Thermal elevation of 60 electrode epiretinal prosthesis for the blind
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24-09-2010, 05:38 PM


Thermal elevation of 60 electrode epiretinal prosthesis for the blind
SUBMITTED BY
SUSMITHA SOMAN
S7 T1
ROLL NO.-67
College Of Engineering, Trivandrum
2007-11 batch


OVERVIEW
INTRODUCTION.
COMPUTATIONAL MODELS.
COMPUTATIONAL METHODS & ASSUMPTIONS.
EXPRIMENTAL METHODS.
COMPUTATIONAL RESULTS.
CONCLUSION.




INTRODUCTION


MARC-multiple-unit artificial retina chip set.
Thermal elevation due to the operation of a dual unit epiretinal prosthesis.
Computational model of 60 electrode device.
ADI-alternating direct implicit &explicit method combined.

RETINAL STRUCTURE
Human eye-allows high resolution imagery
Retina Structure
Multiple cell layered
Photoreceptor cell layer- captures light.
Bipolar cell- transmits signals.
Horizontal cells & amacrine cells
form connections between the neurons.

RETINAL DISORDERS

Retinitis Pigmentosa-group of genetic eye condition.-some people totally blind,sum not.
Age related macular degeneration.-leading cause in individuals 60 year old or more., loss of vision in the central visual field(macula).
Incurable retinal degeneration disease.
Loss of photoreceptor cells.

DISORDERS








RETINAL PROSTHESIS

Retinal prosthesis-two approaches-
60 element electrode.-positioned in .25 mm resolution, increased no of electrodes, better resolution.

COMPUTATIONAL MODELS
HEAD AND EYE MODEL
1 mm resolution slices
Eye region –relatively coarse-classified into 3
Small model extracted from original model.
Bio heat equations .


CNTD…
IMPLANT MODELS.
Obtained from CAD file.
Hermetic case-for electronics & inductive coil-silicon rubber encircles eye.
2 computational models -
.25mm-chip package.
.125mm-finer resolution


THERMAL PROPERTIES
Thermoregulatory mechanism-
Represented by metabolic heat rate & blood perfusion constant.
Temp dependent.
BFC –accounts for effect of blood on tissues
BFC provides nourishment to the retina ,act as heat sink.



COMPUTATIONAL METHODS
NUMERICAL METHODS
ADI- Alternating Direction Implicit –stable.
Explicit finite difference time domain computational methods-conditionally stable.
ADI used in implant region,explicit in rest volume.
Advantage of higher time step with ADI and less computation per time step with explicit.


EXPERIMENTAL VERIFICATION
Experiments performed on canine eyes because of the similarity to human eye.
Package has 2 resistors,3 capacitors.
Open eye
Closed eye-high
temperature.


COMPUTATIONAL RESULTS
Initial temperature is not 37 degree c .
Dual resolution explicit ADI was 11 times faster for same time steps.
Difference in value due to resolution difference.
Hand wound coil-size inaccuracy.


THERMAL EFFECTS OF CHIP PACKAGE


Implant placed on sclera
Small finite gap between.

Two cases.

Negligible distance between chip package and scleral tissue-worst case of thermal elevation.

Finite distance of .5mm to 1mm-this gap is modelled as air.


CONTD..
Hand wound coil used-caused inaccuracies.
Difference in tissue properties,implant positioning,measurement.
Most of the heat flows in vitreous humor,reducing effect on eye tissue.
Because of silicon insulation, effect of heat is less..


THERMAL EFFECT OF SECONDARY COIL
Higher resolution model –for computation of thermal elevation by the secondary coil.
Artificial heat source-current carrying current.
Temp increase is max .39degree c.,max at fat tissue and insulation contact.
CONCLUSION
Detailed analysis of implant,compared canines.
Hybrid method-ADI ,explicit-reduces simulation time,better accuracy
Found-Component positions of prosthesis system –minimize heat diffusion.
Difference –inherent errors,placements,tissue disimilarity,measurement inaccuracies.

REFERENCES

J.D Weiland.W.lui and M.S.Humayun,”Retinal prosthesis “,,Annuv Red biomed eng,,vol 7,2007.

D.Yanai,J.D.Weiland,M.Mahadevappa,R.J.Greenberg,1.Fine and M.S.Humayun,”Visual Performance using retinal prosthesis in three subjects with retinis pigmentosa”,Amer.J.Ophthamol.,vol 143..no.5,pp 820-827,may 2007.

K.Ghosalia,J.Wieland.M.Humayun and G.Lazzi,”Thermal elevation in the human eye and head due to the operation of retinal prosthesis.”,,IEEE Trans.Biomed.Eng;vol 51,aug 2007.

J.Coulombe,M.Sawan,and J.F.Gervais,”A highly flexible system for micro stimulation of the visual cortexBig Grinesign and implementation,” IEEE Trans. Biomed.circuits.Sys..vol 1.,no,4,,pp258-269,Dec 2007.



.ppt   THERMAL ELEVATION OF A 60-ELECTRODE EPIRETINAL PROSTHESIS FOR.ppt (Size: 2.04 MB / Downloads: 46)







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