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Neuron-Semiconductor Chip with Chemical Synapse between Identified Neurons
R. Alexander Kaul,
1
Naweed I. Syed,
2
and Peter Fromherz
1,
*
1
Department of Membrane and Neurophysics, Max Planck Institute for Biochemistry, Martinsried/Munich, Germany 82152
2
Respiratory and Neuroscience Research Groups, University of Calgary, Calgary, Canada T2N 4N1
(Received 29 August 2003; published 23 January 2004)
Noninvasive electrical stimulation and recording of neuronal networks from semiconductor chips is a
prerequisite for the development of neuroelectronic devices. In a proof-of-principle experiment, we
implemented the fundamental element of such future hybrids by joining a silicon chip with an
excitatory chemical synapse between a pair of identified neurons from the pond snail. We stimulated
the presynaptic cell (VD4) with a chip capacitor and recorded the activity of the postsynaptic cell
(LPeD1) with a transistor. We enhanced the strength of the soma-soma synapse by repetitive capacitor
stimulation, establishing a neuronal memory on the silicon chip.
DOI: 10.1103/PhysRevLett.92.038102 PACS numbers: 87.18.Sn, 73.40.Mr, 87.80.Xa
The nervous system’s unique properties to control all
animal functions, including learning and memory, hinge
upon the synaptic connectivity between many neurons
within a network and their ability to exhibit synaptic
plasticity in response to environmental stimuli [1]. For
a development of neuroelectronic systems, the features of
neuronal dynamics and of electronic circuitry have to be
combined. In contrast to approaches that electronically
emulate neurons [2] or that join neuronal nets with elec-
tronics by impaled microelectrodes [3–5], a direct non-
invasive interfacing of semiconductor chips and nerve
cells leads to real hybrids that are microscopically inte-
grated [6 –10]. Here we report on the direct coupling of a
silicon chip with the basic element of neuronal learning.
We interfaced the pre- and postsynaptic neuron of an
excitatory chemical synapse by a silicon chip using iden-
tified respiratory neurons from the pond snail Lymnaea
stagnalis [11–14]. We successfully stimulated the pre-
synaptic neuron VD4 (visceral dorsal 4) by a chip capaci-
tor and recorded postsynaptic excitation in LPeD1 (left
pedal dorsal 1) by a transistor. With the soma-soma paired
neurons the strength of the cholinergic synapse was po-
tentiated by tetanic capacitor stimulation.
Previous studies demonstrated how individual neurons
can be noninvasively coupled to electronic microstruc-
tures of a semiconductor substrate, such as capacitors for
stimulation [7] and transistors for recording [6]. Two
neurons from L. stagnalis were electrically connected
through a chip [9], and a chip was interfaced with two
neurons from L. stagnalis that were electrically joined
through their grown neurites [8]. The implementation of a
neuronal memory on a semiconductor required a micro-
electronic interfacing of two neurons that formed a
chemical synapse as illustrated in Fig. 1(a). It is known
that in vivo the neuron VD4 from L. stagnalis forms a
cholinergic synapse with the neuron LPeD1 [11,12]. That
synapse can be reconstituted in vitro in a soma-soma
configuration [13]. By using soma-soma contacts, we
avoided problems with a displacement of neurons from
their contact sites as caused by neuronal outgrowth [8].
We paired VD4 and LPeD1 neurons on a linear array of
microelectronic contacts for stimulation and recording as
illustrated in Fig. 1(b). Presynaptic action potentials were
FIG. 1 (color). Silicon chip with synapse. (a) Hybrid device
with capacitor (C), chemical synapse, and transistor (gate G,
source S, drain D). Not to scale. (b) Micrograph with presyn-
aptic VD4 neuron (left) and postsynaptic LPeD1 neuron (right)
from Lymnaea stagnalis on a linear array of capacitors and
transistors. Scale bar 20 m .
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elicited by a capacitor; pre- and postsynaptic activities
were recorded by transistors. Capacitor stimulation was
applied to potentiate synaptic strength.
Transistors and capacitors were made by boron doping
of n-type silicon [6,7]. They were insulated from the
electrolyte by a 10 nm layer of silicon dioxide and from
one another by narrow lanes of 600 nm local field oxide
[15–17]. The chips were wire bonded to a standard
package (Spectrum, CPGA 208L, San Jose, CA, USA).
A Perspex chamber was attached for the culture me-
dium. Before each use, the chip was wiped with a 10% so-
lution of detergent (FOR, Dr. Schnell, Munich, Germany)
in milli-Q water of 70
C, rinsed with milli-Q water,
and sterilized with UV light for 15 min. A solution of
poly-L-lysine (MW 84 000, P-1274, Sigma, Munich,
Germany) was applied for 8 h (1mg=ml in 150 mM
Tris, pH8:4). Finally the chip was rinsed three times
with aqua ad (Braun, Melsungen, Germany), once with
antibiotic saline [18], and three times with aqua ad and
dried. To obtain individual VD4 and LPeD1 neurons of
L. stagnalis, the central ring of ganglia of snails (shell
length 15–22 mm) was isolated and the neuronal somata
were extracted with a suction pipette (60–90 m diame-
ter) attached to a syringe (Gilmont GS-1200 2 ml,VWR,
Brisbane, CA, USA) by a polyethylene tubing [18]. Pairs
of VD4 and LPeD1 neurons were incubated for 12–18 h on
the chip in 1 ml conditioned medium [18] at room tem-
perature and 80% humidity.
Figure 2(a) shows a chip with the pair of VD4 and
LPeD1 neurons that was used to carry out a complete set
of experiments as presented below. First, we tested
whether a chemical synapse was formed [11]. Conven-
tional intracellular recordings were made from the pre-
and postsynaptic neurons. The cells were impaled with
microelectrodes made from borosilicate capillaries
(1403547, Hilgenberg, Malsfeld, Germany) with a puller
(Zeitz, Augsburg, Germany), filled with a saturated solu-
tion of K
2
SO
4
(15–20 M), contacted with chlorinated
silver wires and connected to bridge amplifiers (BA-1S,
NPI Instruments, Tamm, Germany). Spontaneous spiking
was suppressed by a hyperpolarizing current. Action
potentials were elicited by depolarizing pulses of 0.3 nA
and 500 ms. We found that presynaptic action potentials in
VD4 generated 1:1 excitatory postsynaptic potentials
(EPSPs) in LPeD1, as shown in Fig. 2(b) (n 6). These
were similar to those seen in vivo [11] and in vitro [13].
We tested whether action potentials in VD4 could be
elicited from the chip by capacitor stimulation, while
keeping the cell impaled with a microelectrode. Using a
waveform generator (33120A, Hewlett-Packard, Palo
Alto, CA, USA) positive voltage pulses were applied to
a capacitor (bulk silicon at 7:5V) with respect to the
bath at ground potential (Ag=AgCl electrode) [7–9]. A
stimulus consisted of two pulses with 3Vamplitude
and 0.5 ms [Fig. 3(a)]. A sequence of three paired pulses
gave rise to short responses of the intracellular voltage
that induced sustained intracellular depolarizations
[Fig. 3(b)]. Action potentials generally occurred after
the second or third stimulus.
Excitatory postsynaptic potentials in LPeD1 were in-
duced by capacitively elicited activity in VD4, as ob-
served by intracellular recording (not shown). The
EPSPs were indistinguishable from EPSPs induced by
intracellular stimulation of VD4. Yet, we focused on the
presynaptic stimulation of postsynaptic action potentials,
because transistor recording of EPSPs was not possible
due to noise. In fact, with intracellular recording we
observed postsynaptic action potentials after two to three
presynaptic spikes elicited by capacitor stimulation
[Fig. 3(d)], in analogy to intracellular presynaptic stimu-
lation [13]. The result provides experimental evidence
that capacitor stimulation is able to trigger synaptic trans-
mission (n 4).
To complete the interfacing of synaptic transmission by
the silicon chip, we tested whether pre- and postsynaptic
activity could be observed with transistors. Before each
measurement, the transistors (source at 2:5V, drains at
0:5V, source-drain current 50–100 A) were cali-
brated by applying defined voltages to the bath. Voltage
pulses were applied to a capacitor beneath a VD4 neuron.
Short transients appeared in both transistor records be-
neath VD4 and LPeD1 [Figs. 3(c) and 3(d)] due to extrac-
ellular voltages beneath the neuron pair and to electrical
cross talk on the chip [8,9]. The action potential in the
presynaptic VD4 neuron was recorded by a transistor as a
positive transient of extracellular voltage with an ampli-
tude around 3 mV [Fig. 3(c)]. The action potential elicited
in the LPeD1 neuron by synaptic transmission was re-
corded by a transistor as a sharp peak of about 3 mV in its
rising phase [Fig. 3(e)]. This experiment demonstrates the
interfacing of a chemical synapse by a semiconductor
chip with presynaptic capacitor stimulation and pre- and
postsynaptic transistor recording. The results with intra-
cellular monitoring of both cells (n 3) were confirmed
by experiments without impaling LPeD1 (n 2)and
without impaling either cell (n 2) (not shown).
A particularly interesting aspect of the VD4-LPeD1
synapse is its capability to exhibit short-term potentiation
[14] that is thought to form the basis of working memory
FIG. 2. Neurons with soma-soma synapse on a silicon chip.
(a) Micrograph of three neurons on a silicon chip with the
central LPeD1 and the left VD4 impaled by micropipette elec-
trodes. (b) Intracellular recording. Upper trace: two action po-
tentials in VD4 elicited by a current injection of 1 nA (holding
voltage 60 mV). Lower trace: excitatory postsynaptic poten-
tials (EPSPs) in LPeD1 (holding voltage VD4 90 mV).
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in animals [1]. Specifically, a presynaptic tetanus in VD4
consisting of five to ten action potentials enhances the
amplitude of subsequent EPSPs which generate post-
synaptic spikes in LPeD1 [14]. At first, we reproduced
that short-term plasticity on the chip with current injec-
tion through intracellular electrodes (n 5, not shown).
Then we tested whether the potentiation could be elicited
by capacitor stimulation and recorded with a transistor.
Since transistor recording of EPSPs was not possible, we
probed the potentiation by the appearance of postsynaptic
action potentials. First as a control, a single action poten-
tial was elicited in VD4 by a pair of voltage pulses applied
to the capacitor. In that case postsynaptic depolarization
was not sufficient to elicit an action potential in LPeD1
[Fig. 4(a)]. Then a capacitive tetanus of six single voltage
pulses (3 V, 0.5 ms) was applied that triggered five action
potentials in VD4 [Fig. 4(b)]. To test for potentiation,
again an action potential was elicited in VD4 a few
seconds after the tetanus [Fig. 4(c)]. The post-tetanic
action potential in the presynaptic cell reproducibly
caused a postsynaptic spike in LPeD1 that was recorded
by the transistor [Fig. 4(c)]. The experiment shows that
the modulation of a soma-soma synapse can be directly
induced and monitored by the silicon chip (n 2).
For an interpretation of pre- and postsynaptic interfac-
ing, we have to consider the nature of the neuron-silicon
junction. With Lymnaea neurons plated on silicon with
poly-L-lysine, the lipid core of the plasma membrane is
separated from the chip by a cleft of about 50 nm width
with the specific conductivity of bulk electrolyte [8,19].
The electrical coupling of cell and chip — of capacitor
stimulation as well as of transistor recording — is deter-
mined by the area specific conductance of the cleft g
J
which is proportional to its width [10]. When a voltage
step of height V
0
S
is applied to the capacitor, an exponen-
tial voltage transient V
J
t is induced in the junction
according Eq. (1) where c
S
and c
M
are the area specific
capacitances of chip and membrane.
V
J
tV
0
S
c
S
c
S
c
M
exp
g
J
c
S
c
M
t
: (1)
At V
0
S
3V with g
J
30 mS=cm
2
, c
S
0:34 F=cm
2
,andc
M
4 F=cm
2
[8] we estimate an
amplitude of 230 mVand a time constant of 110 s. Such
strong, short extracellular voltage pulses may affect the
conductance of the attached membrane with a depolariz-
ing current that gives rise to action potentials. The process
is reversible without persisting damage of the neurons.
Transient electroporation may be involved.
An action potential with the intracellular voltage V
M
t
gives rise to capacitive and ionic currents through the
attached membrane. A transistor records the resulting
extracellular voltage V
J
t between cell and chip [10] as
described by Eq. (2) with the area specific conductances
g
i
JM
of the attached membrane and the reversal volt-
ages V
i
0
.
V
J
t
1
g
J
c
M
dV
M
dt
X
i
g
i
JM
V
M
V
i
0
: (2)
Different weights of capacitive current [first term of
Eq. (2)] and ionic currents [second term of Eq. (2)] give
rise to different shapes of the response V
J
t for VD4
FIG. 3 (color online). Synaptic transmission on silicon chip. (a) Voltage at the capacitor beneath VD4 with three double-pulse
stimuli (blowup). (b) Intracellular voltage of VD4 with four action potentials (holding voltage 60 mV). (c) Transistor record
of VD4 with responses to the presynaptic action potentials. (d) Intracellular voltage of LPeD1 with one postsynaptic action poten-
tial (holding voltage 70 mV). (e) Transistor record of LPeD1 with the response to the postsynaptic action potential (blowup).
The short transients in the transistor records are due to extracellular voltages beneath the neuron pair and to electrical cross talk on
the chip.
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neurons and for LPeD1 neurons (Figs. 3 and 4). A capaci-
tive outward current dominates in the narrow positive
response in LPeD1 matching the rising phase of the
action potential V
M
t. In VD4 the wide positive response
resembles the shape of V
M
t itself. In this case ionic
outward currents dominate through potassium or leak
conductance.
The cholinergic nature of the synapse between VD4
and LPeD1 is well established [11,12]. The electroni-
cally controlled synaptic transmission and potentiation
(Figs. 3 and 4) are consequences of successful interfacing
the pre- and postsynaptic neuron. A presynaptic mecha-
nism of potentiation is likely, induced by calcium ions
[20]. A direct effect of Ca entering the cell through leaks
induced by capacitor stimulation can be excluded, as its
intracellular diffusion (diffusion coefficient <20 m
2
=s)
[21] is too slow to reach the synapse during a potentiation
experiment.
In the present Letter, we have shown that chemical
synapses exhibiting short-term memory can be directly
joined to a semiconductor chip. The study forms the basis
for various directions of research, such as long-term
investigations of memory in neuronal nets, developments
of biochips for chemicals interfering with synaptic activ-
ity, and implementations of hybrid systems that integrate
electronic circuits with typical features of neuronal dy-
namics and memory [22,23]. Stimulators with higher
capacitance and transistors with lower noise will improve
the quality of interfacing and enable a gating of ion
channels and a monitoring of postsynaptic potentials.
Homologous chips fabricated by complementary metal-
oxide-semiconductor (CMOS) technology with two-
dimensional arrays of transistors [24] and capacitors
will allow an interfacing of large neuronal networks.
We t h a n k G u
¨
nther Zeck and Paolo Bonifazi for the
chips, Carool Popelier (Free University of Amsterdam)
for the generous gift of the animals, and Wali Zaidi for
excellent technical assistance. We acknowledge the sup-
port of the Max Planck Society for funding the coopera-
tion between Munich and Calgary.
*To whom correspondence should be addressed.
Electronic address: fromherz@biochem.mpg.de
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FIG. 4 (color online). Synaptic potentiation on a silicon chip.
The upper traces show intracellular voltages in red, the lower
traces capacitor stimuli (left) and transistor records (right) in
black. (a) Control. Capacitor stimulation of VD4 neuron with
action potential in VD4 (left) and no postsynaptic action
potential in LPeD1 (right). (b) Potentiating stimulus. Train of
six capacitor stimuli applied to VD4 with action potentials.
(c) Potentiated response. Capacitor stimulation of VD4 with
action potential in VD4 (left) and postsynaptic action potential
in LPeD1 (right).
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