By means of ad hoc masking and deposition methods, M2Neural aims at developing, multifunctional polymeric coatings for neural interfaces, able to:
- minimize both the mechanical and chemical mismatches between the implanted electrode and the neural tissue;
- maximize electrode biocompatibility and long-term stability, by minimizing scar tissue formation and promoting neural migration;
- provide peripheral nerves with a combination of biochemical (drug-mediated) and physical (nanoparticle-mediated) stimuli, able to greatly enhance axonal health.
These objectives will be achieved by means of an integrated research effort, centered on the study and the development of an engineered multifunctional hydrogel, acting as a coating with a thickness in the micrometer range, on the electrode surface. Polyimide (which is the substrate for the SELINE electrode) is biocompatible, but its stiffness (~ 2.5 GPa) is far from the one of peripheral nerves. The engineered hydrogel Young’s modulus will mimic the typical one of neural tissue (~ 1 kPa), thus minimizing the mechanical mismatch. The hydrogel will be based on a zwitterionic copolymer provided with a photo-labile 4-azidophenyl group, reactive upon light illumination (350 nm). This will allow the formation of a thin biocompatible and non-degradable coating, covalently attached to the polyimide surface (Figure 2).
Figure 2: Chemical structure of the zwitterionic copolymer and light-activated hydrogel formation on the polyimide surface. Piezoelectric nanoparticles and drugs are embedded in the thin multifunctional polymeric coating.
The crosslinking level of the hydrogel (and thus its Young’s modulus) will be tuned by varying the number of crosslinking moieties, the polymer concentration and conditions of photolysis reaction. Piezoelectric nanoparticles will be dispersed in the polymer prior to gel formation, neurotrophic factors and anti-inflammatory molecules will be immobilized by degradable linkers within the polymeric matrix, then the electrode (with its active site being masked) will be coated and homogeneously photopolymerized for the required time. A similar strategy will be applied with poly(2-oxazolines), which will be also tested in the course of the project.
After being carefully characterized, such nanocomposite hydrogels will be tested ex vivo (to assess their stability), in vitro (with neural cell lines) and in vivo (on rats). A set-up for ultrasound stimulation will be developed and the nanocomposites will be periodically stimulated both during cell culture (in vitro) and during the implant (in vivo).
Drugs will be gradually released by the hydrogel at the implant site, thus minimizing the formation of scar tissue and promoting neural migration. Piezoelectric nanoparticles, entrapped in the hydrogel and activated by external US sources, will generate local electrical fields which could trigger or accelerate drug release from the matrix, besides directly stimulating neural tissue. The overall objective is to maximize electrode biocompatibility, neural migration and axonal health and to minimize inflammatory reaction leading to fibrotic tissue formation at the electrode surface.
The different innovative technological components envisioned in the project may provide significantly different benefits to the electrode performance. Moreover, it is hard to predict which could be the combined effect of such improvements. In order to investigate the biological outcomes deriving from both the separated and combined M2Neural technological innovations, we designed an ad hoc experimental protocol. During the integration phase, different prototypes will be developed, namely: a traditional SELINE electrode (Fig. 3a), a SELINE electrode provided with the engineered soft coating on its non-conductive surface (Fig. 3b); a SELINE electrode provided with the engineered soft coating on its non-conductive surface and with embedded anti-inflammatory drugs and neurotrophic factors (Fig. 3c) and a SELINE electrode provided with all the previously mentioned elements, with the addition of piezoelectric nanoparticles which will be stimulated (during in vivo implants) by external ultrasound sources (Fig. 3d).
Figure 3: The different neural electrode prototypes produced in the project ((a) is the already available SELINE prototype).
All these electrodes will be tested, in parallel, in vivo during two phases of the project (first phase: from M12 to M17; second phase: from M31 to M36, see Task 6.2 in Section 4). The “best” prototype will be the one showing the highest signal/noise ratio, biocompatibility (in terms of minimization of fibrotic reaction and maximization of neural health) and signal stability/quality overtime.