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Rotating sphere for opto-mechanical characterization of contact lenses
using nano-indentation
Design Team
Jodi Belz, Dane Rubado,
Paul St. Pierre, Alexander Turovsky
Design Advisor
Prof. Kai-tak Wan
Co-Advisor: Prof. Andrew Gouldstone and Prof. Rifat Sipahi
Sponsor: Johnson & Johnson Vision Care
Abstract
Current mechanical characterization of contact lenses is found using nano-indentation. The current setup
allows the lens sample to lie on its convex surface on a stationary rigid planar substrate, and as a result,
indentation is only possible at the lens apex. This poses a problem when attempting to characterize the
properties over a specific location of the lens surface at specific meridional and azimuthal angles from the
apex. Contact lenses with a specific diopter have non-uniform thickness, and therefore the mechanical
properties will vary throughout the lens. This project aims to solve this problem by designing a
mechanically rotating spherical fixture that will hold the lens. The fixture will mimic the real-life
environment of a contact lens, i.e. the human eye, to acquire useful and accurate property data. The rotating
sphere will allow the user to derive a force vs displacement curve for the lens at any desired location. A
control system with a computer GUI will complement this fixture to allow the user to accurately and
precisely position the indenter over the lens.
Exploded View of Test Stand Assembly
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The Need for Project
To maximize user comfort, a
new rotational fixture is
necessary to test lens properties
at any point on the surface of the
lens.
This project aims to construct an apparatus for characterizing the
mechanical behavior of a soft hydrogel contact lens via nanoindentation. Mechanical properties of lenses directly affect users’
degree of comfort as well as susceptibility to corneal pathologies. In
general, optical transparency is compromised by the material’s
stiffness. For instance, a hard lens is more transparent but less
comfortable. Mechanical characterization thus helps to optimize the
opto-mechanical behavior. Current methods can only measure the
properties at the apex of the lens due to a lack of rotational capability.
A new fixture is necessary because material properties (e.g. stiffness)
Meridional and Azimuthal
vary as a function of azimuthal (φ) and meridional (θ) angles (shown
left) from the apex and are therefore unobtainable with the current
Angles
setup.
The Design Project Objectives and Requirements
There are five main design
Design Objectives
criteria: creation of a rigid
This project has several criteria that must be met in order to be
spherical substrate, lens
successful: create a rigid spherical substrate, create a temperature-
hydration, system realignment,
controlled liquid cell to hydrate the lens, align the indenter with the
rotational control, and graphic
lens apex, control the rotation of the fixture with two degrees of
user interface.
freedom, and create a user interface to control the movement of the
device.
Design Requirements
Contact lenses vary in size, so this project was designed around a
typical Acuvue Oasys lens with a radius of curvature of 8.4mm and
diameter of 14mm. These parameters should be used to design a rigid
spherical substrate to hold the lens during testing with indentation
forces ranging from 100nN to 10mN. The current nano-indenter is
depicted in the figure to the left. The environment of the support stand
should mimic the typical environment of use, so the lens must always
be hydrated and kept within a temperature range between 22 and 37
degrees Celsius for the entirety of the test (about 30 minutes). The
resolution of the rotation must be less than or equal to 0.1mm which
translates to a 0.682 degree step interval. Controlled rotation about the
Nano-indenter
about x-axis (meridional) and y-axis (azimuthal) should be achieved.
Time for rotation, e.g. moving from (θ = 0o, φ = 0o) to (45o, 90o),
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should take less than one minute.
Design Concepts Considered
Two test fixture designs were
considered. The first design
Two main designs were considered. The chosen design must allow
the contact lens sample to rotate with two degrees of freedom (DOF).
mimics a trackball mouse using a
One way to accomplish this would be to mimic the way a track and ball
ball and rollers configuration.
computer mouse works where the ball of the mouse rotates two rollers.
The second design is a two
degree-of-freedom concentric
bucket assembly powered by two
motors.
In the case of the computer mouse, the ball provides the input and the
rotation of the rollers causes the mouse pointer to move on the
computer screen. In the case of the test setup, the rollers would be
attached to a motor and would rotate the ball with the contact lens on it.
Another viable mechanism to provide the 2 DOF necessary would
involve the use of two concentric buckets: an outer bucket providing
meridional rotation as well as an inner bucket providing azimuthal
rotation. A lens support fixture would then be fitted into the top of the
inner bucket to hold the contact lens.
The lens support itself had design considerations that needed to be
accounted for. This portion of the fixture needs to secure the lens
during rotation while it is submerged in liquid. This needs to be
accomplished without constraining a large portion of the lens. There
were three main designs considered for securing the contact to the lens
support. These were pinning, clamping, or utilizing suction to hold the
contact in place.
There were two motor configurations considered for this project.
The project demands precise control with high resolution, and stepper
motors were therefore the best choice. Stepper motors with natively
high resolution are difficult to acquire. The other option was picking
more readily available stepper motors and equipping them with
gearboxes to reach the high resolution needed by the project, but this
would greatly increase the price and geometric size of the assembly.
Recommended Design Concept
The recommended design is a two
The recommended design consists of a two degree of freedom
degree-of-freedom concentric
concentric bucket assembly powered by two independent stepper
bucket assembly powered by two
motors. This design allows for contact lens indentation at any point on
independent stepper motors.
the contact lens while isolating the electrical components from the
aqueous bath that hydrates the lens at all times. An interchangeable
lens support fixture, designed to the contact’s radius of curvature, sits
in the inner bucket and constrains the lens during indentation. The
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inner bucket rotates azimuthally via a stepper motor while
simultaneously acting as a reservoir for the hydration liquid. A thrust
bearing and a roller bearing provide rotational independence from the
outer bucket. The outer bucket provides meridional rotation via two
steel shafts coupled to another independent outer motor. The shafts
are supported by two roller bearings mounted into a U-shaped
aluminum frame.
Design Description
The support bracket is composed of three 6160-T6 aluminum
pieces for ease of assembly and will be used as the support base of the
entire structure. It will be composed of three separate pieces: one base
piece and two wall pieces. The entire U-shaped support measures
Recommended Design
approximately 4”x4”x7”. Each wall piece will have a hole drilled into
Assembly
it in order to hold a deep-groove ball bearing. These bearings are
necessary in order to allow for free rotation of the support shafts that
will be inserted into the bearings.
The shafts have two steps at 1mm increments, the smallest
section being 6mm in diameter. There are two identical 4140 steel
shafts, one connected on either side of the outer bucket.
The outer bucket is a hollow polycarbonate cylinder with steps
designed to hold a thrust bearing and a deep groove ball bearing. Its
purpose is to house the inner bucket as well as provide meridional
rotation.
The inner polycarbonate bucket has a recessed cutout meant to
act as a liquid reservoir to keep the lens hydrated. Its purpose is to
provide azimuthal rotation via the motor mounted to the underside
outer bucket.
The interchangeable lens supports will be 3D printed out of ABS
plastic and will screw into the top of the inner bucket. There are three
Inner bucket with Attached
Lens Support
possible design ideas for how to secure the lens to the lens support:
suction, a pin at the apex, and a clamp at the edges. The best design
will be the one that minimizes the number of points on the lens that
cannot be tested.
Analytical Investigations
A stress analysis was performed on the system based on the
weight of the assembly and the necessary motor torque. Free body
diagrams as well as shear force and bending moment plots can be
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found in Rep. 6.2.
A complete static and fatigue shaft diameter analysis was
performed on both aluminum and steel using Maximum Sheer Stress
Theory (MSS), Distortion Energy Theory (DE), and DE-ASME
Elliptic Theory which guards against fatigue and yielding. Aluminum
was found to have a finite life insufficient for the needs of the project.
Steel was then considered as an alternative. An appropriate steel
diameter with a safety factor of three was found to be approximately
5.25mm. A minimum diameter of 6mm was chosen to obtain a
conservative standard diameter. These calculations are found in Rep
Appendix A.
A finite element analysis via ANSYS was performed on the steel
shaft to determine deflections. Deflections were on the nano-scale and
found to be insignificant.
Key Advantages of Recommended Concept
The main key advantage over other design concepts was
protection of electronic components from the aqueous solution used to
hydrate the lens. Because of this system’s two independent stepper
motors, the challenge of programming a rotational control system is
simplified.
Financial Issues
Total cost of project is $1260.
Sixty percent of the finances were allocated to the two stepper
motors used to drive the fixture. These motors have the highest native
resolution available in a stepper motor, and are equipped with
encoders for positional tracking. The other option was to add
gearboxes to less expensive motors, but the gearboxes were so
expensive that this was not financially plausible. It was also a less
practical design due to the large size of the gearboxes.
Thirty percent of the finances were allocated to bearings to allow
the design to rotate. A large roller bearing accounted for most of this
portion of the budget, costing over two hundred dollars.
The remaining ten percent was allocated to raw materials. Our
design is machined from polycarbonate, steel, aluminum, and ABS
plastic.
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Recommended Improvements
This design could use a better
method of securing the contact to
In this project there were two main areas that could use
improvement: the lens support and the alignment method.
the lens support, as well as
None of the lens support designs solved all of the issues with the
interchangeable lens supports to
design. Ideally the lens support would secure the lens without limiting
accommodate different lens sizes.
any of the available test space on the contact. By satisfying this
This design could also use a fine-
criterion it would be possible to test the contact lens at any point on its
tuned calibration method.
surface in one experiment, allowing for the most effective data
gathering.
Aside from having a better method for securing the contact to the
lens support, it would also be beneficial to have other sized lens
supports for different lenses. The current fixture allows for
interchangeable lens supports, but no other sizes have been designed
at this point.
The assembly could also use a more precise method of alignment
under the indenter. Prior to indentation, the center of the lens support
would ideally be perfectly in-line with the indenter. This would
guarantee perpendicular indentation, and thus the most accurate data.