Abstract
Purpose :
Atomic Force Microscopy (AFM) is an established technique to measure the mechanical properties of cells and tissues. AFM elasticity measurements are dependent on the underlying substrate stiffness - samples can seem overly stiff or soft depending on what is beneath the sample. Therefore, we hypothesized that AFM measurements of the cornea would be dependent on intraocular pressure (IOP) in vivo. The purpose of this current study is to determine if AFM measurements of corneal elasticity change with IOP. This interrelationship could expand the field and scope of AFM, and eventually be integrated into instruments that assess IOP.
Methods :
To avoid variability associated with cadaver tissue, experiments were conducted using a synthetic cornea (Cordelia, Bioniko Models) designed to replicate the material properties of corneas. The model was placed in a custom 3-D printed IOP chamber (Figure 1) that fits into the AFM and allows for IOP control. The chamber functions as a sealed-end manometer and the pressure is recorded using a pressure transducer (Preciseline, KELLER). The pressure chamber was filled completely with deionized water, allowing the mounted artificial cornea to rehydrate and equilibrate for at least 10 minutes. The chamber was then placed in a custom AFM system that has been designed for corneal mechanical property measurements. Mechanical testing was performed using custom cantilevers constructed with 50-100 micron diameter beads (Polysciences, Inc.) onto tipless cantilevers (MikroMasch). Measurements were acquired from 15 to 40 mmHg, with increments of 5mmHg, and recorded 15 times per pressure point. The values were recorded using IgorPro software (WaveMetrics, Inc.) and analyzed using custom MATLAB code to provide elasticity. Statistical outlier analysis was performed on the elasticity values, with outliers omitted.
Results :
The calculated value of Young’s modulus increases linearly in response to increasing pressure beneath the cornea. For every 1mmHg, the elasticity of the artificial cornea increased by 4.26kPa.
Conclusions :
Indentation measurements with a cantilever at the corneal surface change with increasing IOP. Using appropriate models, AFM could potentially be used to determine IOP. Additional experiments are needed to determine how patient-specific data, like corneal thickness and baseline stiffness, impacts the cantilever surface measurements.
This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.