(550e) Transdermal Delivery of Biopharmaceuticals Using Microsecond Thermal Ablation

 SEQ CHAPTER \h \r 1TRANSDERMAL DELIVERY OF
BIOPHARMACEUTICALS USING MICROSECOND THERMAL ABLATION

Jeong Woo Lee¹,
Priya Gadiraju¹, Jung-Hwan Park³, Mark G. Allen^1,2, Mark R. Prausnitz¹

¹School
of Chemical and Biomolecular Engineering, Georgia Institute of Technology,
Atlanta, GA

²School
of Electrical and Computer Engineering, Georgia Institute of Technology,
Atlanta, GA

³Department
of BioNano Technology and Gachon
BioNano Research Institute, Gachon
University,

 Sungnam,
Republic of Korea

Introduction

            The conventional transdermal
patch has provided advantages of non-invasive and simple administration, but it
is generally limited to only small molecular weight and lipophilic therapeutics
due to the protective function of the skin, which mostly resides in the top
superficial layer of epidermis, stratum corneum. Macromolecular
biopharmaceuticals cannot be delivered through the skin at therapeutic rates
due to the stratum corneum barrier, so that various technologies have been investigated
to overcome the skin barrier property such as mechanical disruption, radio
frequency-based heating, ohmic heating, and laser ablation. The key to success
with skin ablation technology is to develop methods to remove the stratum
corneum barrier selectively without causing deeper skin damage ^[1]. Here, we present transdermal
delivery of biopharmaceuticals with microsecond thermal skin ablation
technology assessed by modeling of heat penetration into the skin, fabrication
and characterization of a novel thermal ablation device, and skin permeability measurement.

Methods and Materials

   Heat penetration into the skin was simulated using
ANSYS software employing
a one-dimensional transient semi-infinite
model with the boundary condition ^[2] that the temperature of the skin surface was held constant
at various
temperatures up to 1100 ° during the
thermal exposure
of 1 µs to 100 ms. Simulations were carried out with and without a
conductive mask used
for guiding thermal energy only and with
and without a "windows" mask used for localized ablation effect as shown in Fig. 1.
The thermal ablation device
was designed by modifying a laminated gas generator actuator developed previously
for aeronautical purposes ^[3].
A microchamber consisting of five patterned and integrated layers (two brass electrodes, two
PMMA backing layers, and one microchamber layer) was fabricated using laser micromachining and lamination of each layer as shown in Fig. 2A. Two different
mask designs were fabricated to facilitate mechanistic study and device
optimization. A conductive mask was designed to efficiently conduct heat
without allowing physical contact between the steam and the skin. A windows
mask was designed to allow heat transfer to the skin through 100 µm-diameter
holes in the mask and to insulate the skin everywhere else.
For
the time characterization of thermal ablation, two voltage driver circuits were placed on either side of the
microchamber to measure the voltage drop across the microchamber and the MOSFET
switch. A ring-type piezoelectric force sensor was attached behind the microchamber to measure the force
exerted by the actuation of the device. During device actuation, in
some cases a high-speed video was taken at a rate of 100,000 frames per second
(EKTAPRO 4540, Kodak).

To visualize the
selective and localized removal of stratum corneum, a piece of full-thickness pig cadaver skin prepared
with IACUC approval was ablated and then a hydrophilic
sulforhodamine solution
was applied to the skin surface. To measure
the permeability of thermally abated skin, human cadaver skin was obtained with approval from
the Georgia Tech Institutional Review Board and either left intact or exposed
to thermal ablation. The
skin permeability
was measured for up to 48 h using a Franz diffusion cell, the donor compartment of
which was filled with either
10^-3 M sulforhodamine B or 10^-4 M Texas Red-labeled bovine
serum albumin (BSA) in
PBS.

Results and Discussion

Our
goal was to identify the longest thermal exposure that does not allow
significant heat to reach the viable epidermis using finite element modeling.
In Case I shown in Fig. 1A, when the skin is directly exposed to superheated
steam at 1100 °, the viable epidermis is predicted to have no significant
temperature rise after exposures of 1 ms, 10 ms and 100 ms as shown in Fig. 1B.
Because heat transfer is most delayed in the stratum corneum layer with the lowest
thermal conductivity of all the skin layers, heat barely penetrates into the
viable epidermis during these short exposures. After a 1 ms
exposure, the viable epidermis is predicted to be heated to a temperature of
180 °, which is expected to injure cells and cause pain as shown in Fig. 1E.

The
mask design used in this study as in Case II and III shown in Fig. 1A permits
sufficient heat transfer that the skin surface temperature is almost the same
as the ejectate temperature after millisecond exposures, but is significantly
lower after microsecond exposures as shown in Fig 1C and 1D. These data suggest
that heating the skin for on the order of 100 ms may
be optimal because that exposure time is long enough to allow the skin surface
to be substantially heated, but still short enough to avoid heating below the
stratum corneum.

 SHAPE  \* MERGEFORMAT

Figure 1. Simulation of heat penetration across skin
heated on its surface to 1100^oC for 1 ms
to 100 ms. (A) Schematic showing skin heating with
different schemes. Simulation predictions for heating (B) as direct exposure,
(C) through a conductive mask and (D) through a conductive and a windows mask.
(E) The time required to increase skin temperature by 10^oC at the
interface.

Guided
by the heat transfer simulation, we designed a thermal ablation device to
achieve thermal exposures on the order of 100 µs as shown in Fig. 2. After the
microchamber is connected to a power supply circuit (Fig. 2B), the electrical
energy stored in the capacitor (Fig. 2C) is discharged rapidly with the MOSFET
switch on, generating an electrical discharge between the two electrodes in the
microchamber. This nearly instantaneous event heats, vaporizes and ejects the
saline from the microchamber through the nozzle, indicating the rapid
transformation of electrical energy into thermal and mechanical energy that can
be used for skin ablation.
We found
that the thermal ablation device operates on a timescale on the order of 100
µs. The video image shown in Fig. 3A is the image of ejectate captured in a
single frame, indicating that the steam jet was ejected within 10 µs at the
most. As shown in Fig. 3B, the voltage across the microchamber rapidly rises
and peaks at 7 µs, which is followed by a sudden voltage drop until 50 µs,
which is the primary electrical discharge occurring in the microchamber. As
shown in Fig. 3C, the force exerted during electrical discharge peaks at 100
µs, which corresponds to the timescale over which the primary steam ejection
occurs. The force curve then oscillates due to resonance of the apparatus and
dissipation of the reaction force energy, but is not believed to reflect forces
generated directly by the steam jet ejection.

Figure 2. Microsecond thermal ablation
system. (A) Schematic diagram of the disintegrated layers of the microchamber,
(B) Image of a microchamber connected to a power supply circuit, (C) Diagram of
the electrical circuit used to transform electrical energy into thermal and
mechanical energy in the microchamber

Figure 3. (A) View of the ejectate from the device, (B)
Voltage change across the microchamber, and (C) Force measured during
electrical discharge.
Guided by the design, fabrication, and characterization
of microsecond thermal ablation system, we ablated the skin to cause selective
removal of the stratum corneum as shown in Fig 4A and 4B. The selective removal
of stratum corneum led to a remarkable increase in the transdermal flux of both
molecules studied, as shown in Fig 4C and 4D. After a lag time, skin
permeability to sulforhodamine and BSA was 3.1 (± 0.9) • 10^-2
cm/h and 3.6 (± 0.8) • 10^-3 cm/h, respectively. This
corresponds to a 10⁴-fold increase in permeability to sulforhodamine
and a 10³-fold increase in permeability to BSA after thermal
ablation in comparison with intact and untreated skin.

 SHAPE  \* MERGEFORMAT

Figure 4. (A) The surface of skin ablated with
microsecond thermal ablation device using a windows mask, (B) Histological
image of the skin sample in (B), Cumulative delivery of (C) sulforhodamine and
(D) Texas Red-labeled BSA.

Conclusion

Through
modeling, we determined that thermal ablation should occur on the 100 µs time
scale in order to ablate the stratum corneum selectively. To generate the high
temperatures needed for skin ablation on this time scale, we designed a
microdevice that rapidly heats water by an electrical discharge, thereby
ejecting a superheated steam jet at the skin surface. We found that this device
was able to selectively ablate stratum corneum and, when combined with a
windows mask, enabled three-dimensional control over tissue removal, resulting
in the increase of skin permeability to sulforhodamine and BSA. This study was
funded by NIH and by Tyco Healthcare (now Covidien).

Reference

[1]           J. W. Lee, P. Gadiraju, J. H. Park, M. G. Allen, M. R.
Prausnitz, J Control Release 2011, 154, 58-68.

[2]       M.
L. Cohen, Journal of Investigative
Dermatology 1977, 69, 333-338.

[3]       B.
English, in School of Mechanical
Engineering, Vol. Doctor of Philosophy, Georgia Institute of Technology,
Atlanta 2006.