(229bq) A Microballistic Approach for Treatment of Keratoconus Using Eosin Y

A Microballistic Approach for Treatment of
Keratoconus Using Eosin Y

Benjamin
Laccetti and Professor Julia Kornfield

California
Institute of Technology Department of Chemical Engineering

Introduction:
The Kornfield Lab
at Caltech, in collaboration with the Schwartz lab at UCSF, has devised a new
treatment for kerataconus, a disease in which the human cornea distends under
intraocular pressure. Using the photosensitizer eosin Y and nontoxic green
light, collagen fibers in the corneal stroma (the layer that gives the cornea strength)
can be cross-linked to resist viscoelastic creep and further progression of
this disease. In order to deliver eosin Y to necessary depths in the cornea,
our lab has also conceived a novel way to traverse the epithelium (a tissue
layer resistant to diffusion of small molecules) without debridement, a
treatment that has a high risk for infection and scarification of the necessarily
transparent cornea. Using a pneumatic capillary gun, our research suggests that
~10 um ballistic particles composed of eosin Y can been embedded in the corneal
stroma with little damage to the tissue. Successful embedment of particles in a
gel model of corneal tissue has been demonstrated as well as penetration of porcine
corneal tissue using particles that have equal density compared to eosin Y.

Materials
and Methods: A pneumatic capillary gene gun
developed with Prof. Alex Groisman at UCSD is being used to accelerate
fluorescent, polystyrene microspheres (Fig. 1).¹ 1% w/v agarose gels,
which we have shown to have similar viscoelastic properties compared to the
cornea, are being tested along with corneas from porcine eyes. These samples
are exposed to ballistic particles traveling at high velocity. Penetration
depth and damage to tissue was assessed with transmission and confocal
microscopy (Fig. 2). Wounds in the epithelium are visualized and measured using
fluoroscein topically applied to our ex
vivo model (enucleated porcine eyes kept alive for up to three weeks).

Results
and Discussion: Polystyrene particles ranging from 30
to 108 um diameter were embedded in the agarose model of corneal tissue using
the pneumatic capillary gun. Mean penetration depths are shown in Figure 3. It
was found that the expected penetration depth of particles in the gel scales with
the mass to surface area ratio. Using a MATLAB model, it was shown whether or
not particles would accelerate to the carrier velocity of the expanding gas.
The particles that accelerate to their terminal velocity follow the same linear
trend, but the particles with a higher mass to surface area ratio, which do not
fully accelerate, fail to reach expected penetration depths. This data from the
tissue surrogate suggested that using 30 um particles, the necessary depth of
100 um could be achieved to traverse the corneal epithelium. However, it was
found that in tissue, our scaling relationship no longer applies. Figure 4
shows 10 micron particle penetrating tissue to depths much greater than predicted.
However, particles that are much larger and have higher mass to surface area
ratios cannot penetrate the cornea. It is expected that this unexpected
behavior comes from the strain-dependent mechanical properties of biopolymer
networks.^2,3
Polymer based particles coated in eosin Y have been delivered to necessary
depths in the gel model and a vibrating orifice aerosol generator is being
constructed to form monodisperse eosin Y particles that will be embedded in porcine
corneal tissue for subsequent cross-linking treatment.

Conclusions:
In a gel model, a predictable scaling relationship for gel penetration
was discovered that is dependent on the mass to surface area ratio of
particles. However, in tissue it was found that the total size of particles,
which affects the total strain/deformation, affects penetration. It is expected
that the impermeability of the cornea to larger particles has to do with the
strain stiffening behavior of biopolymer networks. This research is revealing
new aspects of corneal biomechanics and suggests that we will be able to embed
eosin Y particles past the corneal epithelium for subsequent cross-linking treatment.

References: [1] Groisman, Simonnet, and Rinberg.
Pneumatic Capillary Gun for Ballistic
Delivery of Microscopic Particles into Tissue. US Patent 20080206870 (2008).
[2] Sharma, Licup, Jansen, Rens, Sheinman, Koenderink, and Mackintosh. "Strain-controlled Criticality Governs
the Nonlinear Mechanics of Fibre Networks." Nature Physics (2016). [3] Motte and Kaufman. "Strain
Stiffening in Collagen I Networks." Biopolymers
(2012).