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Nano and Micro Technologies

Nano and Micro Technologies - Poster Session A

Poster J10 - 3D Printed Furniture for Bioelectronic Membrane Pull Testing

Thursday, October 24, 2024
10:00 AM - 11:00 AM EST
Location: Exhibit Hall E, F & G

Introduction:  With rising medical interest in neurological devices, such as bioelectronic brain-machine interfaces, the need for both flexible and biocompatible materials also increases. Typically, rigid materials implanted into the brain lead to cases of fibrosis and tissue inflammation, thus reducing in the devices’ performance. Recent advances in material science have enabled soft, porous and conformal bioelectronics that match the mechanical properties of the surrounding tissue and cause minimal inflammatory response. One common geometry is multi-electrode array multi-electrode array (MEA) membranes, composed of biocompatible layers of SU-8 and metal, that have been shown to form stable interfaces with three-dimensional neuronal or cardiac tissue cultures [1]. At an aspect ratio of ~1000:1 and perforated with strain reliefs throughout, the membranes are designed to be flexible enough to deform during culture (Figure A) and can match the mechanical properties of the surrounding tissue in-vivo, thereby minimizing immune responses [2].

In some applications, such as interfaces with cardiac tissue [3], it is desirable to design devices that are highly stretchable, e.g. 200% normalized displacement without permanent deformation. Structures that incorporate serpentine or other coil-like geometries may meet this design criterion because of their nonlinear mechanical properties. It is important to measure these mechanical properties experimentally, but doing so is a challenge because of the scale difference between the membrane and mechanical testing apparatus. Here, we design and print 3D structures to overcome the scale differences between our device and commercial pull-testing machines.

Materials and

Methods:  We utilized a Formlabs Form 2 3D printer with Clear V4 Resin, with a tolerance and layer height of 0.100 mm. The resin has an ultimate tensile strength of 65.0 MPa and tensile modulus of 2.8 GPa (Formlabs). After printing, prints were washed for 10 minutes in isopropyl alcohol in the Form Wash (Formlabs) and cured for 30 minutes in the Form Cure using 405 nm wavelength light and heat. Washing and curing processes cleaned and reduced print malleability; printing supports were clipped, and the print was sanded to smooth imperfections. Whilst defining print dimensions in SolidWorks, printer tolerance and slight shrinkage during the curing process must be considered.

Multiple variations of the parts were printed to tweak the relevant as-printed dimensions. A custom tray (with a footprint of 5 mm x 44 mm and height of 6.35 mm) was made to hold two opposing membrane clips far enough apart that a prefabricated SU-8 membrane could be positioned across the gap (Figure 1A). The clips included hoist points above the center of mass to avoid imparting stress during transfer of clips from the tray onto the mechanical testing apparatus. Additionally, 1/16” diameter stainless steel rails were fed through holes along the short dimension to facilitate mounting and handling during membrane gluing step (Figure 1B). Membranes were glued to the tray using super glue to affix their extents. Mylar tape was used to affix the clips to the tray prior to shipping out for pull testing.

Results, Conclusions, and Discussions: The final design consists of two sliding clips positioned along ridges to accommodate 9.05mm long membranes. The clips positioned in the tray were secured (by hand) and membranes were affixed to the clips. Furthermore, two “leaf” parts (6.35 mm x 44 mm x 1.5 mm, also mounted to the steel rails) were positioned alongside the clips to prevent in-plane shear to the membranes and modularize the design for shipping multiple samples. The clips and leaves were secured to the tray with Kapton tape.

Our structures can enable reproducible mounting between the membrane and pull testing apparatus, thereby reducing instrumental error. They are easily produced, with each system taking about two hours to print and process, and can increase the throughput of experiments for testing different design iterations. These experiments are crucial for understanding the relationship between stress and normalized displacement. They could also be used to understand how the electronic properties of bioelectronic devices change with deformation. Future experiments can now include study of the mechanical properties of membranes under biologically relevant strains to confirm electrical continuity prior to culturing three-dimensional hydrogels laden with neurons that distort the membrane, and both read and stimulate electrical signals within the culture. These studies could inform the design of new bioelectronic membranes for 3D integration with tissues such as brain or heart, and accelerate the development of systems with a wide variety of modalities including stimulating and recording.

Acknowledgements (Optional):  

References:
1. Liu H, Haider B, Fried HR, Ju J, Bolonduro O, Raghuram V, Timko BP. Nanobiotechnology: 1D nanomaterial building blocks for cellular interfaces and hybrid tissues. Nano Res. 2018;11(10):5372-99.
2. Timko BP. Neural implants without brain surgery. Science. 2023; 381(6665): 268-269.
3. Bolonduro OA, Duffy BA, Rao AA, Black LD, Timko BP. From Biomimicry to Bioelectronics: Smart Materials for Cardiac Tissue Engineering. Nano Res. 2020;13(5):1253-67.

Funding (to B.P.T.)
- NSF CAREER Award, 2239557
- American Heart Association Transformational Project Award, 23TPA1057212
- NIH/NIBIB, R21EB034527