When fully submerged under water, geckos exhibit a loss of adhesive capability due to lower van der Waals forces between their toes and a substrate. Thus, examining the effect of temperature and relative humidity on gecko adhesion is useful for the design of gecko-inspired adhesives. This would help synthetic fabricated adhesives to be able to work under real environmental conditions. To date, substantial experiments have been carried out to investigate the effect of temperature on gecko adhesion ^[1][2][89,90]. However, the experimental data sometimes provided erratic results. Peng et al. ^[3][91] mentioned that one study had shown that the adhesion force of geckos constantly reduces with increasing temperature, within the temperature range of their natural habitat (17–40 °C). However, the possibility of fabricating temperature-independent gecko-based adhesive was also presented in the literature ^[1][89].

Niewiarowski et al. ^[4][92] summarized the temperature consequence of gecko adhesion performed by Stark et al. ^[5][93] in 2016. The former research group mentioned that the result of temperature on gecko adhesion was complex, and felt that the relationship between temperature and relative humidity should be considered. They explained three major limitations of gecko-based synthetic adhesives: (a) insufficient theory and data on the mechanics of synthetic adhesives under non-ideal situations (wet, dirty, and rough surfaces); (b) lack of understanding of the contribution by natural gecko-inspired adhesive materials such as skin and lipids as unique compared with their structure; and (c) insufficient qualitative and quantitative data on the working of live geckos on natural substrates. Addressing these limitations will open up more applications for gecko-based synthetic adhesives.

Some of the important parameters that significantly contribute to gecko adhesion are contact van der Waals forces ^[6][94], electrification ^[7][95], and capillary forces ^[8][13]. Considerable research has been conducted on van der Waals force and capillary forces, although not much on contact electrification. All these parameters need to be studied under relative humidity (RH) to better understand gecko adhesion in non-ideal situations ^[4][92]. Lab-related research to understand gecko-based adhesion has been limited to major surface types such as smooth, dry, clean glass, or plastic. However, to better understand gecko adhesion, the effects of surface roughness comprising real surfaces differing in many length scales should be analyzed under wet conditions. Theory and lab studies have focused on the impact of control roughness on the surface adhesion and design of gecko-inspired adhesive, especially related to safety factors ^[9][10][52,84]. The safety factor is the capacity of a system to work higher than normal limits; for a gecko system, this needs to be studied under wet conditions. Gecko setae are naturally hydrophobic in certain conditions; it is very difficult to have a permanent superhydrophobic gecko toe. Badge et al. ^[11][96] performed an experiment in which a thick permanent hydrophobic coating was applied to gecko setae. The setae were later found to be superhydrophobic indefinitely and without any change ^[11][96]. Stark et al. ^[5][93] carried out several studies and analyzed the results of temperature and humidity on adhesion in a gecko-inspired synthetic adhesive. They established that the reaction of humidity must be considered in the study of temperature-dependent adhesion. Adhesion was found to be dependent on humidity, but only when the surrounding temperature was low ^[2][90]. Provided that the temperature effect is not considered, Huber et al. ^[12][97] and Sun et al. ^[13][98] experimentally observed the adhesion force of gecko seta (spatula) increasing with the rise in relative humidity. Puthoff et al. ^[14][99] further experimentally analyzed the impact of surrounding humidity on gecko setae and claimed that due to the wetting of gecko setae, there was a rise in adhesion force. In this same study, they discovered that an increase in RH results in softening of the setae as well as viscoelastic damping, which helps to increase adhesion. Tan et al. ^[15][100] studied the impact of RH on the adhesion of gecko-inspired nanopillar arrays. The nanopillar arrays were fabricated with PS material, where the array tips were shaped into spatula and T-shaped geometries. A superficial layer, softened by water from the humid environment was obtained on the nanopillars. This layer formed a kind of “soft shell-stiff core” geometric structure. This core structure reduced the stress distribution at the interface of each nanopillar and contributed to the enhancement of normal adhesion. In another study, Leckband et al. ^[16][101] showed that the water-repelling property of a lipid–protein monolayer is not a natural property of that surface. Moreover, this property is controlled by the surroundings ^[16][101]. They assumed that proteins on the surface experienced variation in compliance when exposed to a highly humid or wet medium, which helped in repelling water. Based on a similar experiment, Pesika et al. ^[17][14] carried out a study in which keratin protein imbibed setae of the gecko exhibited hydrophilic properties by varying the compliance of surface proteins. The study showed that the adhesion force of the gecko rose with humidity due to the increase in surface energy. However, adhesion force is reduced when the gecko setae are fully submerged in water due to the decrement of van der Waals forces.

Geckos can climb and walk on different surfaces. Adhesion studies performed on geckos highlight the influence of various levels of surface substrate roughness on macroscopic as well as microscopic scales ^[18][102]. In addition to adhesion, frictional forces are an important factor pertaining to geckos. A minimum frictional force can be distinguished for a specific range of surface roughness values due to which geckos can stick to a surface.

When analyzing the adhesion of an elastic body to a rough substrate, two parameters need to be understood: (a) attractive contact due to adhesion energy, and (b) non-attractive interconnection due to the formation of elastic strain energy during contact ^[18][102]. The implication of surface roughness on adhesion for gecko reptiles has been studied by researchers. In order to understand the impact of roughness on adhesion between two elastic bodies, an experiment was carried out by Fuller et al. ^[19][103]. Similar experiments had been carried out on geckos by Persson et al. ^[20][35] and Peressadko et al. ^[21][104], who tested rubber balls pressing against hard rough substrates. The tests highlighted that a net pull-off force could be computed from surface roughness power spectra secured from a measured surface height profile. This case presented an understanding of the adhesion mechanism for geckos, which could be assessed in two experiments: (a) calculation of the adhesion of a single gecko hair by atomic force microscopy (AFM), and (b) the inspection of live geckos sticking to surfaces of various surface roughnesses ^[18][102]. Huber et al. ^[18][102] carried out the above experiments to understand the impact of surface roughness on the adhesion of geckos to bulk materials as well as nanomaterials. The experiments were carried out at a certain room temperature and relative humidity. For the AFM experiment, nine dissimilar surfaces were marked according to increased root-mean-square (RMS) roughness. On the other hand, for checking the sticking ability of geckos to surfaces with various surface roughnesses, asperity sizes were created for the test in a range between 0.3 µm and 12 µm. It was observed that on surfaces with a nominal asperity size of 0.3 µm, geckos were not able to stick to the surface. On the other hand, increasing the asperity size to 1 µm, it was observed that lizards could stick to surfaces only for a while because their toes were slipping off after some time. Then, on further increasing the asperity size to 12 µm, geckos could stick to the surface for about five minutes. Generally, for bulk material, it has been observed that if surface roughness is incremented, then there is a decrement in adhesive force ^[19][103]. However, in the present experiment, for nanoscale, it was observed that the adhesive force of a gecko spatula decreased for intermediate RMS roughness varying from 100 nm to 300 nm. However, above and below this intermediate RMS roughness value, it was found that the detachment force increased. These data corresponded with those of live geckos, which are able to stick to substrates with high and low roughness ^[18][102]. This highlights the fact that when single spatulae are considered, geckos could stick to very smooth or very rough surfaces, but failed with intermediate roughness surfaces. This concept is useful to fabricate artificial gecko-inspired adhesives based on surface roughness.

It has been learned that there are very scant data available on the surface roughness characteristics of surface layers utilized by geckos for climbing. In reality, natural surfaces differ in roughness over distances ranging from seven to eight orders of magnitude ^[22][105]. The statistical tool of RMS has been used to indicate roughness for comparing different results. However, the limitation of this tool is that it makes the surface variation discrete in the landscape of a 3D surface into one entity which hinders analyses of important variations with respect to contact mechanics theory. Therefore, it is important to obtain quantitative data which can help realize the abilities of geckos sticking to different surfaces possessing dissimilar roughness values ^[23][106]. One of the reasons why geckos have fascinated researchers is their ability to stick on rough, dry, dirty, wet, and smooth surfaces, in spite of the fact that approximately 40% of gecko species do not possess an adhesive system ^[23][106]. Geckos are found to generally stick to natural surfaces such as flower petals, leaves, rocks, sand, etc., which have varied surface roughness values. However, most contemporary research on developing gecko-inspired adhesives is for very smooth surfaces. This is to develop artificial adhesives for applications in cleaning building surfaces or climbing smooth surfaces in the form of gloves and shoes. For example, Stanford’s Stickybot and DARPA Z Man project supports the fact that the major challenges in the fabrication of gecko-inspired adhesives are those related to highly smooth surfaces ^[23][106]. However, the contact mechanics of these artificial adhesive systems are dependent on the contact area, i.e., smooth and clean surfaces; in reality, geckos can easily stick to these smooth surfaces. The roughness of natural surfaces on which geckos stick and crawl differs so widely from atomistic to millimeter length ^[21][104]. Hence, how variations in surface roughness affect wild natural geckos has not been studied in detail. How fibrillary adhesives found in geckos create strong adhesion while adjusting variation in surface roughness at different lengths of scales is yet not understood clearly ^[24][107]. Niewiaroski et al. ^[23][106] tried to generate an analogy which can provide a new direction of study. They reported that ecologists had faced two significant problems in understanding the effect of the thermal environment on geckos: (a) how to prototype heat exchange between surrounding and geckos, and (b) the procedure to calculate temperature during heat exchange ^[23][106]. Hence, they developed a physical heat-based model based on thermal ecology to better study the impact of surface roughness on gecko adhesion. Usually, a physical model in thermal ecology is a copy of the gecko with similar size, shape, conductive, convective, and radiative heat exchange rates. This model is generally prepared from a hollow metal cast painted to complement the reflectivity of the live gecko, providing the model with an equilibrium temperature that is almost equal to that of the live gecko. In this model, PDMS is used to create a prototype of the physical model. This model was expected to interact with real surroundings, similar to the way natural geckos behave. From the adhesion tests performed on this model, it was observed that the adhesive force of the model was identical to that of the force produced by natural geckos. To understand this model better, it is also important to analyze its surface mechanics and the results obtained should be similar to those of natural geckos. This model provides future insights into testing fibrillary adhesives on model rough surfaces. Ye et al. ^[25][108] employed the reversible adhesive characteristics of gallium (Ga)-based liquid metal coatings to enhance the adhesion, reduce vulnerability to non-optimal conditions, and increase the maximum–minimum adhesion ratio (switching ratio) of PDMS adhesives. They controlled the phase change of Ga and found that it can be used as a new strong adhesive, possessing two adhesive states: (a) a high adhesion state (solid) and (b) a low adhesion state (liquid). It was observed that the maximum adhesion and the switching ratio were dependent on the roughness of the substrate. The results showed that the Ga-based coating method could be used as a potential adhesive for rough surfaces under both dry and wet conditions. Drotlef et al. ^[26][109] fabricated gecko-inspired skin adhesive films composed of elastomeric microfibers decorated with conformal and mushroom-shaped vinyl siloxane (VS) tips for wearable sensors. Here, it was shown that crosslinking of the VS tips helped in obtaining strong skin adhesion to the multiscale roughness of the skin. It was also observed that the fabricated adhesive could attach to other complex topographies over a wide range of surface roughnesses under both dry and wet conditions. Tan et al. ^[27][110], combining the adhesive advantages of gecko setae and creeper root, fabricated a switchable fibrillar adhesive. This adhesive consisted of PU as the backing layer and graphene/shape memory polymer (GSMP) as the pillar array. It was observed that the photothermal effect of graphene modified the GSMP pillars into a viscoelastic state which enabled enhanced adhesion on surfaces over a wide range of roughness. Controlling the phase state of the GSMP pillar helped in switching between high and low adhesion states conveniently.