1. Characterizing mechanical properties of peripheral nerves during needle insertion, using the viscoelastic standard mechanical body model Ingólfur Pálsson Group 701 Department of Health Science and Technology

2. Introduction to the problem Insertion of multi-site neural recording electrodes into nerves, can result in the structural failure of the electrode. Insertion of multi-site neural recording electrodes into nerves, can result in the structural failure of the electrode. Needle used to guide the electrode into the nerve. Needle used to guide the electrode into the nerve. Characterizing the mechanical properties of needle insertion into nerves, could help reduce trauma to the nerve. Characterizing the mechanical properties of needle insertion into nerves, could help reduce trauma to the nerve. Hoffman et. al has shown that dimple occurs in the rat brain during needle insertion. Hoffman et. al has shown that dimple occurs in the rat brain during needle insertion. Yoshida et. al has fitted a viscoelastic model (SMB-model) to needle insertions into rat brains. Yoshida et. al has fitted a viscoelastic model (SMB-model) to needle insertions into rat brains. Aim: Investigate if the SMB-model could be fitted to needle insertions into peripheral nerves. Aim: Investigate if the SMB-model could be fitted to needle insertions into peripheral nerves. Penetration force and dimple. Penetration force and dimple.

3. Needles The needles are used to penetrate the perineurium The needles are used to penetrate the perineurium The data recorded is used to measure the dimple and the mechanical properties of the needle insertion The data recorded is used to measure the dimple and the mechanical properties of the needle insertion Important factors of the needle design Important factors of the needle design Opening Angle Opening Angle Needle Diameter Needle Diameter

4. SMB - model Models viscoelastic properties of data Models viscoelastic properties of data Must be verified by measuring a known viscoelastic material - Honey Must be verified by measuring a known viscoelastic material - Honey

5. Results The SMB-model was verified using a 200µm needle The SMB-model was verified using a 200µm needle The SMB-model did not fit the data of needle insertion into peripheral nerves, for any populations of needles. The SMB-model did not fit the data of needle insertion into peripheral nerves, for any populations of needles. χ2 and correlation between residuals test confirmed that the model did not fit. χ2 and correlation between residuals test confirmed that the model did not fit. (χ2 = 1263.4±1099 (mean±std) > χ2 005 =14.671

6. Results of needle insertions into nerves The Dimple and penetration force vs. needle diameter The Dimple and penetration force vs. needle diameter ANOVA and Newman-Keul: ANOVA and Newman-Keul: Penetration force was the similar for all needles except the 200µm needle. (p=0.0001) Penetration force was the similar for all needles except the 200µm needle. (p=0.0001) Mean of dimple was equal for all needle diameters. (p=0.048) Mean of dimple was equal for all needle diameters. (p=0.048) The largest opening angles (>24°) differed. (p=0.00009) The largest opening angles (>24°) differed. (p=0.00009)

7. Discussion The SMB-model works The SMB-model works The peripheral nerve is not elastic in accordance with the SMB-model The peripheral nerve is not elastic in accordance with the SMB-model Penetration force differed for the 200 µm needle Penetration force differed for the 200 µm needle Dimple is unaffected by needle diameter Dimple is unaffected by needle diameter The opening angle of the needle is a crucial factor in dimple and penetration force The opening angle of the needle is a crucial factor in dimple and penetration force The mechanical behavior of the peripheral nerve resembles friction The mechanical behavior of the peripheral nerve resembles friction