Page 245 - Optofluidics Fundamentals, Devices, and Applications

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common treatment is to remove the lens from the capsular bag and
replace it with a synthetic lens, called IOL [54,55]. There are several
different designs of IOLs, but the simplest design is to use a fixed lens
with one or multiple focal distances [56,57]. Although the patients
regain their vision after surgical replacement, they lose most of the
accommodation capability. Due to the inconveniences caused by
fixed focus IOLs, efforts have been made to restore accommoda-
tion in human vision. The majority of commercially available
accommodating IOLs achieve accommodation by axial linear motion.
When the IOL is moved closer or further away from the retina via
mechanical coupling to the ciliary muscle [58–60], accommodation is
achieved. This is essentially how conventional optical systems achieve
auto-focusing, a mechanism that has been proved to be less efficient
than animal eyes. Indeed, theoretical analysis shows that 1-mm travel
of a 20-D single-optic IOL can only yield an accommodation of ~1.2
D. Even with a dual-optic IOL (with 32-D anterior-moving lens and
−12-D correcting posterior lens), the same travel only yield an
accommodation of ~2.2 D [61,62]. These results are significantly
worse than the 7.0-D average tuning range of young adults [63].
The large performance gap between human crystalline lens and
artificially made IOL is due to the fundamental fact that optic-shift is
a much less efficient mechanism than change of lens shape.
Capsular bag refilling with gel can be considered as one type of
fluidic lens to restore ocular accommodation. It is achieved by filling
the capsule with an injectable malleable material while preserving
capsule integrity. Though many experimental studies have already
been done [64–66], several problems remain to be solved (e.g., achiev-
ing emmetropia in the relaxed state, adequate accommodative
response upon zonuler relaxation [67]). In this section, we explore the
use of bio-inspired fluidic lens as IOL to restore the clarity and accom-
modation of human vision.

9-3-1 Optical Simulation of Eye Model
A pseudoaphakic eye based on Liou and Brennan’s eye model was
adopted to theoretically investigate the optical performance of fluidic
IOL [68]. In a pseudoaphakic eye, the native crystalline lens is replaced
with an implanted synthetic lens because of cataract or other eye dis-
eases. The corneal lens has radii of curvature of 7.77 mm for the ante-
rior (front) surface and 6.40 mm for the posterior (rear) surface. The
pseudoaphakic eye has an axial length of 23.95 mm. The refractive
index of the corneal lens is 1.376 at the wavelength of 555 nm, which
is the peak of the photopic curve. The pupil is modeled as a circular
aperture stop on the front surface of the crystalline lens with its center
offset by 0.5 mm nasally from the optical axis. The angle between the
optical axis and the visual axis is 5 [69]. The refractive indices of aque-
ous and vitreous humors are 1.336. To simulate the effects of fluidic