title: Compliant intracortical implants reduce strains and strain rates in brain tissue in vivo.
abstract: OBJECTIVE
  The objective of this research is to characterize the mechanical interactions of (1) soft, compliant and (2) non-compliant implants with the surrounding brain tissue in a rodent brain. Understanding such interactions will enable the engineering of novel materials that will improve stability and reliability of brain implants.
  
  
  METHODS
  Acute force measurements were made using a load cell in n = 3 live rats, each with 4 craniotomies. Using an indentation method, brain tissue was tested for changes in force using established protocols. A total of 4 non-compliant, bare silicon microshanks, 3 non-compliant polyvinyl acetate (PVAc)-coated silicon microshanks, and 6 compliant, nanocomposite microshanks were tested. Stress values were calculated by dividing the force by surface area and strain was estimated using a linear stress-strain relationship. Micromotion effects from breathing and vascular pulsatility on tissue stress were estimated from a 5 s interval of steady-state measurements. Viscoelastic properties were estimated using a second-order Prony series expansion of stress-displacement curves for each shank.
  
  
  RESULTS
  The distribution of strain values imposed on brain tissue for both compliant nanocomposite microshanks and PVAc-coated, non-compliant silicon microshanks were significantly lower compared to non-compliant bare silicon shanks. Interestingly, step-indentation experiments also showed that compliant, nanocomposite materials significantly decreased stress relaxation rates in the brain tissue at the interface (p < 0.05) compared to non-compliant silicon and PVAc-coated silicon materials. Furthermore, both PVAc-coated non-compliant silicon and compliant nanocomposite shanks showed significantly reduced (by 4-5 fold) stresses due to tissue micromotion at the interface.
  
  
  CONCLUSIONS
  The results of this study showed that soft, adaptive materials reduce strains and strain rates and micromotion induced stresses in the surrounding brain tissue. Understanding the material behavior at the site of tissue contact will help to improve neural implant design.
authors.affiliation: School of Biological & Health Systems Engineering, Ira A. Fulton School of Engineering, Arizona State University, Tempe, AZ 85287, USA.
authors.forename: Arati
authors.initials: A
authors.lastname: Sridharan
authors.forename: Jessica K
authors.initials: JK
authors.lastname: Nguyen
authors.forename: Jeffrey R
authors.initials: JR
authors.lastname: Capadona
authors.forename: Jit
authors.initials: J
authors.lastname: Muthuswamy
mesh_terms: D000818:Animals; D001921:Brain / Q000502:physiology* / Q000601:surgery*; D055119:Elastic Modulus / Q000502:physiology; D004567:Electrodes, Implanted; D019544:Equipment Failure Analysis; D017276:Friction; D006244:Hardness / Q000502:physiology; D008839:Microelectrodes; D011474:Prosthesis Design; D019919:Prosthesis Implantation / Q000379:methods*; D051381:Rats; D017207:Rats, Sprague-Dawley; D033081:Shear Strength; D013314:Stress, Mechanical; D014783:Viscosity
references.citation: Biomaterials. 2008 Aug;29(23):3289-97
references.pmid: 18485471
references.citation: Epilepsia. 2013 Dec;54 Suppl 9:79-83
references.pmid: 24328878
references.citation: Hum Brain Mapp. 2004 Apr;21(4):257-70
references.pmid: 15038007
references.citation: Nat Rev Neurosci. 2012 Dec;13(12):867-78
references.pmid: 23165263
references.citation: Nature. 2004 Apr 1;428(6982):487-92
references.pmid: 15057821
references.citation: J Neurotrauma. 2003 Nov;20(11):1163-77
references.pmid: 14651804
references.citation: Biophys J. 2006 Apr 15;90(8):3012-8
references.pmid: 16461391
references.citation: Brain. 2011 Aug;134(Pt 8):2248-60
references.pmid: 21764818
references.citation: J Biomed Mater Res A. 2007 Jul;82(1):169-78
references.pmid: 17266019
references.citation: J Neural Eng. 2013 Dec;10(6):066001
references.pmid: 24099854
references.citation: J Neural Eng. 2008 Dec;5(4):455-76
references.pmid: 19015583
references.citation: ACS Appl Mater Interfaces. 2010 Jan;2(1):165-74
references.pmid: 20305827
references.citation: Nat Rev Neurosci. 2014 May;15(5):313-25
references.pmid: 24739786
references.citation: Biophys J. 2014 Mar 4;106(5):1123-33
references.pmid: 24606936
references.citation: Nature. 2012 May 17;485(7398):372-5
references.pmid: 22596161
references.citation: J Neural Eng. 2014 Oct;11(5):056014
references.pmid: 25125443
references.citation: Annu Rev Biomed Eng. 2008;10:275-309
references.pmid: 18429704
references.citation: Electroencephalogr Clin Neurophysiol. 1974 Mar;36(3):314-8
references.pmid: 4130612
references.citation: J Neural Eng. 2012 Aug;9(4):046020
references.pmid: 22832283
references.citation: Brain Res. 2007 Jul 16;1158:103-15
references.pmid: 17555726
references.citation: J Neural Eng. 2011 Dec;8(6):066011
references.pmid: 22049097
references.citation: Biomaterials. 2012 Sep;33(26):5983-96
references.pmid: 22681976
references.citation: J Neural Eng. 2006 Dec;3(4):L23-30
references.pmid: 17124327
references.citation: J Neural Eng. 2009 Oct;6(5):056003
references.pmid: 19700815
references.citation: J Biomech. 2005 May;38(5):1093-105
references.pmid: 15797591
references.citation: J Neural Eng. 2006 Mar;3(1):59-70
references.pmid: 16510943
references.citation: Neuron. 1994 Sep;13(3):645-55
references.pmid: 7917295
references.citation: Biomaterials. 2004 May;25(12):2229-37
references.pmid: 14741588
references.citation: Science. 2008 Mar 7;319(5868):1370-4
references.pmid: 18323449
references.citation: Biomaterials. 2014 Apr;35(13):3919-25
references.pmid: 24529901
references.citation: J Neurosci. 2013 Aug 21;33(34):13873-81
references.pmid: 23966706
references.citation: IEEE Trans Biomed Eng. 1973 Jul;20(4):260-9
references.pmid: 4196687
references.citation: Biomaterials. 2013 Jul;34(20):4703-13
references.pmid: 23562053
references.citation: Exp Neurol. 2005 Sep;195(1):115-26
references.pmid: 16045910
references.citation: J Neural Eng. 2005 Dec;2(4):103-13
references.pmid: 16317234
references.citation: J Physiol. 2007 Mar 15;579(Pt 3):603-11
references.pmid: 17272345
references.citation: J Neurosci Methods. 2000 Apr 15;97(2):93-101
references.pmid: 10788663
references.citation: Proc Natl Acad Sci U S A. 2006 Nov 21;103(47):17759-64
references.pmid: 17093050
references.citation: IEEE Trans Neural Syst Rehabil Eng. 2006 Mar;14(1):91-100
references.pmid: 16562636
references.citation: Biomaterials. 2013 Sep;34(29):7001-15
references.pmid: 23791503
references.citation: Conf Proc IEEE Eng Med Biol Soc. 2006;1:3588-91
references.pmid: 17947041
references.citation: Ann Biomed Eng. 2010 Dec;38(12):3733-43
references.pmid: 20614247
references.citation: IEEE Trans Biomed Eng. 2007 Jun;54(6 Pt 1):1089-96
references.pmid: 17554827
references.citation: J Neural Eng. 2013 Dec;10(6):066014
references.pmid: 24216311
references.citation: J Neural Eng. 2006 Sep;3(3):189-95
references.pmid: 16921202
references.citation: J Neural Eng. 2011 Apr;8(2):025027
references.pmid: 21436513
references.citation: Front Neuroeng. 2014 Feb 04;7:2
references.pmid: 24550823
references.citation: Annu Rev Neurosci. 2004;27:487-507
references.pmid: 15217341
references.citation: Acta Biomater. 2014 May;10(5):2209-22
references.pmid: 24468582
references.citation: Behav Brain Sci. 2010 Aug;33(4):245-66; discussion 266-313
references.pmid: 20964882
references.citation: J Neural Eng. 2009 Apr;6(2):024002
references.pmid: 19255461
references.citation: J Neurosci Methods. 2009 Nov 15;184(2):199-205
references.pmid: 19666051
references.citation: J Neural Eng. 2012 Oct;9(5):056015
references.pmid: 23010756