaneous remodeling and photorejuvenation using radiofrequency devices. Indian J Dermatol. PMID 20161847 (opens in new window).
6. Ko K, et al. (2025). Computational modeling and histologic analysis of 6.78- and 2-MHz monopolar radiofrequency-induced thermal reactions. Lasers Med Sci. PMID 41315066 (opens in new window).
7. Zelickson BD, et al. (2004). Histological and ultrastructural evaluation of the effects of a radiofrequency-based nonablative dermal remodeling device: a pilot study. Arch Dermatol. PMID 14967794 (opens in new window).
8. Fitzpatrick R, et al. (2003). Multicenter study of noninvasive radiofrequency for periorbital tissue tightening. Lasers Surg Med. PMID 14571447 (opens in new window).