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ramé-hart Newsletter

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November 2017

  Even Cyborgs Need Contact Angle

Cyborgs exist mostly in science fiction - Darth Vader in Star Wars and The Borg in Star Trek, for example. The term cyborg is short for cybernetic organism. According to the strict definition, any implant or physical attachment could make a human a cyborg. My late grandfather, for example, would qualify since he had a pacemaker installed. Somehow we never thought of grandpa as a cyborg. Increasingly, however, internal implants are moving from rather simple devices like the pacemaker to more advanced technological devices like brain implants - also called neural implants.

Currently, a lot of resources are being expended to develop biomedical prostheses which bypass portions of the brain that have been damaged by a stroke or accident. This would include eye implants for vision and cochlear implants (shown below) for hearing. Additionally, neural implants, such as neurostimulators, are installed in persons suffering from Parkinson's disease and other maladies such as depression, PTSD, and chronic pain. Many of these devices feature neural interfaces for communicating with the nervous system through implantable electrodes. Yet additional neural implants are being developed to interface with computer processors and memory chips.

Baby with Cochlear Implant
by Bjorn Knetsch and licensed under Creative Commons v2.0

In all cases, it's important that these implants are not rejected by the body, that they integrate well with living tissue, that they last a long time, and that any other negative effects that an implant may have on the brain are minimized. To facilitate this research, a lot of work is being done in the area of surface chemistry of neural implants and intracranial electrodes. Surface modifications can reduce complications such as fibrous tissue encapsulation, extend the life of the implant, promote improved biocompatibility, and improve the integration of implant materials with living tissue.

For permanently installed implants with electrodes, tissue encapsulation and cellular response are important variables. While the mechanical and biocompatible properties are also important, it's the surface layer of the materials that drives biological responses. It's critical that the mismatch between the rather rigid, dry, static, electronic, and hydrophobic implant surface and the soft, wet, ionic, dynamic, and hydrophilic biological tissue is ameliorated. A number of innovative biomaterials (e.g., graphene, conductive polymers, carbon nanotubes, and silicon nanofibers) coupled with a variety of advanced surface modifications offer researchers great promise in their quest to integrate neural tissue with man-made parts.

Hydrogel coatings, for instance, are used on neural implants to increase wetting properties of the surface. When the implant surface's contact angle is lowered, protein adsorption is hindered which, in turn, limits the body's ability to detect foreign matter and reject it. Additionally, greater hydrophilicity improves the electrical signal transfer of electrodes. Hydrogels help turn a dry rigid surface into something that more closely matches soft, wet and hydrophilic neural tissue.

Implants are also often coated with NPCs (or, neural progenitor cells). These cells promote biocompatibility and, like the hydrogel coatings, reduce the foreign body reaction. However, in order for NPCs to attach well to the implant surface, the surface must be made more hydrophobic. Researchers at the University of Pittsburg have had success treating silicon implant surfaces with laminin, an extracellular matrix. In tests, untreated silicon implant surfaces with an average water contact angle of 27° have been made to improve to 85° after being treated with laminin.1 See graphic below. The laminin-treated surface is now more able to accept NPCs and promote their attachment and growth.

The figure on the left shows the original silicon surface with a contact angle of 27.3° +/- 1.4°. The figure on the right shows the surface with the laminin that had a contact angle of 85.0° +/- 1.6°.
by Schacko91 and licensed under Creative Commons v2.0

Graphene2 is another material often used in neural implants due to its physicochemical properties. However, to improve integration with surrounding tissue, graphene-coated surfaces are treated by steam plasma which improves wetting properties and reduces contact angle. This surface treatment makes the surface more hydrophilic which decreases scar formation and promotes protein adsorption at the neural interface.

Honeycomb Structure of Graphene

A variety of novel surface modification techniques are being studied and developed in an effort to improve biocompatibility of implants at neural interfaces, reduce tissue damage, extend the life of the implant, and minimize the hostility of foreign material on living cells.

1 Azemi, E; et al. (2010). "Seeding neural progenitor cells on silicon-based neural probes". Neuroscience: 673–681.
2 For more on Graphene and contact angle, see our August 2012 Newsletter.

  Product of the Month: Environmental Fixture

This month's featured product is our Environmental Fixture, p/n 100-14. This useful accessory is optimized for liquid/liquid and captive1 bubble studies. An inverted needle is used to produce both inverted pendant drops as well as inverted sessile drops on the underside of the substrate. Inverted pendant drops with an immiscible external liquid phase are produced to measure interfacial tension (IFT).

Environmental Fixture p/n 100-14

Captive bubble is an alternate method for measuring wetting properties on solids with high surface energy. The captive bubble method is also used on hydrogels like the ones discussed in the article above which cannot be measured in the traditional solid/liquid/gas environment. Note that the Environmental Fixture is supported on all current ramé-hart models. For more information or to request a quotation, please contact us today.

1 For more information on captive bubble, see our July 2015 Newsletter.


Carl Clegg
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Phone 973-448-0305
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