Finger Paper Grip
1. Introduction
Grasping an object between the pads of the thumb and the index finger is the prototype grip used for precision-handling studies. Precision grip must be controlled in order to achieve the optimal minimum force necessary to prevent the slip of an object. In perceptual tasks such as surface discrimination, the normal loading must be modulated to provoke a controlled slip. The precise control of finger pressure derives from the responses of strain-sensitive cutaneous mechanoreceptors at the tips of the digits, as well as from motor control systems that sense muscle length and power based on sensory input from both cutaneous and muscle mechanoreceptors [1,2]. The dynamic tactile signals from the cutaneous mechanoreceptors reliably encode various aspects of contact events around which most object manipulation tasks are organized [3,4]. In 1984, Westling & Johansson [5] published the results of an ingenious paradigm to study the control of grip force during the grasping and lifting of objects. They reported that the normal component of the grip force is influenced by three important factors: (i) the weight of the object, (ii) the friction between the object and the skin, and (iii) the safety margin set by the individual based on prior experience. Moreover, data from studies involving healthy participants with experimentally induced sensory deficits and from patients with sensory loss, because of peripheral nerve damage or central brain lesions, clearly demonstrate the central role of somatosensory feedback for dexterous manipulation. Several methods have been used to transiently interrupt sensory information from the hands of healthy subjects: the use of gloves [6], cooling with sprays or gels [7] and injections of local anaesthetics [8,9]. An almost invariable effect of these manipulations was an increase in the grip force applied against the grasped object. One logical primary reason for the increases in the motor output is a strategic response of the nervous system to ensure against slippage of the object despite a deficit of sensory information. Excessive grip forces have also been observed for the paretic hands of both children with hemiplegic cerebral palsy [10] and stroke patients [11], and also for patients with strong compression of the median nerve [12]. The excessive grip forces were generally attributed to the perturbed feedback of sensory information.
Tactile exploration involves the movement of a finger pad across a counter body, typically at smaller normal loads than those used in grip. The subjective assessment of the roughness of fine but not coarse textures is greatly enhanced by sliding [13]. This was considered as evidence for the duplex theory of texture perception, which was originally proposed by Katz [14]. He argued that coarse textures involve spatial coding as a result of the response of the low-threshold cutaneous mechanoreceptors in the finger pad while fine texture perception relies on a temporal coding, which has been termed vibrotaction. Essentially, the movement of the finger pad over such surfaces causes vibrations that have been measured directly by proximity sensing [15]. A similar mechanism applies to indirect touch in which a probe is moved across a surface causing vibrations to be propagated along the probe to the fingers [16,17]. These studies suggest that the friction of the finger pad may not play a primary role in assessing the surface roughness. However, the analysis of oscillations in the frictional force has shown that there is some correlation with roughness [18], and that the oscillation amplitude depends on both the orientation of the fingerprint ridges and any load dependence of the coefficient of friction [19,20]. Moreover, lubrication can reduce the perceived magnitude of the roughness [18,21].
In texture perception, the frictional and normal forces are adjusted optimally in a way that depends on the topography of the surface [22], which supports the contention that friction is a significant factor in tactile appraisal. Data from such active touch studies on rough surfaces, which involve the subject stroking the surface rather than by an imposed sliding of the surface against the finger pad (passive touch), are difficult to interpret because of this tendency to optimize the friction by changing the normal load in a way that is probably governed by pleasantness. There is not compelling evidence to support a feedback mechanism based on pleasantness. However, Skedung et al. [23] found that, for test papers having different roughnesses, the subjects reduced the normal force as the coefficient of friction increased. Correlations with perceived roughness have been found with both the measured roughness and the coefficient of friction, which is further evidence of the importance of friction [24].
The ranking of roughness is a relatively restricted attribute of a tactile response and is an example of discriminatory touch. For example, Gwosdow et al. [25] investigated the influence of perspiration on fabrics and found that the resulting increase in skin friction enhanced the perception of roughness. The increase in friction correlated with a reduction in comfort, which is arguably more important. Gerhardt et al. [26] also observed an increase in the friction of skin against fabrics as a function of increasing epidermal moisture. They pointed out the relevance of this work on textiles to skin damage such as blisters, abrasion and decubitus ulcers. Similar types of damage can result from sports activities that involve, for example, sliding contacts with equipment, grass or artificial playing surfaces.
Simultaneous measurements of vibration and friction would establish whether tribological interactions play a significant role in modifying the vibratory response, which is currently regarded as the primary sensory cue in assessing fine surface texture. However, it is clear from a recent work [27] that subjects are capable of ranking friction quite accurately. It was found that the Weber index for the coefficient of friction of a glass surface is 0.18. That is, subjects could distinguish a difference of about an 18 per cent reduction in the coefficient of friction, which was achieved by increasing the amplitude of ultrasonic vibrations applied to the glass; this method of reducing the friction is well established in metal forming, for example [28]. These results are consistent with earlier work by Smith & Scott [29], who showed that subjects can scale the friction of smooth surfaces, for which vibrotaction is not applicable.
Thus, it is reasonable to conclude that friction is a significant factor in discriminatory touch. It is also an important factor in affective and hedonic touch, and in associated emotional attributes such as pleasantness and comfort. Although affective touch is commonly connected with the unmyelinated mechanoreceptive afferents that innervate hairy skin (the C tactile or CT-afferent system) [30], it is clearly an important aspect in the context of the finger pads where such afferents are not thought to exist. It is also obvious that a lubricated surface is felt quite differently to one that is unlubricated even if the presence of a lubricant, owing to sensory compensation mechanisms, has a minor effect on the discrimination of the roughness. On the other hand, in the case of relatively smooth surfaces, friction must be a major source of sensory information. For example, Guest et al. [31] investigated the sensory attributes of a wide range of lubricants on a slightly textured polypropylene (PP) sheet and found that there were correlations between the measured friction and sensory dimensions such as watery. Even for unlubricated surfaces, the perception of dryness for a wide range of materials was found to increase as the friction decreased [32]. It has also been observed for dry surfaces that unpleasant tactile sensations increased with the extent of stick-slip motion [33].
Nakano et al. [34] simulated the application of cosmetic foundations by measuring the sliding friction between silicone elastomer surfaces in the presence of such products. They processed the data using artificial neural networks and were able to predict the emotional tactile comfort with relatively high accuracy based on the frictional data. However, despite such observations and that intuitively it might be expected that friction is a major factor in touch, attempts to objectively deconvolve the unique role of friction have proved to be difficult. For example, in one study, the human tactile evaluation of a range of surfaces was dominated by the rough/smooth and soft/hard dimensions with the stick/slippery dimension being ranked as a much weaker contribution [35]. More recently, Chen et al. [36] explored a wider range of dimensions and only a strong correlation between the coefficient of friction and the wet/dry dimension was observed. The interactions between the various dimensions, e.g. with the surface topography, will have a considerable effect on the friction, which is one complicating factor that was clearly recognized in this study and others [37]. Another is the sensitivity of the coefficient of friction to variables such as normal load, sliding velocity and occlusion, which will be discussed in the current review. As a further indication of the role of friction, virtual reality studies have shown that subjects can readily identify complex textured surfaces on the basis of vibrations alone [38]. While both low- and high-frequency components of the finger-surface interaction were due to friction, subjective reports indicated that absence of the low-frequency components in the simulation, i.e. the absence of net friction, decreased the level of realism.
The detection of slip on the surface of the skin and sudden changes in the load force during object manipulation have been attributed to the fast-adapting low-threshold mechanoreceptors [39-41]. The extremely high densities of these units in the fingertips, together with their small receptive fields, certainly provide a high spatial acuity to the fingertips [3]. The early adjustment to a new frictional condition, which may appear soon after the object is initially touched (approx. 0.1-0.2 s), depends on the vigorous responses of the mechanoreceptors during the initial phase of lifting an object [39]. In order to prevent slip, the grip/load force ratio must exceed a minimal value determined by the coefficient of friction between the skin and the object, i.e. the critical ratio at which slips occurs will increase with the slipperiness of the object. Moreover, the responsiveness of especially the fast-adapting units to localized slips, which are not accompanied by acceleration events, suggests that they are also sensitive to other aspects of the mechanical changes reported by Johansson & Westling [39]. These include ‘local redistributions of the strain/stress pattern of the field related to the sliding of the surface structure over the skin’ [39, p. 151]. These authors also note that ‘one important factor contributing to the low frequency of localised slip responses actually observed … might have been the spotty appearance of the slip zones’ that portends widespread slip.
In seminal work, Phillips & Johnson [42] applied a simple analytical model to estimate the compressive strains that were developed at the locations of the slow-adapting mechanoreceptors as a result of a grating being indented into a finger pad. Linear expressions were derived in order to relate these values to the discharge rates of the afferents. This concept of neuromechanical coupling has since been adopted by a number of researchers using more complex finite-element models of the finger pad, e.g. Maeno et al. [43] and Shao et al. [44] or closed-form solutions [45]. Such work should lead to a more quantitative understanding of tactile perception and grip function based on the principles of contact mechanics and the critical neurophysiological factors. However, the formulation of the stress boundary conditions to prescribe the frictional interactions in such models is simplistic compared with the actual behaviour of the finger pads. An aim of the current paper is to critically review the current knowledge about the friction of the finger pad. This will provide a basis for developing more realistic stress boundary conditions in order to improved simulations of touch and grip and, hence, more accurately predict the response of the low-threshold mechanoreceptors located just beneath the skin surface.
The fingerprint ridges and the large number density of sweat pores that are located in the ridges [46,47] are the main physiological characteristics that explain the tribological complexity of a finger pad (figure 1). In particular, the continuous eccrine sweat secretion causes an accumulation of moisture at the sliding interface for a finger pad in an enduring contact with an extended impermeable surface. This phenomenon is termed occlusion in the present paper. The dominant mechanism is the reduction in the evaporation of the secreted sweat because of the large decrease in the free surface area when such a contact is made. The kinetics of occlusion will be reviewed as will be the influence of the nature of the countersurface and the addition of excess water, when the contact is defined as being in the wet state. The contact mechanics of the finger pad, which is dominated by the fingerprint ridges, will be considered in terms of the influence of the load dependence of the contact area and friction. The mechanoreceptors respond to dynamic as well as static perturbations. Consequently, the evolution of a finger pad contact, when subjected to tangential loading, will be discussed in addition to the changes in the friction that occur when the sliding velocity is varied.