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The Basic Science of Radiofrequency-Based Devices

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This chapter outlines the basic science and specific principles of operation for radiofrequency (RF) technologies with a focus on minimally-invasive applications enhancing liposuction procedure. Before discussing the parameters, settings and techniques for radiofrequency-assisted lipolysis (RFAL) and fractional RF subdermal treatment, it is important to understand the fundamentals of the basic science of RF technologies and applications. The chapter accurately describes the physics of the processes occurring during RF-based treatment, and the factors affecting its safe and efficacious outcome. The discussion of RF-based devices will use terminology and definitions provided by FDA guidance for electrosurgical devices. Measurements and computer simulations conducted by the authors to illustrate importance of different parameters for the specific treatments of skin and subcutaneous fat are also presented.
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Chapter
The Basic Science of
Radiofrequency-Based Devices
Michael Kreindel and Stephen Mulholland
Abstract
This chapter outlines the basic science and specific principles of operation for
radiofrequency (RF) technologies with a focus on minimally-invasive applications
enhancing liposuction procedure. Before discussing the parameters, settings and tech-
niques for radiofrequency-assisted lipolysis (RFAL) and fractional RF subdermal
treatment, it is important to understand the fundamentals of the basic science of RF
technologies and applications. The chapter accurately describes the physics of the
processes occurring during RF-based treatment, and the factors affecting its safe and
efficacious outcome. The discussion of RF-based devices will use terminology and
definitions provided by FDA guidance for electrosurgical devices. Measurements and
computer simulations conducted by the authors to illustrate importance of different
parameters for the specific treatments of skin and subcutaneous fat are also presented.
Keywords: radiofrequency, monopolar, bipolar, RFAL, micro-needling
1. RF treatment effect
The method of operation for the vast majority of esthetic energy-based devices
(EBDs) is through the generation of heat causing physiologic modifications to the
human tissue. RF energy is a method to deliver heat into the human body at a level
and distribution required for the specific application. For sub-necrotic thermal
applications, this heat can be a relatively low temperature for fibroblast stimulation
and metabolism acceleration (hands free RF devices). Alternatively, the heat can be
more aggressive, ablative coagulative and necrotic in nature (RF assisted lipolysis or
Fractional micro-needling technology). It may occur that during the same treat-
ment, RF energy effects will be both non-ablative on the skin and ablative-
coagulative sub-dermally.
In most instances with RF, microwave and light-based technologies, heat is the
result of a common pathway for the desired thermal effects. This understanding has
given rise to an entire generic category of esthetic and medical EBDs.A variety of
technologies and devices have been developed based on thermal treatment of tissue,
either ablative or non-ablative, selective, or non-selective, using optical energy, RF
electrical current, focused ultrasound to generate the heat. The common outcome of
these devices leads to some heat-assisted transformation of local tissue. This
thermally stimulated tissue alteration or remodeling typically results from:
Selective thermal targeting of tissue by focusing energy at the desired spot
internally or externally. Energy can be delivered to the selected volume in a
minimally invasive manner by focusing energy to penetrate the tissue under the
1
skin surface. An example of a minimally invasive treatment is electro-surgical
devices which deliver thermal energy into the body via a tiny cannula or needle.
Alternatively, electrocautery devices focus the energy on the tissue surface,
ablating the tissue in proximity of the tip of the instrument to dissect the a soft
tissue.
Non-selective bulk heating, used mostly for sub-necrotic heating to stimulate
natural processes in the body leading to increased production of collagen,
elastin and ground substances. The result may include tissue tightening,
circumferential reduction and wrinkle reduction.
RF energy is an important part of the armamentarium for treatment options
comprising tissue cutting and coagulation, minimally invasive selective tissue
targeting and bulk heating. RF current is the accepted type of energy used in four
out of five surgeries conducted in the world and most industry leaders in the
aesthetic field employ RF energy in at least one of their applications.
2. What is electromagnetic energy?
Electromagnetic (EM) energy travels in waves and spans a very broad wavelength
spectrum from DC voltage, to very short wavelengths in gamma radiation (Figure 1).
RF energy is small part of electromagnetic spectrum having frequencies in the
range of Kilohertz to Gigahertz. The shorter wavelength and the higher frequency,
the more energetic are quanta of EM radiation and the more destructive it can be for
the tissue. RF energy, Microwaves, Infrared and Visible Light has relatively low
frequencies and represent non-ionizing radiation which is not able to modify the
DNA (genes) inside the cells. High frequency radiation as UV, X-ray and Gamma
are ionizing radiation which in natural conditions is generated by plasma or by
radiative isotopes.
A very small part of RF spectrum range is used in EBD, and its properties will be
the primary focus of the current chapter.
3. The history of RF
RF energy has been used in medicine for over 100 years. Nikola Tesla, (1856
1943), Croatia-born electrical and mechanical engineer, is reputed as being the
father of alternating high frequency current. But it was Dr. William Bovie (yes, of
Figure 1.
Electromagnetic Spectrum.
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Enhanced Liposuction - New Perspectives and Techniques
the Bovie cautery fame) that developed the first electrosurgical device during the
period of 19141927 at Harvard University [1]. The first reported use of an electro-
surgical generator in an operating room occurred on October 1st, 1926 in a surgery
performed by Dr. Harvey Williams Cushing [2, 3]. Since Dr. Bovie introduced RF
energy and the electrocautery, RF had been used for ablation [4] and coagulation
[5] in surgery and medicine. Over the past 20 years, RF energy has evolved and
come to dominate esthetic medicine (for good reasons, as will be explained in this
chapter). RF was first being used in non-ablative form for skin collagen remodeling
and other esthetic applications (Figure 2) [6, 7].
4. Radiofrequency in medicine
The specificity of RF energy in medicine is that it acts as an electrical current
flowing through the tissue but differently than radiation. RF energy is associated
with electro-surgical devices and can be defined as high frequency alternating
electrical current heating soft tissue without significant electrical nerve stimulation.
It is critical to minimize nerve impact to avoid electric shock which may cause
muscle spasm and cardiac arrest.
It is important to remember that tissue has ion conductivity with the most
prominent varieties being Na+, K+, and Cl(sodium, potassium, and chlorine ions
respectively). Nerves are affected as a result of ion penetration through the mem-
brane of neuron. Under normal conditions the nerve is surrounded by electrically
neutral liquid where ions with positive and negative charge compensate each other
and bound by Coulomb force preventing free diffusion of the electrical charge. As
an electrical field is applied the ion starts to move and the nerve stimulating effect
depends on ion displacement (D) in alternating electrical field that can be presented
as following:
DμE
f(1)
where μis mobility of ions which is proportional to conductivity of tissue σ;Eis
electrical field strength; fis frequency of electric field.
Figure 2.
The early pioneers of RF energy in medicine, [8] (A), [9] (B) and [10] (C).
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It is obvious the displacement of the ions is higher when electrical field is
stronger and it is applied for longer time (Figure 3).
In general, polarity of RF voltage is changed so fast that ions vibrate in the
same place without significant movement. However, users of RF may
occasionally observe small muscle tweaking when high RF parameters are used.
Therefore, RF energy used in electrosurgery is limited by lowest frequency of
100 kHz, while the recently developed esthetic devices operate at frequencies above
300 kHz.
The typical range of RF is 100 kHz to 5 MHz according to the FDA guidance
[11]. This is intended to exclude other frequencies that may technically fall within
the RF portion of the electromagnetic spectrum, but operate in a fundamentally
different manner. However, there are few products with higher RF frequency of up
to 40 MHz. If RF is higher than 5 MHz there is significant radiative component with
reduced capability to predict the distribution in the patients body and can even
potentially affect the treatment attendant.
The ions oscillating in RF field interact with the surrounding tissue, losing its
kinetic energy and generating the heat. The heat generated by electrical current in
conductive media is described by Joules law:
H¼σE2(2)
The heat generated in each point of tissue is proportional to tissue conductivity
(σÞand square of electric field (E).
The Ohms low in vector form allows to calculate the density of RF current ( j)in
each point of tissue:
j¼σE(3)
While continuity equation allows to analyze RF current distribution in the tissue
j¼0(4)
The Eq. 4 states that electrical current coming into any volume of tissue is equal
to the current going out of the same volume (Figure 4).
Figure 3.
Ione displacement for a) low amplitude and high frequency of electric field; b) low amplitude and low
frequency; c) high amplitude and low frequency.
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The other conclusion from the charge continuity equation is that all RF current
emanating from one electrode into the tissue flows to the other electrode. The
current density on the electrode surface depends on the size of the electrode.
5. RF penetration depth
Penetration depth of RF energy depends on the electrode geometry and diver-
gence of the RF current inside the tissue. We will determine RF penetration depth
as the depth where RF energy is decreased by exponential factor (e = 2.71 ) and
analyze a few typical cases (Figure 5).
The first case in Figure 5 illustrates small electrode distant from the return
electrode. The RF current density and consequently electric field in vicinity of the
electrode diverges spherically and current density drops as square of distance from
electrodes. Taking into the account that heat is proportional to square of electric
field. Therefore, heat created by RF energy can be represented as following:
H¼σEo
r0
r0þd

4
(5)
Where E
0
is electric field on the surface of semispherical electrode, r
0
is radius of
electrode and d is distance from the electrode. It is easy to calculate that heating
drops by exponential factor at d = 0.28 r
0
. For the electrosurgical electrode having
tip with radius about 0.5 mm the RF penetration into the tissue is about 140
microns. Such small RF penetration depth allows to cut the tissue with minimal
thermal damage.
Figure 4.
Schematic illustration of continuity law.
Figure 5.
RF current distribution for typical geometries of electrodes.
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Figure 5b shows two long electrodes having cylindrical surface contacting the
tissue. The distance between the electrodes is larger than an electrode size. In this
case the heat distribution near the electrode can be calculated using the following
equation:
H¼σEo
r0
r0þd

2
(6)
The heating drops by exponential factor at the distance of d = 0.64 r
0
. Such
configuration of esthetic devices is commonly used, but the penetration depth is
limited and most of the energy is concentrated near the electrode.
The case shown in Figure 5c represents two parallel electrodes having size
comparable with the distance between them. Analysis of heat distribution required
computer simulation but RF penetration depth can be estimated as half distance
between the electrodes [7].
The thermal measurements conducted for the three cases described above are
shown in Figure 6.
Thermal experiments were conducted using porcine tissue and a RF generator
with the frequency of 1 MHz and 50 W power. The thermal camera FLIR A320 was
used for thermography of tissue during RF application.
Heat conductivity, real geometry of electrodes and non-uniformity of tissue
effect the thermal imaging but measurements correlate well with theoretical con-
sideration.
6. Tissue conductivity and impedance
The electrical properties of tissue play important role in understanding of RF-
tissue interaction.
Tissue conductivity is a strong function of tissue type. The fundamental article
of Gabriel et al. [12] summarized data on electrical conductivity for different types
of tissue. Figure 7 shows tissue conductivity of fat and skin in broad range of
frequencies.
In the RF range, the tissue conductivity is a weak function of frequency. The
tissue has resistive and capacitive properties. The capacitance of tissue in RF diapa-
son is determined by recharging of cell membrane.
The properties of different types of tissue are presented in Table 1.
Our measurements in-vivo for tumesced adipose tissue show that fats conduc-
tivity is very similar to the one of skin and is in the range of 1 to 2 S m
1
.
Conductivity of tissue is a function of temperature and is changed in the range of
sub-necrotic heating by 2%/
o
C [13]. Our measurements of tissue conductivity
Figure 6.
Thermal measurements of tissue temperature generated by RF current for typical geometries of electrodes.
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Enhanced Liposuction - New Perspectives and Techniques
between two electrodes in-vivo showed smaller change for the temperature close to
the normal body temperature and larger change when tissue temperature deviated
more (Figure 8). The tissue was pre-heated to 43 °C during 15 min and then tissue
impedance was measured for short RF pulses during two hours as skin cooled down.
As tissue is heated to higher temperatures resulting in tissue coagulation and
dehydration, the tissue impedance is increased dramatically [7]. Schematic change
of tissue impedance as function of temperature is shown in Figure 9.
Figure 7.
Electrical conductivity of skin and fat as a function of frequency of electrical current.
Tissue Conductivity, S m
1
Blood 0.7
Skin 0.25
Fat 0.03
Bone 0.02
Table 1.
Tissue conductivity at 1 MHz [12].
Figure 8.
Impedance of tissue measured between two electrodes applied to the skin surface.
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As mentioned above regarding conductivity, heating of tissue reduces its
impedance with a rate of about 2% per degree Centigrade [13]. This change is
related to reduction of tissue viscosity which is reduced with temperature increase.
Coagulation of the tissue causes a chemical change in tissue structure, subsequently
changing the trend of impedance behavior. When heating up to 100 °C, the evapo-
ration of liquids dehydrates the tissue, dramatically increasing tissue impedance.
Additional heating of the tissue leads to its carbonization. Dependence of tissue
conductivity on temperature is utilized by ELOS (Electro optical synergy) technol-
ogy where tissue is preheated using optical energy creating a preferable path for RF
current [14, 15]. This can provide treatment advantages for some applications.
7. RF waveform
The RF energy can be delivered in continuous wave (CW) mode, burst mode
and pulsed mode (Figure 10).
Figure 9.
Schematic impedance behavior as function of temperature.
Figure 10.
Typical RF waveforms.
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Enhanced Liposuction - New Perspectives and Techniques
For gradual treatment of large areas, the CW mode is most useful, allowing for
the slow increase in temperature in large tissue volume. It is used for treatment of
cellulite, subcutaneous fat and skin tightening. CW mode typically delivered in
device intended for moving over the treatment area.
The burst mode delivers RF energy with repetitive pulses of RF energy. It is used
in applications where peak power is important while average power should be
limited. Such an example would be blood coagulation. Also, it is used in hands
free devices where energy is added by small portions maintaining the required
temperature.
Pulsed mode is optimal when small tissue volume should be affected without
heat spreading to the surrounding tissue. Pulsed mode is used in micro-needling
devices.
8. Effect of spot size
In order to create tissue ablation, very high energy density is required. In
electro-surgical cutting instruments, a very small electrode, or needle type electrode
is used to concentrate electrical current to very small area, which increases the
energy density to ablative levels. Coagulation instruments, which create energy and
thermal profiles coagulating the cells and shrinking the collagen, usually have larger
surface area electrodes than ablative devices. Typically, the surface area of such
electrodes is a few square millimeters to generate heat in larger volume but at a
lower level to create coagulation rather then ablation. Sub-necrotic heating is usu-
ally used for treatments related to stimulation of natural processes in the tissue,
such as collagen remodeling, revascularization, speeding fat metabolism. In this
case the spot size is about 1 square centimeter or larger. Schematical illustration of
spot size effect is shown in Figure 11.
Generally, the smaller the electrode, the higher the energy density and the effect
tends to be ablative (e.g., cutting cautery tips), whereas larger sized electrodes,
have a gentler tissue effect, either coagulation (hemostasis) or sub-necrotic tissue
heating [16].
Figure 11.
The effect of electrode size, or spot size on the energy and power density.
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9. Monopolar RF systems
RF current always flows between two electrodes having opposite polarity. The
FDA definition of monopolar devices relates to the size and position of electrodes in
respect to patient during the treatment. According to FDA guidance [11],
monopolar is an electrosurgical technique in which the current flows from a single
active electrode at the surgical site, through the patient, to a relatively distant return
electrode.
The most common feature of a mono-polar device is a single electrode applied in
the treatment area while the return electrode has a much larger contact surface and is
placed outside of the treatment zone, usually in the form of a grounding pad. In this
electrode geometry, the high RF current density is created near the active electrode
and RF current diverges toward the large return electrode. The heat zone for this
geometry can be estimated using analytic spherical model for continuity equation
stating that electrical current flows continuously from one electrode to another.
rj¼0(7)
Taking into account Ohms law in differential form (Eq. 3) and the definition of
an electric field, Eq. 5 can be rewritten as:
1
r2
rr2φ
r¼0(8)
Where φis the potential of the electric field. The solution for this equation
provides RF current density distribution between electrodes.
j¼σVr0R
r2Rr0
ðÞ (9)
Where σis tissue conductivity, Vis voltage between electrodes, r
0
is radius of
small electrode and Ris the radius of the large electrode.
For the instance when the return electrode is much larger than the active
electrode, the equation can be simplified as:
j¼σVr0
r2(10)
Correspondently, heat power according to Joules law can be estimated as:
P¼σV2r2
0
r4(11)
This simple equation leads to a few interesting conclusions:
Heat generated by RF current near the active electrode does not depend on
position of the return electrode when return electrode is much larger in size than
the active electrode and located at a distance which is much larger than the active
electrode size.
Heating decreases dramatically as distance increases from the active electrode.
As was shown before, RF energy penetration depth is about one third of electrode
radius (Figures 5 and 6). However, heating temperature on the electrode surface
may reach hundreds of degrees centigrade and coagulation effect may be extended
much larger than RF penetration depth. The other factor enlarging thermal zone is
heat conductivity spreading heat around.
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Enhanced Liposuction - New Perspectives and Techniques
RF current behavior in the body for monopolar systems is visualized schemati-
cally in Figure 12.
RF current is concentrated on the active RF electrode and rapidly diverges
toward the return electrode.
Monopolar devices are most commonly used for tissue cutting. Schematically,
the RF current flow through the patient for monopolar devices is shown in
Figure 13.
The RF current is always flowing through a closed loop via the human body. As
we showed above, the current density out of the vicinity of the return electrode is
negligible. However, a malfunction where some low frequency current escapes out
of a monopolar configuration holds high risk because the entire body is exposed to
the electrical energy. Most commercially available devices have isolated output to
avoid any unexpected RF current path to the surrounding metal equipment.
Treatment effects with monopolar devices depend on RF power and size of
electrode. The classic use of monopolar technique is tissue cutting and ablation
while occasionally it is used for soft tissue coagulation or sub-necrotic heating
[6, 1719].
The main features of monopolar devices are:
Figure 12.
Schematic RF current distribution between electrodes for monopolar system.
Figure 13.
Electrical current flowing through the patient and monopolar electrosurgical device.
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Predictability of thermal effect near the active electrode
Ability to concentrate energy on a very small area
High non-uniformity of heat distribution which is strong at the surface of the
active electrode and is reduced dramatically at a distance exceeding the size of
electrode, thereby limiting penetration depth
10. Bipolar RF systems
According to FDA [11], bipolar is an electrosurgical device in which the current
flows between two active electrodes placed in close proximity. In bipolar devices
both electrodes create a similar thermal effect and are applied to the tissue treat-
ment area (Figure 14). Bipolar devices create larger thermal zones and this circuit is
used in electro-coagulators. The advantage of bipolar systems is the localization of
all RF energy in the treatment zone (Figure 14).
Bipolar devices concentrate all RF energy between electrodes in the treatment
area. This geometry is more suitable than a monopolar system to create uniform
heating in larger volume of tissue. In order to understand heat distribution between
electrodes the following three rules should be taken into the account:
Heating is always higher near the electrodes surface and reduces with a
distance because of current divergence. Divergence of RF current between
electrodes reduces current density and correspondently generated heat.
For any geometry, RF current density is higher along the line of shortest
distance between the electrodes and reduced with distance from the electrodes.
RF current is concentrated on part of the electrode having high curvature
creating the hot spots.
A schematic distribution of electrical currents in uniform media in bipolar
device is shown in Figure 15.
In bipolar devices, both electrodes create an equal thermal effect near each of the
electrodes and the divergence of RF current is not as strong because of the small
Figure 14.
Electrical current flowing through the patient and bipolar electrosurgical device.
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Enhanced Liposuction - New Perspectives and Techniques
distance between the electrodes. For bipolar systems shown in Figure 15, most of
the heat is concentrated between the electrodes.
Penetration depth of RF for bipolar devices is a function of electrode size and the
distance between them. By increasing the distance between the electrodes, the elec-
trical current can go deeper, but divergence is also increased. In case the distance
between the electrodes is much larger than the electrode size, the heating profile will
be similar to two monopolar electrodes. Schematically, bipolar current distribution
and measured thermal effect are presented in Figures 5b and 6b,respectively.
The most uniform distribution of RF current is obtained in planar geometry
when tissue is placed between two large parallel electrodes. This can be realized
when negative pressure forces the tissue to fill the cavity between the parallel
electrodes. Measured RF energy distribution for the cavity filled with the tissue is
shown in Figure 16.
11. Capacitive coupling of RF energy
High frequency current is able to penetrate through the dielectric material which
behaves as capacitor. This effect is used to isolate metal electrode from patient. This
Figure 15.
Electrical current distribution for bipolar system.
Figure 16.
Thermal image of heat distribution created in the skin folded between two parallel electrodes.
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method is called capacitive coupling. There are a number of devices in the medical
esthetic market that use this technology for RF delivery [18, 19].
The capacitance of planar dielectric layer is described by the following equation:
C¼εε0S
L(12)
Where εis dielectric constant of dielectric material, ε0is the vacuum permittivity,
Sis area of dielectric and L is thickness of the layer.
Impedance of the dielectric layer depends on frequency of current (f)
R¼1
2πfC (13)
For example, polyimide layer with area of 4cm
2
and thickness of 100micron has
capacitance of about 106 pF and impedance of this layer is 1.5 kOhm at 1 MHz and
375 Ohm at 4 MHz.
For cylindrical geometry capacitance is represented by the following equation
C¼2πεε0L
ln b
a
 (14)
Where ais inner diameter and bis outer diameter of dielectric coating.
The leakage of RF current through the dielectric coating should be taken into the
account at design of electro-surgical instruments.
12. Thermal relaxation time
The temperature dissipation is characterized by thermal relaxation time (TRT)
of the targeted area. For localized treatment, in order to avoid significant heat
transfer, the pulse duration should be less than the TRT.
The TRT is a function of tissue thermal properties, heated volume shape and
size. Soft tissue has thermal properties close to the water.
For the planar object the TRT can be estimated as [20].
TRT ¼d2
4a(15)
Where dis thickness of layer, and ais tissue diffusivity. Diffusivity is equal to
tissue conductivity divided by the heat capacitance and measured in cm
2
s
1
.
For a cylindrical object, such as a blood vessel or hair, a similar equation can be
used with different geometrical factors.
TRT ¼d2
16 a(16)
where dis object diameter; one can see that cooling time is square function of
the size.
Thermal relaxation time should be taken in to the account when thermal
effect should be localized. It is critical in fractional RF technologies when thermal
coagulation should by limited by small zones around the needle electrodes.
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13. Tissue modification by RF energy
The thermal effect of RF on tissue is not different from laser or any other heating
method. Multiple studies [21, 22] discuss the temperature effect on tissue. Since
treatment effect is not only a function of temperature, but also of the period of time
(when this temperature is applied), it is known that in the millisecond range the
coagulation temperature is 70-90 °C, while if temperature is applied for a few
seconds, the temperature of 45 °C causes irreversible damage. Hyperthermia stud-
ied intensively for treatment of cancer confirms strong dependance of tissue vitality
on time that temperature is maintained [23]. RF induced hyperthermia was mea-
sured for adipocytes in a clinical study [24]. The fat cell viability was 89% after RF
heating for 1 min at 45 °C while after heating during 3 min the vitality dropped
down to 40% (Table 2).
There is extensive data on the correlation between tissue temperature,
pulse duration and treatment effect. Moritz and Henriques demonstrated that
the skin thermal damage threshold is a function of temperature and time [25].
Later it was demonstrated that skin damage function can be described by
Arrhenius equation where pre-exponential factor is a linear function of pulse
duration [22].
D¼At exp E
RT
 (17)
Pulse duration is one of the most critical parameters when utilizing RF energy in
order to achieve a clinical response. It affects treatment results because timing
influences the thermo-chemical process in tissue. The other effect of pulse duration
is energy dissipation away from the treatment zone due to heat conductivity from
the exposed area to the surrounding tissue.
In other words, the degree of damage is a linear function of pulse duration and
an exponential factor of tissue temperature. This means that tissue temperature is
more influential on the degree of damage than the pulse duration.
It is well known that sustained hyperthermia at 42 °C for tens of minutes causes
death of most sensitive cells such as in the brain [26]. In laser medicine the pulse
duration in the millisecond range causes tissue to burn at a temperature above
60-70 °C.
Dehydration and carbonization of the ablated treated tissue may cause the
accumulation of a non-conductive tissue layer on the electrode surface. This tissue
is sometimes called eschar and if it accumulates on the surface of the treatment
electrode, it may affect significantly the energy delivery to the electrode and hence
Temperature Tissue effect
37-44 °C Acceleration of metabolism and other natural processes.
4550 °C Conformational changes, hyperthermia (cell death)
5080 °C Coagulation of soft tissue
5080 °C Collagen contraction
90100 °C Formation of extracellular vacuoles, evaporation of liquids
>100 °C Thermal ablation, carbonization
Table 2.
Tissue thermal effect.
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the treatment zone or even damage the hand piece. Carbonization or Eschar reduces
or totally blocks the working area of electrodes and affects treatment efficiency,
reducing the electrical current flow to the tissue (Figure 17).
Usually, the surgeon must clean an electro-surgical instrument periodically during
the treatment to remove any eschar from the treatment electrode. Alternatively,
companies, like InMode created a technological solution avoiding this problem. In
InMode devices, impedance monitoring measures the increase resistance to flow
(increased impendence) caused by eschar on one of the electrodes and cuts off the RF
energy and flow of RF current briefly, minimizing the risk of the eschar built up at all.
The most important tissue modification induced by RF heating is a contraction
of collagenous tissue. This effect is known for decades and is used intensively in
orthopedy [27, 28] and ophthalmology [29].
Skin contraction was a primary focus for the first RF devices in esthetic medi-
cine [6, 15, 17, 19]. Only in the last decade there is the understanding that the skin
appearance is more affected by collagen in the reticular dermis and fibro septal
network (FSN) binding skin with superficial fascia and muscles. A study published
in 2011 [30] showed that skin has very dense collagenous tissue and shrinkage of
collagen fibers is limited, while connective tissue in the subdermal space may
contract above 30% during a few seconds of heating. The threshold temperature for
collagen contraction was measured in the range of 60-70 °C.
In the experiments in our facility, the contraction of FSN was quantified on ex-
vivo post abdominoplasty human tissue. The area was marked proximal to the
RFAL cannula tip and monitored during RF energy application. The resulting mea-
surements are presented in Figure 18.
One can see that thermal exposure of subcutaneous tissue with RF energy during
three seconds resulted in area contraction by 42%.
14. Radiofrequency assisted lipolysis (RFAL)
RFAL technology was developed by InMode Ltd. to improve treatment results
during liposuction procedure. The thermal contraction of collagen in dermis and
subdermal FSN allows treatment of patients with saggy skin and patients for whom
previously excessive skin was a main concern [31].
Figure 17.
Cutting Bovie electrocautery, with an eschar built upon the fine needle tip.
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Enhanced Liposuction - New Perspectives and Techniques
The uniqueness of RFAL technology is that it does not fall under any standard
device definitions. It combines features of monopolar and bipolar technologies,
minimally-invasive and non-invasive technologies, creating very specific energy
profile treating simultaneously subcutaneous fat, connective tissue forming FSN
and dermis. Each of these tissue components requires different thermal exposure.
Adipose tissue should be destroyed, FSN should be remodeled without denaturation
of collagen while skin should be exposed to sub-necrotic heat to modify it without
superficial burn [3133].
The RFAL device geometry is shown in Figure 19. The RF current flows back
and forth from the internal electrode (cannula tip), where the thermal effect is
coagulative, to a larger, external electrode. The external electrode moves along the
skin surface, in tandem with the internal electrode and creates a gentle, non-
ablative bulk heating effect on the dermis. Ratio between size of internal and
external electrode is selected to limit skin heating at sub-necrotic heating while
temperature in the fat should reach 50-70 °C.
Moving the hand piece back and forth through the intended treatment area,
uniform coagulation of adipose and vascular tissue is achieved. While the external
electrode is always moved over the skin surface, the internal electrode should pass
through the deep, intermediate and/or superficial fat layers to treat the adipose
Figure 18.
Subcutaneous fat before and after application of RF energy.
Figure 19.
Schematic depiction of RFAL treatment geometry.
17
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DOI: http://dx.doi.org/10.5772/intechopen.96652
tissue up to the depth of 5 cm. The Lipo-coagulation, results in liquefaction of the
adipose tissue, hemostasis and stimulated contraction of adjacent vertical, oblique
and horizontal fibers of the FSN, that connects the overlying soft tissue to the
underlying muscle.
Figure 20 shows thermal profile created by RFAL cannula inside porcine tissue.
The temperature around the internal electrode is 70 °C. The volume exposed to
high temperature around the cannula. The tissue between internal and external
electrode is exposed to directional RF flowing between the electrodes.
Computer simulation shows similar thermal profile (Figure 21) to the measured
thermography.
One of the advantages of RF energy is that it is can be delivered into the body
though the very tiny sub-millimeter cannula. That allows to minimize incision and
mechanical trauma at treatment of such delicate zones as face and neck [33]. Large
size cannula results in higher non-uniformity and especially for subcutaneous fat.
Figure 20.
Thermal profile in the tissue created by RFAL device.
Figure 21.
Computer simulation of temperature field created by RFAL device.
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Enhanced Liposuction - New Perspectives and Techniques
15. Micro needling RF
Another RF based technology enhancing liposuction results is micro needling
RF. The fractional coagulation of subcutaneous tissue helps tight the skin and
reduce skin sagginess after liposuction [34].
Fractional skin treatment was introduced in esthetic medicine about two
decades ago and has become one of the most popular modalities for the improve-
ment of skin quality. This procedure is based on the coagulation of multiple small
spots with a size of 100 microns to 0.5 millimeter. This allows the procedure to be
very tolerable and with relatively short down-time. Focused laser beams or needle
sized RF electrodes are used for ablation of micro-spots resulting in high efficiency
and consistency of the treatment, with low risk of side effects and fast skin healing.
In contrast to lasers where the thermal effect is limited by the ablation crater, the
RF energy flows through the whole dermis, adding volumetric heating to fractional
treatment. This volumetric bulk heating adds a skin tightening effect to the more
superficial improvement generated by tissue ablation.
RF fractional technologies are differentiated by needle length and size. The flat
electrodes provide a more superficial effect improving texture and fine lines
[34, 35] while longer needles penetrate deeper, providing deeper dermis remodeling
and causing substantial skin tightening [36].
The needles can penetrate to the different depths allowing epidermal ablation
and deep subdermal treatment. Recently the FDA cleared Morpheus8 device of
InMode Ltd. for treatment up to depth of 7 mm.
Figure 22 shows Morpheus8 tip schematically with needles extended to the
subdermal fat.
Needles coated with polymer and releasing RF energy only at the needle end
provide better protection of epidermis and provide lower down time.
A microscope image of a coated needle is shown in Figure 23. The gold plated
needle has diameter of 0.3 mm and coated with polymer of 20 microns thickness.
There are several different configurations of RF electrodes for micro-needling
devices. The most common configuration is by applying RF energy between adja-
cent rows of needle electrodes. This method creates a coagulation zone in vicinity of
the needle end.
Figure 22.
Schematic illustration of Morpheus8 tip with needles penetrating into the sub-dermal space.
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DOI: http://dx.doi.org/10.5772/intechopen.96652
The alternative technology is used in the InMode Morpheus8 device where RF
energy is applied between the needle and an external electrode applied to the skin
surface. Each needle has a strong thermal effect near the needle end and gradient of
bulk heating toward the external electrode, similar to RFAL technology. Each nee-
dle generates small bulk heating but superposition of the heat from multiple needles
results in essential thermal effect. Morpheus8 device automatically treats tissue in
multiple layers delivering RF energy sequentially during needle retraction. This
burst mode creates three-dimensional matrix of coagulation zones and strong bulk
heating. Schematically the burst mode treatment is shown in Figure 24.
Micro needling technology was developed for treatment of facial wrinkles but
further development of the technology has extended its use to treat the body as
well.
The micro needling technology supplements both regular liposuction and
energy-based minimally invasive technologies and addresses the first few
millimeters of body coagulating adipose tissue and tightening FSN.
Figure 23.
Coated needle.
Figure 24.
Schematic illustration of burst mode treatment using Morpheus8 device.
20
Enhanced Liposuction - New Perspectives and Techniques
16. Treatment control
One of the risks of any thermal treatment (laser, ultrasound, plasma or RF) is the
possibility of a thermal skin injury. Thermal treatment in subcutaneous or subdermal
layers may create full thickness skin burn. Therefore, monitoring of delivered energy,
predictability of energy distribution and accurate measurement of tissue parameters
during the treatment has crucial importance for the energy-based devices.
16.1 Tissue temperature measurements
Non uniform treatment or over-heating the treatment area may result in the risk
of unwanted thermal damage to the skin during the treatment. To avoid or mini-
mize this risk of a skin burn, real time thermal measurements are necessary. There
are two basic methods of skin temperature measurements:
Infrared (IR) thermometers measuring IR radiation of heated object.
Contact measurements using a thermocouple, thermistor or
thermo-transistors.
Advantages of IR thermometers is the speed of measurements and that they do
not need to be built into the device thus are independent of the treatment. The
obvious weakness of this method is collecting IR radiation from relatively large area
which depends on distance from the measured area. You are also relying on a third
party that is not linked in time of space to the thermal treatment being performed.
Most importantly, you are not measuring the internal thermal profile.
A typical IR thermometer measures area which depends on distance between
skin and thermometer and it varies from 1cm
2
to a few square inches at large
distance from the patient. It allows you to monitor average skin temperature in
treatment area but does not protect from appearance of small hot spots that lead to
the full thickness skin burns.
The thermistors or thermocouples are extremely miniature and can be embed-
ded into the electro-surgical instrument. Limitation of such contact measurements
is response time which depends on heat transfer from the tissue to the sensor.
However, special design allows to reduce response time to sub-second range.
Ideally, the user should know the temperature inside the body where energy is
utilized for the fat coagulation and FSN tightening, and temperature on the skin
surface above the treatment zone to ensure skin safety.
In addition, during the procedure sophisticated mechanisms monitor the tissue
temperature together with its dynamic characteristics as the speed of temperature
rise, allowing precautional measures before the critical temperature is reached.
Temperature monitoring for EBD is important not only for safety but also for
treatment efficacy. Collagen contraction occurs in relatively narrow range of tem-
peratures from 50 °C to 80 °C and overheating may result in denaturation of
collagenous tissue and uncontrolled scar formation.
RFAL technology has maximum thermal safety measurements including:
Skin temperature monitoring;
Fat temperature monitoring;
Temperature surge protection catching fast temperature changes.
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16.2 Monitoring of delivered energy
Most types of energy cannot be monitored directly but rather electrical supply to
the energy source is monitored. RF energy has unique properties resulting from
continuity Eq. (4) allowing to measure RF voltage and RF current flowing through
the tissue and get in real time all information about energy deposition in the tissue.
Measurement of electrical RF parameters is not difficult engineering project and it
can be performed every micro-second that allows to control the RF energy delivery
even for very short pulses.
Measurements of RF current (I) and RF voltage (V) allows to calculate RF
power (P) and RF impedance (R) using Ohms law
R¼V
I(18)
and Joules law
P¼VI (19)
The RF energy can be calculated as integral of RF power measurement over the time:
E¼ðt
0
Pdt (20)
RFAL and Morpheus8 technologies of InMode Ltd. utilize all these measure-
ments to control the treatment safety and efficacy.
16.3 Impedance sensing and control of RF output
Measurements of tissue impedance should be considered separately because of
importance of this parameter for different aspects of treatment. The most obvious
use of the impedance measurements is indication of contact between electrodes and
treated tissue. Contact measurements are important to avoid poor coupling of the
RF device with patient and avoid arcing. Contact monitoring has become a common
feature for most RF-based devices.
Referring to Figure 9 one can see that coagulation, dehydration of tissue and
eschar formation result in impedance increase. Monitoring of tissue impedance can
be used to limit heating process and avoid undesired treatment effect.
Another use of impedance monitoring is to control the lower limit, which may
indicate that the distance is too small between electrodes. In RFAL technology it is
used to reduce the risk of the cannula coming too close to the skin surface.
16.4 Safety features of the RF devices
All above mentioned measurements of RF parameters worth nothing if its not
used for enhanced treatment safety helping physician to optimize the procedure.
The BodyTite device from InMode Ltd. uses RFAL technology, combines the
maximal number of safety features, and should be used as the gold standard for
safety features for RF devices.
Performing liposuction, the physician should be concentrated on safe manipula-
tion with the minimally invasive accessory. Safety features related to the thermal
component of the treatment should be implemented in automatic or in a very
intuitive way not disturbing physician attention.
22
Enhanced Liposuction - New Perspectives and Techniques
The skin impedance for each patient is different and may vary for the different
treatment zones, amount of tumescent applied or treatment depth. RF energy is
adjusted by the device automatically to provide the required optimal energy to the
patient.
Tissue impedance is monitored constantly by the BodyTite and the device
automatically cuts RF energy if some of the limits are exceeded.
The user may set desired temperature cut-off limits for skin and internal elec-
trode. The device applies full power when the temperature is significantly below the
threshold and starts to reduce power automatically as treatment approaches the
required target temperature. This scheme allows to avoid thermal overshooting and
maintains desired heat profile. RF energy delivery is accompanied by an audible
signal which speeds up as the cut-off temperature is approached, similar to modern
car approaching wall while parking. RF power is switched on and off automatically
to maintain the desired temperature as the user scans the treatment area with the
cannula.
If the cannula accidentally comes too close to the dermis, the tissue volume
between the electrodes is reduced and the applied RF power heats the tissue
extremely fast. To address this issue, a temperature surge protection is
implemented in BodyTite device. When the temperature sensor measures a
temperature increase as too fast, the device automatically shuts RF energy and
produces an audible sound to attract the physicians attention.
17. Summary
RF based medical devices are a common tool for plastic surgeons, used during
most surgical procedures. RFAL and RF fractional technologies have become impor-
tant modalities for about 20% of plastic surgeons, for enhancing liposuction results or
by its own for patients for whom reduction of adipose compound is not a main
esthetic goal. Over the last 100 years extensive knowledge has been acquired about
RF technology and RF-tissue interaction. The information in this chapter can help a
potential buyer of new equipment make a rational choice, based on goals of treatment
and physics of the RF device in question. Even more importantly, expanding the
physicians understanding of his or her devices already in use can maximize treatment
outcomes and minimize unwanted side effects and complications.
Author details
Michael Kreindel
1
* and Stephen Mulholland
2
1 Inmode Ltd., Richmond Hill, Ontario, Canada
2 Private Practice, Toronto, Canada
*Address all correspondence to: michael.kreindel@inmodemd.com
© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms
of the Creative Commons Attribution License (http://creativecommons.org/licenses/
by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
23
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DOI: http://dx.doi.org/10.5772/intechopen.96652
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Enhanced Liposuction - New Perspectives and Techniques
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Chapter
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