Graefe's Archive for Clinical and Experimental Ophthalmology
Incorporating German Journal of Ophthalmology
© Springer-Verlag 2004

Laboratory Investigation

Subretinal implantation and testing of polyimide film electrodes in cats

Helmut G. SachsContact Information, Thomas Schanze2, Marcus Wilms2, Andreas Rentzos2, Ursula Brunner1, Florian Gekeler3 and Lutz Hesse4, 5

(1)  University Eye Clinic, University of Regensburg, Franz Josef Strauß Allee 11, 93042 Regensburg, Germany
(2)  Applied Physics-NeuroPhysics Group, Department of Physics, Philipps University Marburg, Marburg, Germany
(3)  Department of Neuroophthalmology, University Eye Hospital, University of Tuebingen, Tuebingen, Germany
(4)  Department of Ophthalmology, Philipps University Marburg, Marburg, Germany
(5)  Klinikum am Gesundbrunnen-Augenklinik, SLK-Kliniken Heilbronn GmbH, Heilbronn, Germany

Contact Information Helmut G. Sachs
Phone: +49-941-9449205
Fax: +49-941-9449281

Received: 10 March 2004  Revised: 8 August 2004  Accepted: 13 September 2004  Published online: 1 December 2004

Background  Progress in the field of microelectronics has led to the development of visual prostheses for the treatment of blinding diseases. One concept under investigation is an electronic subretinal prosthesis to replace the function of lost photoreceptors in degenerative diseases, such as retinitis pigmentosa.
Methods  In the subretinal prosthesis design concept, an array of stimulation electrodes is placed in the subretinal space. To test the feasibility of the concept and to determine basic stimulation parameters, wire-bound stimulation devices were used in acute trials for up to 12 h in three eyes in anaesthetised cats. These wire-bound stimulation elements were based on strips of polyimide film. The film strips were introduced through a sclerostomy into the vitreous cavity and via a retinotomy into the subretinal space during a modification of the standard three-port vitrectomy procedure. On entry through the retinotomy, the film was advanced mechanically to the desired position in the area centralis. Perfluorocarbon liquid (PFCL) was used to establish close contact between the electrode array and the outer retina. Stimulation was performed with computer-generated sequences of current waveforms in acute trials immediately after surgical implantation of the stimulation film. Cortical recordings in the primary visual cortex were performed with electrodes placed in locations corresponding to the retinal stimulus site.
Results  All three implantations were carried out successfully with the stimulation array implanted beneath the outer retina of the area centralis of the operated eye. The retina was attached over the stimulation array in all cases. No cortical responses were recorded in one of the stimulation sessions. The results from another session revealed clear intracortical responses to subretinal stimulation with polyimide films. Following single-site retina stimulation, the estimates of spatial cortical resolution and temporal resolution were approximately 1 mm and 20–50 ms, respectively.
Discussion  Our results indicate that focal subretinal stimulation evokes localised spatio-temporal distribution of cortical responses. These findings offer hope that coarse restoration of vision may be feasible by subretinal electrical stimulation.
The first two authors, H.G. Sachs and T. Schanze, contributed equally to this paper.


Millions of people worldwide suffer photoreceptor loss due to degenerative retinal diseases [20]. In Germany alone, 17,000 people become blind every year [22], and to date there is no effective treatment or cure. About 50% of all cases of blindness are caused by damage to the retina [22], and this population includes patients with macular degeneration and retinitis pigmentosa—conditions that cause progressive degeneration of the outer retina.

A number of approaches, including gene therapy and pharmacological measures, are currently being pursued in the hope of preventing blindness [2, 29]. However, once vision is lost, only two approaches have shown promise for restoring vision: retinal transplantation [29] and bioelectronic visual prostheses [1, 39, 1114, 1618, 20, 21, 2326, 28, 3032, 3437].

In this latter category, two kinds of retinal implants are under development. Epiretinal implants are designed to stimulate retinal ganglion cells with an electrode array that is implanted on top of the inner limiting membrane. Subretinal implants are inserted between the pigment epithelium layer and the outer layer of the retina and attempt to stimulate intact remaining retinal cells by photodiode-generated currents. Previous experiments have shown that microphotodiode arrays (MPDAs) do not deliver sufficient energy to stimulate the retina [35].

The objective of the present study was to implant polyimide film electrodes subretinally in anaesthetised cats and to record the intracortical responses evoked by subretinal electrical stimulation in order to draw conclusions concerning the feasibility of this approach.

Materials and methods

All surgical and electrophysiological procedures were performed in Marburg with anaesthetised cats. The procedures complied with the guidelines of the European Communities Council Directive (86/609/EEC) and were approved by an official German Animal Care and Use Committee. In addition, the ldquoPrinciples of laboratory animal carerdquo (NIH publication No. 85-23, revised 1985), the OPRR Public Health Service Policy on the Humane Care and Use of Laboratory Animals (revised 1986), the U.S. Animal Welfare Act, as amended, and the Association for Research in Vision and Ophthalmology (ARVO) guidelines were followed.

Electrode design
Subretinal stimulation electrodes should be highly flexible, smooth, thin and light to avoid causing mechanical damage to the retina. They need to be robust to guarantee function over time, biocompatible to ensure long-term tissue acceptability, and implantable in surgical terms. In addition, to achieve stimulation, the electrodes should be capable of delivering sufficient electrical current within safe charge injection limits. These requirements are satisfactorily fulfilled by the platinized titanium nitrite electrodes on a polyimide film used in our experiments [15, 30]. A single electrode is round, flat and has a diameter of 50 mgrm with a spacing of 100 mgrm in a given row. Figure 1 illustrates the electrode design.
Fig. 1  Tip of polyimide film electrode for subretinal stimulation (arrow electrode)

Electrode implantation


Three adult cats (body weight 3–4 kg) received atropine sulphate (0.03–0.05 mg/kg) to reduce salivation. Anaesthesia was induced by intramuscular injection of a mixture of ketamine hydrochloride (Ketanest, 10–20 mg/kg) and xylazine hydrochloride (Rompun, 1–2 mg/kg). During eye surgery the cats were fixed on their back in a modified Horsely–Clarke support and anaesthesia was maintained with ketamine hydrochloride (Ketanest, 1–10 mg/kg). Local anaesthetics (lidocaine, bupivacaine) were used in addition. The level of anaesthesia was controlled by monitoring body respiration, ECG, and reflexes. Body temperature was maintained at about 38°C.

Surgical procedures

Polyimide film electrodes were implanted into the left eyes of three cats using a modified standard three-port vitrectomy procedure. The anatomy of the feline eye necessitated the following modifications. Lateral access permitted manipulation in the pars plana region. A deep lateral canthotomy was necessary to access this region as far posteriorly as 6.5–7.0 mm in order to spare the voluminous feline lens. A small surgical wound speculum proved helpful for widening the canthotomy and for gaining sufficient space for the required sclerostomies, which were are all carried out in the accessible temporal field. The infusion port was fixed to the sclera in the 3 orsquoclock position between the access for the vitrectomy probe and necessary intraocular instrumentation and the access for the stimulation film. No fiberoptic intraocular light source was necessary to illuminate the intraocular surgical field. The coaxial illumination from the operating microscope falling through the noncontact wide-field lens system (BIOM II, Ocular Instruments, California, USA) was bright enough to perform the entire surgical procedure. This is due to the highly reflecting tapetum lucidum in the cat. A very few disturbing light reflexes appeared in the observation lens systems but these did not hamper intraocular surgery, thus permitting bimanual handling of instrumentation and film. The vitrectomy was carried out using standard surgical parameters. Only the nasal and anterior parts of the vitreous cavity are difficult to access through this lateral port without touching the lens. Next the retina was prepared for film implantation: a retinotomy was performed in an inferior temporal mid-peripheral area by injecting balanced salt solution (BSS) via a Teflon cannula into the subretinal space and conventionally opening the retina with vitreous scissors in the bleb area thus created. A viscoelastic substance (Healon 0.1–0.3 ml) was introduced via the retinotomy into the subretinal space to ease the subretinal introduction of the stimulation film from the vitreous cavity. To stiffen the highly flexible stimulation film, two more rigid guide films similar in shape to the stimulation film were used during introduction into the vitreous cavity. The stimulation film was sandwiched between the guide films during scleral passage. The guide films were withdrawn when the stimulation film in the premacular region was grasped with vitreous forceps. With the aid of these forceps the stimulation film was advanced through the retinotomy into the subretinal space as far as the desired macular area (Fig. 2). The inferior temporal mid-peripheral area for the retinotomy was chosen corresponding to the sclerostomy for the foil implantation. This allowed placement of most of the electrodes, which are distributed in rows of logarithmic ascending distances on the stimulation film, in the central posterior area by advancing the foil through the retinotomy. The viscoelastic solution and subretinal fluid in the bleb area were aspirated and PFCL was injected carefully. 7.0 Vicryl was used to suture the extraocular film onto the sclera and to seal the sclerostomies. The cats were then repositioned for subretinal electrical stimulation and simultaneous cortical recording. Therefore, the globes were filled with PFCL to a maximum extent. Repositioning has to be performed with extreme care to avoid any dislocation of the subretinal film due to movement of the PFCL. The cats were repositioned in a way that the PFCL was covering the implanted stimulation film completely at any time during this manoeuvre. Film position was then checked ophthalmoscopically prior to the stimulation experiments and photodocumentation was performed with a handheld fundus camera (KOWA, Kowa Ltd, Tokyo, Japan). An standard exchange of PFCL against silicone oil was carried out after the recording session. The reopened sclerostomies were closed again and the extraocular foil portion was hidden under the conjunctiva. The retinotomies and the accidental retinal perforation were not treated by retinopexy. The operated eyes were controlled according clinical demands in a follow-up period of 3 months ophthalmoscopically.
Fig. 2  Film electrode in position in subretinal space of cat immediately after implantation with some electrodes positioned in an upper central position

In vivo testing


For cortical recording the cats were positioned in a standard Horsely–Clarke support. Anaesthesia was maintained by ventilation with N2O/O2 (70%/30%) and halothane (0.3–0.8%) (n=1) or isoflurane (0.5–1.5%) (n=2). Continuous monitoring of rectal temperature (38°C), end-expiratory CO2 (3.8–4.2%), ECG, EEG, and reflexes was used to control the level of anaesthesia. In addition to general anaesthesia, local anaesthetics (lidocaine) were applied during the skull surgery required to insert up to seven fibre electrodes into the primary visual cortex.

Stimulation and recording

Electrical stimulation was performed in three cats with computer-generated sequences of voltage waveforms and fast voltage–current converters with zero offset current [27]. The applied stimulation currents consisted of short charge-balanced rectangular impulses (400 mgrs) with amplitudes ranging from 10 to 100 mgrA. The currents were applied by the subretinally placed film electrodes against a distant large reference electrode located at the rectal body temperature sensor.

Cortical recordings in the primary visual cortex were performed with up to seven quartz-isolated platinum–tungsten electrodes (80 mgrm diameter) placed in locations corresponding to retinal stimulation sites using an Eckhorn electrode drive [10]. Three types of signals were extracted: (1) single unit activity; (2) multiple unit activity; and (3) local field potentials. Alternatively, broadband cortical signals were band-passed (1–4000 Hz) and stored on hard disk/CD for subsequent data analysis. For details see ref. [27].

Data analysis

Averages of the cortical responses evoked by focal electrical retinal stimulation were computed in order to assess electrical retinal stimulation. Cortical activation distributions were then computed according to the method described by Schanze et al. [27]. To estimate the temporal resolution of cortical responses to retinal stimulation by a waveform approach, all values were renormalised to a retinal eccentricity of 2° visual angle. In this case the duration of the first cortical excitatory deflection was estimated.

Implantation results

All three implantations were performed successfully. In all three feline left eyes operated, the film electrodes were completely implanted subretinally in the area centralis.

By comparison with human eyes, the feline retina is considerably more difficult to detach surgically. The partial detachment required for subretinal placement of the stimulation film was achieved in all three eyes. Manipulation of the subretinal film for submacular placement caused unintentional perforation of the retina out of the subretinal space in one animal. Satisfactory submacular placement was achieved in this case by retracting and readvancing the film. Once the film is in a subretinal position and has been stabilised with PFCL, the extraocular part of the stimulation film can be carefully fixed to the sclera without causing dislocation. This permits further manipulation of extraocular film components during the stimulation procedure, such as connecting the film to the stimulation unit with a microplug. During all three experiments neither displacement of the film nor any other adverse events, especially inflammatory reactions, were detected over a 12-h period. At the end of the experiments the stimulation unit was disconnected and the films were mobilised and withdrawn. PFCL was removed and the sclerostomy, conjunctiva and canthotomy were sealed. All cats regained consciousness without problems within 2 h after skull closure. No adverse events were detected for the operated eyes during the 3-month follow-up period.

Cortical responses
No explicit cortical responses were evoked by retinal stimulation in one of the three experiments. The reason for this is unclear because artifacts induced by electrical stimulation were measured. However, it is assumed that retinal sensitivity to electrical stimulation was reduced and was unable to recover within the short interval between implantation and stimulation. The second stimulation experiment yielded moderate and the third experiment unequivocal intracortical responses to subretinal stimulation with polyimide film electrodes. Stimulation thresholds around 50 mgrA were detected for biphasic charge-balanced stimulation currents (Fig. 3). The excitatory cortical group responses peaked at about 20 ms following electrical stimulation. As expected, the distribution of the cortical responses was retinotopically related to retinal stimulation sites. Estimation of the average full width at half height of the cortical activation distributions yielded about 1 mm cortex, corresponding to 1° visual angle. The temporal resolutions ranged from 20 ms to 50 ms.
Fig. 3  Cortical response and activation distribution to stimulation with subretinally implanted film electrodes (modified from ref. [33]). a Time course of stimulation current and mean cortical local field potential response. b Cortical activation distribution in response to focal subretinal stimulation. The circles denote corresponding data points


The ocular surgery for implanting subretinal film-bound stimulation arrays in feline eyes was developed by applying surgical techniques and instruments normally used in humans for routine pars plana vitrectomy and buckle surgery.

Low-threshold stimulation is important for the safety of retinal tissue. Stimulation thresholds around 50 mgrA were measured in our experiments approximately 2 to 5 h after retinal electrode implantation. The surgical trauma might be an explanation for the relative high thresholds of around 50 mgrA. A successful retinal stimulation may have a lower threshold in an intact retina or might be assumed if the retina were allowed to recover for a longer period. The observed differences in the animals may be caused by different retinal conditions shortly after surgery and seems to be worth looking at in further experiments. The calculated charge densities were approx. 2 mC/cm2 for the platinum-coated titanium nitrite electrode with its fractal surface geometry and are therefore in a reasonable range.

The measured spatial and temporal resolutions of cortical responses evoked by subretinal electrical stimulation were in the same range as those reported for epiretinal stimulation with fibre or polyimide film electrodes [16, 27].

These experiments have demonstrated the feasibility of film-bound subretinal electrical stimulation. The model suggests a variety of possible further experiments to determine electrical and technical parameters in subretinal visual prosthesis development. Because of the accumulated wealth of knowledge concerning the feline visual system and the availability of the model, it appears reasonable to use this experimental design to collect extensive and essential data in preparation for electrical subretinal stimulation experiments in humans.

On anatomical grounds, the cat may not be the ideal model for the development of surgical techniques in connection with subretinal implants. However, the feline model does provide extensive additional information if interest is focused not only on the surgical aspects but also on the desired function of a stable implant. Therefore the feline model should not be rejected in the context of visual prosthesis development.

Our results further indicate that short focal subretinal stimulation evokes localised spatio-temporal cortical responses. While coarse restoration of vision should be feasible with the subretinal implant, further experiments are necessary to determine the potential and limitations of this visual prosthesis approach.

Acknowledgements  The authors gratefully acknowledge the excellent technical assistance of W. Gerber, M. Grosch, C. Csellner, and P. Muth, Applied Physics-NeuroPhysics Group, Department of Physics, Philipps University Marburg, Germany. We thank our colleagues in the MPDA Team for their ongoing cooperation, especially W. Nisch and H. Sailer from the Natural Science Institute (NMI) in Reutlingen, Germany for providing us with the stimulation film electrodes. Special thanks to R. Eckhorn, V.-P. Gabel, and E. Zrenner for valuable discussions and encouragement. Supported by grants from the German Federal Ministry of Education, Science, Research, and Technology (BMBF) 01 KP 0006 and 01 KP 0012.


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