Resorbable PLGA Microneedles to Insert Ultra-fine Electrode Arrays

in Neural Tissue for Chronic Recording

Frederik Ceyssens

, Marta Bovet Carmona

, Dries Kil

, Marjolijn Deprez

, Bart Nuttin

Aya Takeoka

, Detlef Balschun

and Robert Puers

ESAT-MICAS, KULeuven, Kasteelpark Arenberg 10, Leuven, Belgium

²Dept. of Psychology, KULeuven, Leuven, Belgium

Experimental Neurosurgery and Neuroanatomy, KULeuven, Leuven, Belgium

NERF, Leuven, Belgium

1 OBJECTIVES

It has been shown that the mechanical rigidity of

neural implants is a key factor that causes the

formation of scar tissue around the implant. Clearly,

this reduces performance and lifetime. Therefore,

recent work has focused on very compliant,

polymer-based implants (Weltman, 2016). However,

such implants need a temporary reinforcement to aid

their insertion in neural tissue (Lecomte 2018).

Recent work has even indicated that it is possible

to keep polymer thin-film neural electrode arrays in

close contact with neural tissue over chronic

timescales, without the formation of scar tissue

(Zhou 2017). To achieve this, injection of an

electrode array suspended in liquid was used, which

is hard to upscale to higher electrode counts and

relatively cumbersome.

In this work, we are investigating the use of

microneedles fabricated out of short-chain, fast

resorbing polylactic-co-glycolic acid (PLGA,

Purasorb PLDG 5002A) as a temporary

reinforcement.

This technique improves the existing arrays,

injected using a capillary, in terms of controllability

and upscalability to larger electrode counts.

Therefore, the objective was to device a

fabrication process, that allows to micromachine

needle-shaped PLGA structures and to embed ultra

fine electrode arrays in those needles. A second

objective was the long-term in vivo testing of the

electrode arrays in rats.

2 METHODS

The electrode arrays are fabricated by a lithography

–based processing technology published earlier

(Ceyssens, 2015). The process was adapted to

reduce the implant thickness to only 1 µm, yielding

an ultra-flexible implant backbone.

The resulting arrays were designed to contain a

linear array of 16 iridium oxide electrodes, aimed at

single neuron recording. The electrodes are 15

micrometer in diameter. Polyimide

(HDMicrosystems PI2611) is used as insulation

material. After fabrication, an Omnetics Nano

connector is attached to connect an external

amplifier during testing. The micromachined wires

between the connector and the electrodes are only 10

µm wide.

Separate microneedles for support are fabricated

out of a short chain PLGA, that resorbs over a period

of 2-3 weeks after implantation. Molding or

picosecond UV laser machining is used. The needles

have a cross-section of 0.35 x 0.25 mm². In a final

step, the needles are bonded to the electrode array

using thermocompression.

For in vitro testing, the needle arrays were

implanted transdurally in the right sensorimotor

cortex of four rats, 4 mm right from Bregma. At

intervals, at least one week apart, the rats were

anesthetized using usoflurane. The spontaneous

spiking activity was recorded. In a second test,

biphasic electric pulses (800 µA amplitude, 0.1 ms

per phase) were applied subcutaneously to the left

forepaw to evoke a muscle contraction. Meanwhile,

the presence and strength of the evoked potential in

the brain was monitored.

Ceyssens, F., Carmona, M., Kil, D., Deprez, M., Nuttin, B., Takeoka, A., Balschun, D. and Puers, R.

Resorbable PLGA Microneedles to Insert Ultra-ﬁne Electrode Arrays in Neural Tissue for Chronic Recording.

In Extended Abstracts (NEUROTECHNIX 2018), pages 6-9

To quantify the electric recordings, the results

were filtered (FIR HP filter, 200 Hz stopband, 250

Hz passband) and a spike counting algorithm based

on a moving window was adopted. Spike threshold

was set to 4 standard deviations below the window

average. To quantify the evoked potential, a FIR HP

filter with 10 Hz stopband was used and post-

stimulation peak-to-peak (PTP) values were

recorded. Also, the RMS ratio after/before

stimulation was determined.

After approximately four months, the rats were

sacrificed and a histological evaluation of the

implantation sites was performed (GFAP and NeuN

staining).

3 RESULTS

3.1 Fabrication and in Vitro Test

The fabrication process yielded about 50% devices

without defects (Figure 1: example from early

fabrication run). The arrays are 1 µm thick in total,

and the metal tracks are 4 µm wide, with 3.5 µm of

insulating PI on every side. An example needle is

shown in Figure 1. The measured electrode

impedance was around 200 kOhm at 1 kHz.

As an insertion test in Agar gel proved

insufficient adhesion between the implant backbone,

a second version was fabricated in which the

backbone was squeezed between two PLGA needles

of half the thickness. This proved to have sufficient

adhesion.

Figure 1: PLGA needle with electrodes.

3.2 In Vivo Testing

The implantations (Figure 2) went as planned. No

wound inflammation or abnormal behaviour of the

animals was observed.

Figure 2: Implantation, showing transparent PLGA

microneedle in burr hole (lower left), connecting cable and

Omnetics connector (upper right).

About 50 days after implantation, the headstage

of rat 3 disconnected from the skull, after which no

measurements were possible on the animal. In all

other animals, it was possible to observe signals

throughout the duration of the experiment.

In general, action potentials and evoked

potentials could be clearly observed over the entire

course of the 4 month time span. There were no

channels dropping out. A typical measurement of

spontaneous action potentials (4 months after the

start of the experiment) is shown in Figure 3.

Figure 3: Sample of observed spontaneous spikes (rat 4, 4

months after implantation). Red crosses show peaks

detected by the algorithm used.

Figure 4 shows an example of an evoked

potential recording, also after 4 months.

The number of observed spontaneous spikes per

second, and the size of the evoked potential are

shown in Figure 5. The average PTP value (over all

channels) is around 2 mV, and about 70 spikes per

second can be seen per channel. Especially the latter

measurement shows a relatively large variation,

though.

Resorbable PLGA Microneedles to Insert Ultra-ﬁne Electrode Arrays in Neural Tissue for Chronic Recording

Figure 4: Example of evoked potential recording (rat 4,

channel 11, 4 months after implantation). The largest peak

is the stimulation artifact, which is followed by a train of

relatively low-frequency responses.

3.3 Histology

Due to the shallow insertion depth, it was not

possible to remove the rat brains for histology

without pulling out the implant, though it was still

possible to inspect the glial scar.

Histology revealed that there was still limited

glial scarring present. The remaining scar is a factor

4 – 6 times smaller in cross sectional area than the

original size of the PLGA needle. An example is

given in figure 6. In later work, a full analysis of all

histological data will be included.

4 DISCUSSION

Briefly, though no full integration (i.e. the electrode

array floating in between neurons without any scar

in between) with the neural tissue was achieved, the

observed astrocytic scar was minimal. Viable

neurons were seen to be present inside the volume

that used to be taken up by the resorbable structure.

Qualitatively, the measurements were stable and

a similar response was seen over the four months of

the experiment.

Quantitatively, the number of spikes per second

and the peak-to-peak values of the evoked potential

was seen to vary about a factor of 2 in between

experiments. As there is no clear upwards or

downwards trend and this variation is even present

between measurements just a few days apart on the

same rat, we can assume this is likely due to inherent

variability in the experimental setup.

This includes the depth of the anesthesia, the

exact positioning of the electrode used for

stimulation and natural variations.

Figure 5: Top: measured PTP value (per rat, averaged over

all 16 channels) of the evoked potential over time. Bottom:

Number of spontaneous spikes per second.

Figure 6: Scar after end of experiment. Red: GFAP stain.

Green: NeuN stain, revealing the presence of viable

neurons up to 100 µm close the center axis of

implantation. The dashed rectangle indicates the

approximate boundary of the original PLGA needle.

5 CONCLUSION

We conclude that this method of inserting ultra-fine

electrodes arrays is practical and yields only minor

tissue damage. Combined with the polyimide-based

neural electrode array presented, we were able to

NEUROTECHNIX 2018 - 6th International Congress on Neurotechnology, Electronics and Informatics

record evoked potentials and action potentials for at

least four months.

ACKNOWLEDGEMENTS

We would like to thank Ester Tooten (UCL,

Belgium) and Karin Jonckers (VIB, Belgium) for

their help in obtaining these results.

Furthermore, our gratitude goes to our funding

sources: FWO-Flanders for Frederik Ceyssens’

research fellowship, KULeuven IDO fund and the

Hercules Foundation for heavy equipment (AKUL

034 and ZW1115). The research leading to these

results has received funding from the European

Research Council under the European Union’s

Seventh Framework Programme (FP7/2007-

2013)/ERC grant agreement n° 340931.

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Journal of neural engineering 12.5 054001.

Resorbable PLGA Microneedles to Insert Ultra-ﬁne Electrode Arrays in Neural Tissue for Chronic Recording