This document summarizes research on developing a polymeric artificial retina using organic photovoltaic materials. The researchers investigated whether the photovoltaic materials generate action potentials in neurons through electric or thermal effects when illuminated. Through experiments and mathematical modeling, they determined it is an electric effect where light absorption generates charge carriers that induce a capacitive effect and cause ion rearrangement in the electrolyte, depolarizing the cell membrane. They developed continuum-based device and thermal models and a cell membrane model that agree well with experimental measurements and could help optimize the design of a biohybrid artificial retina.

1. Mathematical Modeling of Bio-Hybrid Devices:
Towards Polymeric Artificial Retina
Matteo Porro1,2∗
, Nicola Martino1,3
, Sebastiano Bellani1,3
, Caterina Bossio1
,
Riccardo Sacco2
, Guglielmo Lanzani1,3
and Maria Rosa Antognazza1
1
Center for Nano Science and Technology, Istituto Italiano di Tecnologia, Italy - 2
Dipartimento di Matematica,
Politecnico di Milano, Italy - 3
Dipartimento di Fisica, Politecnico di Milano, Italy - ∗
email: matteo.porro@iit.it
Problem
Recently neuroprosthetic interfaces based
on organic photovoltaic materials have been
demonstrated to generate upon illumination
action potentials, with spatial and temporal
resolution, both in cultured neurons [1] and
in blinded dissected rat retina [2].
Working principle: electric or thermal phenomenon?
Electric effect Thermal effect
Active layer
ITO/Glass
Solution
Cell
Pipette
OH‒
Cl‒
Cl‒
OH‒
Na+
K+
Na+
H+
H+
‒
‒
‒ ‒ ‒ ‒
+
+
+
++
hν
K+
+
OH‒ Cl‒OH‒
Na+ K+
Na+
H+
hν
K+
Cl‒
OH‒
Cl‒
Heat
Light is absorbed and free charge carri-
ers are generated in the active layer
Charge displacement determines
capacitive effects in the solution cleft
Light is absorbed and heat is produced
The temperature of the solution in-
creases and the cell capacitance and
ion channel conductances change
Ions rearrange in the electrolyte
The membrane is depolarized and an action potential is generated
Substrate electrical model
Transient photovoltage measurements have been performed chang-
ing device thickness, illumination intensity and direction.
0 100 200 300
−150
−100
−50
0
Time [ms]
Voltage[mV]
0 100 200 300
−150
−100
−50
0
Time [ms]
Voltage[mV]
65
115
161
187
400
20
23
26
26
33
0 100 200 300
−120
−100
−80
−60
−40
−20
0
Time [ms]
Voltage[mV]
4.68
8.08
18.5
39.2
76.4
121.0
197.5
267.5
Change thickness
Light from ITO
Change thickness
Light from solution
Change intensity
Light from ITO
From these evidences we propose the following working principle:
ITO
Electrolyte
+
_
+
_
Exciton
hυ
+
_
Diffusion
Electron
Hole
Electric
field
Exciton dissociation at the ITO in-
terface
Electrons are trapped at the surface
and holes diffuse in the polymer
Charge displacement determines
the photovoltage
When light is switched off, holes
move back to the ITO interface, re-
combine with electrons and the pho-
tovoltage vanishes.
Continuum-based device model with the following assumptions:
Drift-Diffusion fluxes
No reactions and negligible
electric field at the interface
with the solution
Bimolecular recombination
Light absorption:
Beer-Lambert law
Trapped electrons deter-
mine a surface charge dis-
tribution at the ITO.
0 100 200 300 400
50
75
100
125
150
175
Thickness [nm]
Peakvoltage[mV]
0 100 200 300 400
0
5
10
15
Thickness [nm]
Risetime[ms]
ITO side exp
ITO side sim
Solution side exp
Solution side sim
0 50 100 150 200 250 300
0
25
50
75
100
125
Intensity [µW mm−2
]
Peakvoltage[mV]
0 50 100 150 200 250 300
0
5
10
15
20
25
30
35
Intensity [µW mm−2
]
Characteristictime[ms]
Experiment
Simulation
With light from the solution
side, most of the excitons are
generated far from the dissoci-
ation region. Hence for thick
devices charge generation is
less efficient.
Changing light intensity Ilight,
photovoltage peak values and
rise time scale as Ilight and
1/ Ilight respectively.
Thermal and cell model
Temperature is probed with a
pipette close to the polymer
interface.
The phenomenon is modeled
using standard heat diffusion
theory and assuming that all
the absorbed energy is trans-
ferred to the solution as heat. 0 100 200 300 400 500
22
24
26
28
30
Time [ms]
T[°C]
0 100 200 300 400 500
−1.5
−1
−0.5
0
0.5
1
1.5
Time [ms]
ΔV[mV]
Patch clamp technique is used
to measure changes in membrane
voltage, resistance and capaci-
tance of several cells (n=51).
Normalizedcapacitance[−]
Time [ms]
0 200 400 600 800
0.8
0.9
1
1.1
Normalizedresistance[−]
1
1.01
1.02
1.03
0 200 400 600 800
Time [ms]
Cell membrane is
modeled with an RC
circuit and temper-
ature dependence
of the parameters is
included.
gM(T) = gM,0
T − T0
10
Q10
VR(T) = VR,0
T
T0
αVR
CM(T) = CM,0 1 + αCM
(T − T0)
cM
VC
gM
VR
I =0P
Vσ
0 100 200 300 400 500
−33
−32.5
−32
−31.5
−31
Time [ms]
V
C
[mV]
Measure
Resting voltage
Capacitive
Model
Good agreement between mea-
surements and model results.
Fitted value of αCM
(0.0031 K−1
) is
consistent with previous results in
literature [3].
Funding
This work was supported by EU project OLIMPIA, FP7-PEOPLE- 212-ITN 316832
References
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zani. A hybrid bioorganic interface for neuronal photoactivation. Nature Communi-
cations, 2, 166, 2011.
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M. Mete, G. Pertile, S. Bisti, G. Lanzani and F. Benfenati. A polymer optoelectronic
interface restores light sensitivity in blind rat retinas. Nature Photonics, 7(5), 400-
406, 2013.
[3] M.G. Shapiro, K. Homma, S. Villarreal, C.P. Richter, F. Bezanilla. Infrared light
excites cells by changing their electrical capacitance. Nature communications, 3,
736, 2012.