Table 7.

Comparison of insertion methods.

Methoda	Surgical difficulty	Alignment and targeting	Tissue response	Limitations
No aid [7, 8, 125, 133–136]	Very easy	Inaccurate: Probe can easily be deviated by anatomical features	Minimal: Only due to probe size/material	Only works with short, stiff, and/or large cross section probes
Shuttle	Moderate: Requires some skill to separate probe from shuttle	Very accurate: Stiff shuttle unlikely to be bent/flexed during insertion	Moderate to high: Shuttle increases cross section, which increases trauma	Probe may shift as shuttle retracts
SS rod: [100, 142, 143]
W rod: [55, 140, 141, 178, 179]
Si plate: [118, 120, 130, 137–139, 180, 181]
Diamond plate: [56]
Dissolvable stiffener	Very easy to moderate: Stiffener may be brittle and require delicate handling	Semi-accurate: Probe can be deviated by anatomical features as stiffener dissolves during insertion	Moderate to high: Stiffener increases cross section, which increases trauma	Cannot retract and re-insert array if positioning is incorrect
PEG: [58, 121, 144–146]
PEG/silk: [121]
PEG/gelatin: [116]
Silk: [62, 112, 121, 147, 148]
Sugars: [60, 61, 149–151]
CMC: [79, 126]
TDP: [59, 106]
Guide [9, 124]	Moderate to hard: Requires precise alignment of probe and guide	Inaccurate: Material can easily be deviated by anatomical features	Minimal: Only due to probe size/material	Only works with relatively short, stiff, and/or large cross section probes
Dissolvable brace	Easy to moderate: Dissolving brace requires close monitoring	Inaccurate: Material can easily be deviated by anatomical features	Minimal: Only due to probe size/material	Cannot retract and re-insert array if positioning is incorrect
PEG: [35, 54, 156]
Silk: [155]
Stiff tipped [157]	Very easy	Semi-accurate: The flexible shank can deviate during insertion	Moderate: Only due to probe size/material, but probe tip (where recordings occur) is a stiff material	Only works with relatively short, stiff, and/or large cross section probes
Stiffness changing material	Very easy to moderate: Can require removal of stiffener	Semi-accurate: Probe can be deviated by anatomical features as it warms and softens during insertion	Minimal: Only due to probe size/material	Current published materials are relatively soft—short and/or large cross section probes are required
SMP coating: [115, 160–163]
PVAc backbone: [129, 131, 164, 165]
Pressurized Gallium: [158]
Pressurized Water: [159]
Engineered cross section	Very easy	Semi-accurate: Probe can be slightly deviated by anatomical features	Minimal to moderate: May maintain stiffness after insertion	Fabrication can be prohibitively difficult
Rolled: [114, 167]
Vertical Struts: [128]
Magnetically guided [123, 168, 169]	Moderate to hard: Requires dedicated equipment and planning	Very accurate: Probe unlikely to deviate from planned path due to stiff tip and high speed insertion	Moderate: Only due to probe size/material, but probe tip (where recordings occur) is a stiff material	Most magnetic materials are not approved for long term implantation
Injection [110, 111, 170–172]	Easy to moderate: Requires simultaneous injection and retraction of needle	Accurate: Needle can be accurately placed, but small deviations may occur during needle retraction	High: Needle is generally large and stiff, causing increased trauma	Only works for devices with no bulky parts which can be fully inserted through a needle

^aBCB = benzocyclobutene, CF = carbon fiber, CMC = carboxymethylcellulose, Ga = gallium, LCP = liquid crystal polymer, Pa = parylene, PE = polyethylene, PEG = polyethylene glycol, PI = polyimide, Pt = platinum, PVAc = polyvinyl acetate nanocomposite, Si = silicon, SMP = shape memory polymer, SS = stainless steel, TDP = tyrosine-derived polymer, W = tungsten.