A sneaker that has better traction even in the rain can be made by taking cues from snakes. This is the concept behind the research of Hisham Abdel-Aal, PhD, an associate teaching professor from Drexel University’s College of Engineering who has been analyzing snake skin to assist engineers in enhancing the design of textured surfaces, such as prosthetic joints and engine cylinder liners—and perhaps even footwear.
As part of an attempt to perceive and quantify the manner in which snakes produce friction when they move, Abdel-Aal, a mechanical engineer who is an expert in tribology (the study of friction), has been gathering and examining snake skins for nearly 10 years. Abdel-Aal has described the process - known as “bio-inspired surface engineering” - for porting “natural data” into the design of commercial products that slip and stick, in a paper recently published in the Journal of the Mechanical Behavior of Biomedical Materials.
Nature has informed many areas of engineering and design, but tribology is one field of study that has been somewhat overlooked when it comes to learning from nature. Snakes in particular have a lot to teach us about optimizing slip and grip. Their existence is dependent on efficiency of motion in very specific environments. The snakes we are studying today are the result of an evolutionary process that has fully adapted the micro-structure of their skin and their body structure to moving and surviving in their habitat from day one. These environments can be brutal on even our most advanced machinery, so applying what we know about snake texturing could help our technology adapt as well.
However, to perceive the design cues from nature, good amount of interpretation is needed. Abdel-Aal’s study in this field is rapidly turning out to be the standard for assisting engineers in unearthing the capability of snake friction control for surface design.
In his latest study, the textural characteristics of snakeskin—collected through the analysis of 350 complete snake skins cast off from 40 distinctive species—were extracted and matched with the standard attributes of textured industrial surfaces, and suggestions were provided on using this framework for developing “smart surfaces” with innovative frictional capabilities.
Guess and Check
Despite the fact that it is a continual force of nature that engineers, scientists, and designers have analyzed and dealt with as background noise for much more years, as far as the question of actually manipulating friction for various uses arises, much of our contemporary knowledge is still obscure.
According to Abdel-Aal, this is partly due to the fact that our grip over friction has evolved by continuous attempts to nullify it using lubricants or to increase it with texture—however nearly always in the quest for on-off goals. Upon achieving that particular goal - such as making a football cleat to work on a muddy field, or an engine piston to generate a specific amount of horsepower - the efforts that were put hardly helps in gaining a wide knowledge of friction.
“Design of texture is still viewed as a ‘black art’ to the effect that currently there exists a gap between available enabling texturing technologies and a conceptual texture-design paradigm,” he wrote in a review of functional surfaces.
According to him, apart from enhancing the efficiency of these particular design challenges, such an insight could also encourage widespread application of friction in designing innovative surfaces. The guide presented by Abdel-Aal takes a good amount of guesswork off texturing and instead enables designers to make deliberate choices - supported by input from the slithering tribology experts.
Finding the Pattern
In order to recognize the elements providing a snake its ability to manage friction, Abdel-Aal examined his collection of skin samples with the detail, and regard for topography, of a cartographer who plots a map.
His collection of skins began as a handful of samples from friends with a Royal python and has increased to hundreds with a little assistance from the Philadelphia Zoo and the Academy of Natural Sciences.
Since it is crucial to analyze the skin the way it was worn by the snake, whenever Abdel-Aal receives a new sample, it is first soaked in water to render it more durable. Subsequently, it is turned right-side out because the majority of the snakes shed their skin similar to a hurriedly removed tube sock.
As a next step, it is mounted on a graphing paper and scanned to make a permanent record with a visual frame of reference. After this, he and his colleagues can start detailed measurements related to the size and shape of the scales, as well as their positioning, with respect to one another and about the body of the snake.
Lastly, a scanning electron microscope is used to investigate the skin to generate an image of the microscopic features that form its texture. Invisibly small, hair-like structures, known as fibrils, are found on snake scales. Although their length is just about 1 μm (i.e. nearly 1/100th of the width of a strand of human hair) the fibrils, and the way they are positioned on the underside of the snake, are crucial to its potential to produce friction.
In addition to the shape, size, distribution, and stiffness of the scales, the arrangement of the fibrils form a distinctive friction profile for every snake—Abdel-Aal has spent efforts to capture and catalog precisely this.
By “mapping” the snakeskin, Abdel-Aaland his associates can identify all the important patterns of texture attributes that help the snake to move along in its surroundings.
Adaptation to local requirements requires specialization in shape, geometry and mechanical properties of the skin building blocks. The implications of adaptation to local conditions are intriguing because they provide a venue for decoding elements of surface design in snakes – such a process has the potential to yield many lessons applicable to the design of technological surfaces.
Apart from categorizing patterns of scales and fibril distribution related the body of the snake, Abdel-Aal’s study produces volumes of studies on the physics behind snake movement and magnitude of the friction forces exerted by the snakes when they undulate, slither, slide, and side-wind.
Abdel-Aal can cross-reference these measurements with the texture profile that he developed for every snake to be able to connect the physical traits to their influence on the mechanics of the snake.
For instance, the musculature and scale texture of large snakes (e.g. pythons and boas) have been optimized for straight-line, or rectilinear, motion. To be able to perform this kind of movement, part of the body is primarily lifted by the snake and it lurches forward by pushing against the ground with sections of its scales. A close at these sections of the skin reveals that there are more fibrils on the “pushing” parts of the body of the snake, which produce adequate friction to enable it to slide forward on the other scales.
Scales to Chevrons
For directly relating the engineered surfaces and the skins, Abdel-Aal reviewed studies related to laser-textured surfaces in which a similar microscopic inspection and inventory of surface features were performed. These texturing methods—such as deposition, sand-blasting, and chemical and laser etching - develop surfaces with very particular friction profiles for items such as hydraulic components in machinery and engine cylinders.
However, they share a significant detail with the texturing observed in nature.
The basic building block in the case of both snakeskin and textured engineered surfaces is a textural element that is repeated in an array distribution. Spacing, length, orientation, and shape of denticulation is, in general, common to a particular family of snakes. Engineered surfaces, on the other hand, feature textural building blocks such as cones, dimples, and chevrons, distributed on the surface. Therefore, both types of surfaces share a common constructional origin.
The chief physical attributes of the textured surfaces are protrusions, dimples, and microscopic channels, which are arrayed to guarantee consistent friction in a lubricated system. Engineers depict surface textures as the average of the measurements of these attributes. Therefore, quantification of “roughness” is performed by averaging the height of the protrusions, ascertaining their slenderness by comparing the height of the protrusions with the area of their base, or by calculating the total area occupied by them.
Abdel-Aal was able to directly relate the fibrils and the protrusions through microscopic measurements of the texture attributes of the snakeskin. Hence, it is possible to apply the same roughness measures to the snakes by simply computing fibril slenderness, height, and overall distribution on the scales.
According to Abdel-Aal, with this advancement, it is possible to combine the functional patterning from a snake on engineered surfaces to develop textures with predictable functions.
“For bio-inspired surface design to be effective, we needed to develop a common vocabulary to describe texturing features,” wrote Abdel-Aal. “We found that three main parameters seemed to translate broadly between the protrusions and dimples of textured surfaces and the fibrils of snakeskin: total area of the feature, feature-to-surface ratio, protrusion/height and height-to-base ratio.”
A fascinating pattern materialized when the snake skins were classified in accordance with these measures. Majority of the “recommended texturing ratios” found by researchers by means of the production and testing of engineered surfaces are identical to those that already exist in snakes.
“It’s striking that engineering research over the last 25 years came to the same design solution, in terms of customization of surface features to promote the efficiency of motion, that snakes have evolved over millions of years,” stated Abdel-Aal. “While it means engineers have probably arrived at the right answer, it also suggests that data from studying snakes could guide us to those conclusions much more efficiently—thus accelerating the development of new surface construction paradigms that can take advantages of the rapidly evolving manufacturing tools.”
Since Abdel-Aal’s study has been enabling engineers to compare snake and surface characteristics—dimples to dimples—some of them have already started to use it to enhance the performance of systems that are reliant on prudent management of friction.
Collaborators from Colombia developed and investigated a surface for a prosthetic hip joint with the help of the tribological data collected from Abdel-Aal’s investigation of Royal Python skin. Taking cues from the study of Abdel-Aal and his associates, scientists in the United Kingdom have been creating texturing schemes for tool inserts applied for dry machining of titanium. Such bio-inspired insert designs reduce the residual heat in the process and maximize the friction. Moreover, engineers from Germany have recently reported a study on snake-inspired cylinder liners that enable the surfaces to reduce friction while moving forward or backward.
Abdel-Aal has published his datasets to help any engineers to use them. However, he also intends to develop them into an algorithm with the ability to smoothly fit into the surface design process.
“Constructing bio-inspired surfaces has a broader goal than merely replication of bio-texturing. In essence, it seeks to extend the potential tribological benefits of reptilian surfaces to the domain of human-engineered surfaces,” Abdel-Aal has written in the journal. “Although that the field is rapidly developing there is a pressing need for more in-depth cooperation between stakeholder communities. I believe this common language between biology and tribology will enable the cross-communication necessary for this cooperation.”