Comparison of Gecko-inspired Fibrillar Adhesives

Comparison of Fibrillar Adhesives (to glass)

(please email if you have updates to this data)
NM= not measured

There has been much confusion in the press recently about the performance of various ``gecko-like'' products. One point of confusion is the comparison of nano-scale contacts, typically done with an atomic force microscope, to large patch areas. This is an apples-to-oranges comparison, as sophisticated hierarchical structures are used in the gecko to get adhesion over large areas, and any material in a nanoscopic contact will have a huge pressure. (Pressure is force divided by area, so as area goes to zero, the pressure tends to infinity.) Another source of confusion is the nature of what makes a sticky material. Soft rubbers and the soft materials used in pressure sensitive adhesives such as Scotch tape are naturally sticky because they can make conformal contacts with hard surfaces, as well as having appropriate viscosity properties. Soft sticky materials can make strong attachments, and by making fibers and stalks of soft material, patches with interesting adhesive properties can be made.
A second point is the attachment and detachment of a gecko adhesive. A gecko adhesive does not require being pressed into a surface- the fibers engage by being dragged parllel to the surface with minimal normal force. A gecko adhesive exhibits ``frictional adhesion'' where the fibers push off the surface if there is not a force parallel to the surface, giving automatic release.

Comparison Tables:

Hard polymer

Soft Polymer

Carbon Nanotube

Product	normal stress (N/sq.cm.) (perp. pulloff)	shear stress (N/sq.cm.)	shear adhesion coefficient (1)	frictional adhesion? (2)
gecko lamella (Lee et al. 2008)	~0	54	>5	yes
HDPE GSA (Gillies et al. 2011)	~0	30	>10	yes
polypropylene GSA (Lee et al. 2008)	~0	1.6	~30	yes
PUA inclined nanofibers (Jeong et al. 2009)	~5	26	~70	yes (?)
polypropylene GSA angled fibers Lee et al. APL 2008	~0	4.5	~45	yes
carbon nanotubes (Qu et al. 2008)	20	100	0.8	no
carbon nanotubes (Maeno and Nakayama APL 2009)	?	45	0.9	?

Notes:
1) Shear adhesion coefficient is the ratio of pull-off shear stress to normal preload stress. A high adhesion coefficient means only a light contact will be needed to engage the adhesive.
2) Frictional adhesion is the key property where the normal pulloff force goes to zero when shear force is removed. In this way, the gecko can remove its foot with zero detachment force.

Macroscopic Patches
Common tests for measuring gecko adhesive performance include the peel test, the normal pulloff test, and shear adhesion strength.
	Hard Polymer Arrays
Product	Material	Modulus	90 degree Peel (N/m)	Pull-off (N)	Shear (N)	Area sq. mm.	Normal preload (N/sq.cm)	adhesion coeff.	Effective Modulus (kPa)
Natural Gecko, Autumn et al Nature 2000	beta keratin	2 GPa	~0	1	10	100	nil (<0.01) (shear engaged)	8-16	100
Gecko Lamellar prep, Lee et al JRS Interface 2008	beta keratin	1.5 GPa	NM	~0	0.27	0.5	<0.05	>5	100
Davies et al. Int. J. Adh. Adh. 2008	polyimide on SEM tape	2 GPa	NM	0.53	~0	196	0.25	2	NM
Geim Nature Materials 2003	polyimide on Scotch tape	3 GPa?	NM	3	NM	100	50	0.06	3000 (w/o buckling)
Lee et al. JRS Interface 2008	polypropylene	1 GPa	< 0.1 N/cm	~0	4 N	240	0.05	~30 (shear)	~200
Lee et al. APL 2008	polyproylene	1 GPa	<0.1 N/cm	~0	9N	200	<0.1	~45 (shear)	~200
Schubert et al. J. Adhesion Sci. Tech 2007	polypropylene	1 GPa	< 0.1 N/cm	~0	1 N	1000	0.05	~2 (shear)	~200
Kustandi et al. Adv. Funct. Mat. 2007	parylene	2.8 GPa	NM	0.7	NM	100	1	0.7	NM
Jeong et al. Coll. Surf. 2008	PMMA	2.8 GPa	NM	~0	3	100	<1	3 (shear)	NM
	Carbon Nanotube Arrays
Product	Material	Modulus	90 degree Peel (N/m)	Pull-off (N)	Shear (N)	Area sq. mm.	Normal preload (N/sq.cm)	adhesion coeff.	Effective Modulus (kPa)
Ge et al PNAS 2007	carbon nanotube on Scotch tape	1000 GPa	2-5	0.8	1-6	16	25-50	<.1	~200
Qu and Dai Advanced Materials 2007	carbon nanotube on Si	1000 GPa	NM	~5	~2.5	16	125	<.2	NM
Qu et al. Science Oct. 2008	carbon nanotube on Si	1000 GPa	NM	~3	~16	16	125	<0.1	NM
Maeno and Nakayama APL 2009	multiwall carbon nanotube on polypropylene	1000 GPa	NM	NM	45	100	50	0.9	NM
Zhao et al J. Vac. Sci. Tech. 2006	carbon nanotube on silicon	1000 GPa	0.08	0.5	0.6	8	>500	<.01	~200
	Soft Polymer Fiber Arrays
Product	Material	Modulus	90 degree Peel (N/m)	Pull-off (N)	Shear (N)	Area sq. mm.	Normal preload (N/sq.cm)	adhesion coeff.	Effective Modulus (kPa)
Sameoto and Menon JMM 2009	PDMS (Sylgard 184)	1.8 MPa	NM	9mN	NM	1	2	5	~400
Yoon et al. Nano Today 2009	polyurthane acrylate + Pt	19.8 MPa	NM	NM	31	100	0.3	100	NM
Jeong et al. PNAS 2009	polyurethane acrylate	19.8 MPa	NM	15	78	300	0.3	70	~26
Kim and Sitti Applied Physics Letters 2006	polyurethane	3 MPa	0.07	0.07	NM	0.4	12	1.5	~300
Murphy, Aksak, Sitti, Small 2008	polyurethane	3 MPa	NM	5	10	100	NM	NM	~20
Gorb et al JRS Interface, 2006	PVS	3 MPa	~1	0.4	NM	7	2	2.9	~300
Parness et al. JRSI 2009	PDMS	1.75 MPa	NM	0.5	1.7	100	0.25	2.1	~50(?)
Santos et al. J. Adh. Sci. Tech. 2007	polyurethane	0.3 MPa	NM	1	~1	390	0.25	4-13	~100(?)
Sitti and Fearing IEEE ICRA 2003	PDMS	0.5 MPa	NM	0.003	NM	100	0.025	0.1	~100
Davies et al. Int. J. Adh. Adh. 2008	PDMS	0.6 MPa	NM	1.2 N	NM	123	0.25	40	~100
Ge et al PNAS 2007	Scotch brand tape	0.1 MPa?	NM	NM	6	16	NM	NM	<100
Shan et al IEEE Nano/Micro 2006	PDMS	2.5 MPa	NM	2.01	NM	100	NM	NM	~240

Spherical Probe Tests
Load, drag, pull test				Spherical indentation test Load then Pull off
Product	Material	Modulus	Probe Radius (R)	Preload	Load-Pull Normal Force and F/R (N/m)	Load-Drag-Pull Normal Force and F/R (N/m)	Adhesion Coefficient
Schubert et al JRS Interface 2008	polypropylene	1 GPa	5 cm	2 mN	< 0.05 mN (< 0.001 N/m)	0.8 mN (0.016 N/m)	0.4
Northen and Turner Current Apl. Phys 2006	organorods	~1 GPa	0.3 cm	12 mN	0.45 mN (0.15 N/m)	NM	0.04
Kim and Sitti APL 2006	polyurethane	3 MPa	0.3 cm	22 mN	65 mN (20 N/m)	NM	3
Murphy et al. App. Mat. and Int. 2009	polyurethane	3 MPa	1.2 cm	256 mN	600 mN (50 N/m)	NM	2.3
Murphy et al JAST 2007	polyurethane	3 MPa	0.3 cm	4 mN	8 mN (3 N/m)	6 mN (2 N/m)	1.5
Greiner, del Campo, Arzt Langmuir 2007	silicone rubber	2.6 MPa	0.25 cm	2 mN	1.1 mN (0.5 N/m)	NM	0.5
Greiner, Arzt, del Campo Adv. Materials 2009	silicone rubber	1.9 MPa	0.25 cm	0.5 mN	0.07 mN (0.03 N/m)	NM	0.14

Nanoscopic Patches
These are measurements of very small numbers of contacts over a very small area. The results at the nanoscale can not be directly extrapolated to performance of macroscopic patches. In many cases an atomic force microscope is used to make contact with one to several fibers. Notes: (1) For natural gecko, we are assuming ``frictional adhesion'' mode, where a shear force is necessary for the pulloff force to be non-zero. (See Fig. 4 of Autumn et al Nature 2000). The shear and normal is estimated by assuming 100 to 1000 spatulae are in contact. Each spatula is assumed to be 200x200 nm. (2) Haeshin Lee, Bruce P. Lee & Phillip B. Messersmith, ``A reversible wet dry adhesive inspired by mussels and geckos, '' Nature,448, 338-341 (19 July 2007)
Product	Material	Modulus	Pull-off (nN)	Shear (nN)	Area sq. um.	Normal stress MPa	Shear stress MPa
(1) Natural Gecko, Autumn et al Nature 2000	beta keratin	2 GPa	20-200	35-350 nN	0.04	0.5-5	0.9-9
Sitti and Fearing IEEE Nano 2002	polyester	0.85 GPa	280	NM	9	0.03	NM
Sitti and Fearing IEEE Nano 2002	PDMS	0.5 MPa	200	NM	9	0.02	NM
Yurdumakan et al. Chemical Comm. 2005	carbon nanotube	1000 GPa	10	NM	0.0006	16	NM
(natural gecko) Huber et al. PNAS 2005	beta keratin	2 GPa	20	NM	0.04	0.5	NM
(2) Lee et al Nature, 2007	PDMS+ p(DMA-co-MEA)	0.5 MPa?	120	NM	1	0.12	NM

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