% IMPORTANT: The following is UTF-8 encoded.  This means that in the presence
% of non-ASCII characters, it will not work with BibTeX 0.99 or older.
% Instead, you should use an up-to-date BibTeX implementation like “bibtex8” or
% “biber”.

@INPROCEEDINGS{RincnMontes:865223,
      author       = {Rincón Montes, Viviana and Srikantharajah, Kagithiri and
                      Kireev, Dmitry and Lange, Jaqueline and Offenhäusser,
                      Andreas},
      title        = {{F}rom stiff silicon to compliant retinal and cortical
                      multisite penetrating implants},
      volume       = {12},
      issn         = {1662-5102},
      address      = {Lausanne},
      publisher    = {Frontiers Research Foundation},
      reportid     = {FZJ-2019-04757},
      pages        = {0},
      year         = {2018},
      abstract     = {Motivation: For years, silicon has been successfully used
                      as the standard substrate material for the fabrication of
                      penetrating neural probes that perform electrical recording
                      as well as stimulation of single or multiple neuronal cells
                      in vivo. Nevertheless, the mechanical mismatch between the
                      stiff silicon and the surrounding soft tissue enhances acute
                      inflammatory responses [1] and hampers the long-term
                      stability and performance of such implants due to glial
                      scarring as a consequence of foreign body responses [2].
                      With the aim to fabricate more compliant neural probes, the
                      flexibility and softness of different polymer materials are
                      considered in this work for the optimization of the design
                      and fabrication of compliant retinal and cortical multisite
                      penetrating microelectrode arrays (MEAs). Furthermore, the
                      insertion of the proposed flexible systems is assessed
                      theoretically and tested experimentally for both retina and
                      brain tissues. In this work, we show the possibility to
                      perform electrical recording and stimulation with multisite
                      penetrating MEAs from both: thick brain tissue and thin
                      retina.Materials and Methods: Flexible substrate materials,
                      such as polyimide (Pi), parylene-C (Pa-C), and
                      polydimethylsiloxane (PDMS) are used for the fabrication of
                      compliant neural probes. The thickness (3-10 µm), the width
                      (50-100 µm), and the length (140-1000 µm) of a multi-shank
                      design were optimized in order to increase the theoretical
                      buckling force threshold for short/medium and long
                      penetrating shanks. Flexible structures with the
                      aforementioned thicknesses are compared with Si-based
                      structures through insertion tests in phantom neural
                      tissues, which are made out of PDMS mixed at different
                      polymer/curing agent ratios to achieve a Young’s modulus
                      akin to brain and retina (10kPa and 23kPa respectively).
                      Following the established designs, photolithography and
                      reactive-ion etching (RIE) techniques are used for the
                      microfabrication of such flexible devices. In addition, to
                      further increase the flexibility of the compliant probes,
                      conductive polymers and stretchable conductive layers with
                      serpentine and mesh designs are tested. Furthermore, the
                      feasibility to perform electrical recording and stimulation
                      is first tested with silicon based probes in light-adapted
                      wildtype retina in vitro.Results: Optimization of
                      cross-sectional dimensions show a higher buckling force
                      threshold for lower aspect ratio designs (e.g. 140 µm long
                      and 100 µm wide). Likewise, the calculation of buckling
                      force threshold for the different materials show that a
                      thickness of 6-10 µm should withstand the insertion of
                      silicon, Pi, and Pa-C probes when applying a tethering force
                      of 1mN, as is the case for brain tissue [1, 2]. In contrast,
                      PDMS would require a minimal thickness of approximately 70
                      µm. The theoretical calculations are also confirmed
                      experimentally with the probes penetration into phantom and
                      real tissue. Moreover, the use of penetrating MEAs and
                      recording inside the thin retinal tissue is proved to be
                      feasible. Discussion and Conclusion: The development of
                      penetrating and at the same time compliant neural probes
                      requires a trade-off between design considerations (length,
                      width, and thickness) and the mechanical properties of the
                      materials to be used during fabrication. On one hand,
                      flexible materials in an optimized cross-sectional design
                      might be used, however these are constrained to the
                      necessary length and maximum accepted thickness of the
                      implant to fulfill the requirements of the target
                      application. For example, while retinal applications demand
                      short shafts, long ones are required for brain. On the other
                      hand, the use of soft materials like PDMS is limited in
                      their use because their high softness drags out its handling
                      during fabrication and usage. Although an increase in
                      thickness might overcome some processing difficulties, the
                      required thickness for it might generate even higher trauma
                      than when using a stiff material, since a higher amount of
                      tissue is displaced during insertion. Hence, when working
                      with flexible and ultrathin implants, a proper strategy to
                      insert such systems into the tissue must be developed. Even
                      though different stiff and biodegradable shuttle solutions
                      have been proposed in the literature [1, 2], wafer scale
                      processes compatible with MEMS technology are still a
                      challenge. Therefore, in this work, further fabrication
                      steps comprising the integration of dry patterning and
                      lift-off techniques, as well as micro-molding, and
                      bio-degradable polymers are investigated for the development
                      of neural probes with built-in insertion shuttles.},
      month         = {Jul},
      date          = {2018-07-04},
      organization  = {11th Int. Meeting on Substrate
                       Integrated Microelectrode Arrays,
                       Reutlingen (Germany), 4 Jul 2018 - 6
                       Jul 2018},
      cin          = {ICS-8},
      ddc          = {610},
      cid          = {I:(DE-Juel1)ICS-8-20110106},
      pnm          = {552 - Engineering Cell Function (POF3-552)},
      pid          = {G:(DE-HGF)POF3-552},
      typ          = {PUB:(DE-HGF)8},
      doi          = {10.3389/conf.fncel.2018.38.00072},
      url          = {https://juser.fz-juelich.de/record/865223},
}