Density · XERF · Oligio — All Radiofrequency, So What Differs?

Ultrasound lifting concentrates energy at a single point, the way a lens concentrates sunlight, creating coagulation points around 1mm across. Radiofrequency works on a different principle. Current is passed through the tissue so that the tissue itself generates heat through resistance.

So radiofrequency heats not points but a volume. When dermal collagen reaches the denaturation temperature range it contracts immediately, and the goal is for new collagen to be produced over the following months. If ultrasound places dots, radiofrequency heats a plane.

One distinction is needed here. Among the devices now grouped under the name “radiofrequency” there is also microneedle radiofrequency, in which needles penetrate the epidermis and deliver energy at a set depth. Its main targets are acne scars · pores · surface irregularity. The three devices in this article are all non-invasive surface radiofrequency using no needles, and their target is loss of elasticity and sagging. What Oligio Kiss adds is not needles either, but ultrasound.

The first axis separating the three devices is monopolar versus bipolar. The names are awkward but the principle is simple.

• Monopolar — current from the handpiece electrode passes through the body to a separate return pad. The current spreads deep into the tissue, and heating is strongest directly beneath the electrode and then decreases with depth. It goes deep.
• Bipolar — both electrodes sit within the handpiece, so the current is confined between them. The effective depth typically stays at about half the distance between the two electrodes. It is shallow and predictable.

Electrode size adds to this. With monopolar, penetration depth is broadly proportional to electrode size. A small electrode gives a high surface current density and acts shallowly and focally; a large electrode acts broadly and relatively deeply. This is why there is a separate tip for the eye area.

Some of this has been confirmed in people. In a split-face trial (31 subjects) in which 3cm² and 4cm² monopolar tips were randomly assigned to the left and right sides of the same face, the smaller tip was statistically significantly superior around the eyes (p<0.001). This topic is written up in detail in Lifting treatment should vary with skin thickness and fat volume.

What changes when the frequency differs? On this point a good study appeared recently: a 2025 study combining finite element simulation with histology of porcine skin.

The principle is this. The electrical properties of tissue vary with frequency. The lower the frequency, the more broadly current spreads even into poorly conducting tissue such as fat; the higher it is, the more current gathers into water-rich, well-conducting structures — the fibrous septa and the dermis — instead.

What matters is the interpretation. This study did not show that “6.78MHz is superior to 2MHz.” It showed that “the two frequencies heat different layers.” And the fact that researchers affiliated with the manufacturer took part must be read alongside it.

Because radiofrequency heats the whole tissue, if nothing is done the epidermis closest to the electrode becomes the hottest part. So cooling is not an option but the mechanism that determines which layer gets heated.

Cooling the surface with cryogen or gas inverts the temperature curve so that the peak temperature point moves down into the dermis beneath the epidermis. It is the only way to reach the dermal collagen denaturation temperature range without an epidermal burn. There is measured evidence too — in XERF’s porcine preclinical work, surface temperature was kept below 43°C while increases in dermal collagen density and elastin were obtained without tissue damage.

Impedance feedback addresses a different problem. The electrical resistance of skin varies considerably with site · stratum corneum hydration · thickness · how closely the electrode is applied · how much gel is used. When resistance is high, the energy actually delivered falls at the same setting; when it is low, there is a risk of overheating. Reading resistance on every shot and correcting the output secures shot-to-shot uniformity and avoids local overheating.

An honest addition. We could not find a peer-reviewed study demonstrating an improvement in safety from impedance feedback taken on its own. What is verified goes as far as this being a mechanistically sound design and a feature the manufacturer promotes.

Organizing the three devices along the axes above gives the following. Only items verified in official manufacturer materials are included.

Here is what to read from the table.

• Density divides depth using a single frequency. It goes deep with monopolar and finishes superficially with bipolar.
• XERF divides layers using a single path. It uses monopolar only, but 2MHz and 6.78MHz each heat a different layer.
• Oligio Kiss adds an entirely different energy. Ultrasound places dots where radiofrequency has heated a plane.

It is accurate to see them as three different routes toward the same destination.

More important than specifications is evidence. Here is what has been published for each device, as it stands.

Stated honestly, the strengths of each device are as follows.

• XERF — it has the largest human study of the three (39 subjects), and it is multicenter. It received US FDA clearance in August 2025 (K251327), and porcine preclinical work measured surface temperature held below 43°C.
• Density — it is the only one with a study measuring change in millimeters with 3D imaging. There is also a result in an aged mouse model in which sequential monopolar-bipolar delivery gave 2.4 times the collagen density of monopolar alone, but that is an animal study and it was manufacturer-funded.
• Oligio — it has the methodologically cleanest single study. It had evaluator blinding and no external funding, maintenance to week 24 was confirmed, and pain was reported lowest at 0.4/10.

One thing must be made clear. The Cosmetics 2024 study above was performed with ‘Oligio’, and the ‘Oligio Kiss’ we have is a separate model that adds focused ultrasound to it. Radiofrequency alone and radiofrequency plus ultrasound are different procedures, so that study cannot simply be carried over as evidence for the Kiss model. We could not find a peer-reviewed clinical study specifying the product name Oligio Kiss. This point is also noted in our Oligio Kiss treatment information.

It is the question we are asked most, and it is also a question with a clear answer.

No randomized trial ranking radiofrequency devices against one another exists.

A systematic review published in 2026 states this explicitly. Searching PubMed · Embase · CENTRAL · LILACS for 2015–2025, it gathered 15 studies with 1,230 patients, of which only one was a randomized controlled trial, the rest being 3 prospective cohorts and 11 case series. Because the heterogeneity of technologies · parameters · outcome measures · populations was too great, the review did not calculate a pooled effect size (meta-analysis) at all, and wrote that “no direct device-to-device comparison studies were identified.”

All the “comparison” data appearing in the literature on these three devices are comparisons between modes within the same device.

• XERF: dual-frequency versus single frequency — porcine preclinical, performed by the manufacturer
• Density: sequential monopolar-bipolar versus monopolar alone — mice, manufacturer-funded

“Mode A of our device is better than mode B” is internal evidence, not evidence of superiority between brands.

And the numbers in the previous section — 92.3%, 0.82mm, maintenance to week 24 — must not be placed side by side and compared. The designs, the samples and the outcome measures all differ. An investigator-assessed improvement rate and a 3D imaging measurement are not in the same units to begin with. It is a comparison that does not hold up statistically.

If there is no ranking, it is natural to ask why we have three. The answer is that they are tools, not options.

1. We look at the thickness of the area first. For thin areas such as around the eyes and the temples we use a small electrode and shallow settings; for thick areas such as the cheek and the jawline we use a large electrode. In the split-face randomized trial, the small tip was significantly superior around the eyes.
2. For areas with a thick fat layer we consider a configuration that reaches with a lower frequency. That 2MHz acts more broadly and deeply in the fat layer was observed in simulation and histology.
3. When both superficial and deep layers must be addressed, we use sequential monopolar-bipolar delivery. Two depths can be divided in a single pass.
4. For sagging that radiofrequency alone does not cover, we add ultrasound on top. Dots are placed where a plane has been heated. We do say at the same time, however, that there is as yet no published clinical evidence for this combination itself.
5. After a deep pass we leave an interval. We do not repeat the same area on a short cycle.
6. We tell you in advance how far a device can go. Lost volume, excision of excess skin and pigmented lesions are not problems radiofrequency addresses. If density and texture are the main problem, skin boosters carry more weight.

Detailed material on each device is written up separately on the XERF, Density and Oligio Kiss pages.

• Verified specifications — Density: 6.78MHz, sequential mono + bipolar · 400W · 5-level gas cooling · impedance correction and skin temperature display. XERF: 6.78 + 2MHz monopolar · cryogen spray · impedance feedback · FDA K251327. All verified in official manufacturer materials.
• What we could not verify — for all three devices, penetration depth per tip (mm) is absent from the official materials. For Oligio Kiss we could not access the manufacturer page and so could not verify frequency · focal depth · cooling method.
• Physics — monopolar deep, bipolar shallow. 2MHz broad and deep in the fat layer, 6.78MHz focal along the fibrous septa. It is a difference in target layer, not a ranking.
• Clinical evidence — XERF 39 subjects (multicenter, manufacturer-supported), Density 16 subjects (3D measurement), Oligio 20 subjects (evaluator-blinded · no external funding). None of the three studies has a control group.
• What does not exist — a randomized trial directly comparing radiofrequency devices. The 2026 systematic review (15 studies · 1,230 patients) states explicitly that none exists.

So the conclusion of this article is not “which device is best” but “which route suits this area, this thickness, this goal.” Choosing the device is our job, not the patient’s, and we think it is right to set out the basis for that judgment openly like this.

Written and reviewed by Lee Chi-Hak, MD, medical director of Miso Clinic in Daegu, South Korea. Every study cited above is given together with its design, its size and the limitations the authors themselves recorded, and where we could not find data, we have said that we could not find any.