Podiatry & Blister Material Friction Testing Using the Inclined Plane Method

Rebecca Rushton introduced the concept of the coefficient of friction as a measure of how slippery or grippy two surfaces are in contact. She explained that slipperiness equates to a low coefficient of friction, while grip or traction means a high coefficient of friction. The discussion focused on how a reduced coefficient of friction, achieved by choosing materials that are more slippery against each other, can prevent blisters by allowing skin to move in sync with the bone at an earlier moment, thereby reducing shear distortion magnitudes. Watch the video replay to discover the results of podiatry-specific and blister material friction testing using the inclined plane method.

Blister Prevention Strategies and Coefficient of Friction

Rebecca outlined several blister prevention strategies that reduce the coefficient of friction. These include methods to keep skin dry, such as moisture-wicking socks, breathable shoes, antiperspirants, and powders, as dry skin generally has a lower coefficient of friction. Lubricants, double socks (aiming for a slippery sock-sock interface), and Engo blister patches (making the sock-shoe interface slippery) were also identified as strategies that reduce friction to prevent blisters.

Material Interfaces and Coefficient of Friction within Footwear

Rebecca Rushton elaborated on the various material interfaces within a shoe, each with its own coefficient of friction, such as ground-outsole, sock-shoe, and skin-sock. She noted that while high grip is needed for the ground-outsole interface, the aim within the shoe is generally a moderate friction level to allow for easy foot insertion without compromising the mechanical efficiencies of gait. Significantly, reducing the coefficient of friction is primarily beneficial for shear-related skin lesions like painful calluses, blisters and even ulcers. Slip occurs at the interface with the lowest coefficient of friction, and for blister prevention, this can be achieved via various means, at various interfaces, depending on the specifics of the anatomical location – the aim is to enable early skin-bone movement synchronization.

Testing Methodology and Materials

Rebecca described the material testing methodology employed, which involved the inclined plane method to measure the static coefficient of friction. She detailed the test setup, where a sock-covered nut (10cm x 10cm, 2.33kg) was placed on various test materials applied to wooden planks, and the angle of inclination at which movement first occurred was recorded. The materials tested included various tapes, insole/orthotic covers (e.g., vinyl, Engo patches, Spenco), double sock combinations (cotton, nylon, armaskin), silicone gel, and loose-packed wool. Six different test socks were used, including cotton, wool, Coolmax/moisture-wicking, specialized athletic socks like Trusox (designed for grip) and Steigen nylon socks.

Key Findings from Material Testing

Rebecca presented the results, highlighting that Engo (Shearban) exhibited the lowest coefficient of friction by far, requiring minimal tilt for movement. Conversely, silicon gel had the highest coefficient of friction, followed closely by the abraded Poron. Trusox, designed for grip, generally showed a higher coefficient of friction among the test socks, especially against tapes, while Stegan socks tended to have a lower coefficient of friction.

Static vs. Dynamic Coefficient of Friction

Rebecca distinguished between static and dynamic (kinetic) coefficients of friction, emphasizing that static friction, occurring when there is no relative movement, is the primary determinant of peak shear and thus critical for blister prevention. The goal of reduced friction mechanisms in blister prevention is to achieve movement earlier, thereby decreasing the peak shear deformation. She noted that duct tape, while having a relatively high static coefficient of friction, showed a significantly lower kinetic coefficient of friction once movement began

Limitations and Future Research

Rebecca acknowledged limitations of the study, including the assessment of only dry conditions, which are not representative of athletic situations or typical blister environments, and the inability to test the prevailing skin coefficient of friction. She suggested that more accurate methods, such as the horizontal pull method, could be used in future research. Rebecca also highlighted existing research by Sanders and colleagues (1998) on orthotic and prosthetic materials; by Polliack and Scheinberg (2006) on blister dressings; and by Marty Carlson (2006) on Engo patches that compared friction levels at different moisture contents from 0% to 100% providing a potential avenue for future research on podiatry-focused materials.

Blister Prevention University and Anatomical Specificity

Rebecca introduced “Blister Prevention University,” a course she co-created with Doug Richie, as a resource for understanding blister prevention mechanisms and strategies. She stressed the importance of anatomical location in blister prevention, advocating against a “one-size-fits-all” approach and emphasizing that effective prevention requires tailored solutions based on bone movement and specific anatomical sites.

5 Audience Questions

Blister Care for Children with Epidermolysis Bullosa (EB)

Rebecca addressed a question regarding blister care for children with EB, explaining that EB blisters are shear-related injuries, and the aim of care is to minimize shear. She suggested reducing bone movement, reducing friction force by encouraging slip between the sock and shoe (rather than skin), absorbing shear and reducing pressure. She cautioned against excessive use of adhesives on EB skin due to the risk of damage upon removal.

How Can Marathoners Keep Their Skin Comfortable & Intact During Events?

Rebecca emplored podiatrists to help their patients move past the over-reliance on hope, luck and bravado for managing friction blisters on their feet. The lowest hanging fruit in helping runners proactively prevent blisters is to focus on their blister history. Simply, any blister the runner has ever had before they should expect to get these blisters, and therefore research and implement the very best prevention for that anatomical site, and have it in place from the start line – not wait until the blister has occurred.

How Do Bamboo Socks Rate

Rebecca explained there is insufficient information to make a determination on this question.

Is Poor Fitting Footwear the Main Cause of Blisters?

Rebecca discussed the role of footwear fit in blister prevention, stating that poor-fitting footwear is not necessarily the main cause of blisters, especially as individuals tend to optimize shoe fit over time. She noted that while many people incorrectly attribute persistent blisters to “the wrong shoes,” the issue often lies outside of the shoe itself. She also highlighted that shoe fit is highly dependent on lacing, especially during activities like ultramarathons where foot swelling can lead to loose laces and toe blisters, even in a perfectly sized shoe.

Do Injinji Toesocks Help Toe Blisters?

Rebecca explained how toesocks, either Injinji (predominantly Coolmax) or Creepers (wool), have the potential to reduce interdigital and possibly pinch blister formation, but not really impact any differently on any other toe blister (dorsum, apical, subungual).