Considerations on Friction & Shear in Human Soft Tissue

A pressure sore or pressure ulcer is mostly caused by three combined extrinsic factors: Pressure, shear and a non-physiologic microclimate (National Pressure Ulcer Advisory Panel, npuap.org). We generally understand that pressure is an important causative parameter for pressure ulcers, but we might not know that the other two factors are commonly present as well — it’s not necessarily that they’re less critical, but they have not as well been researched yet. Therefore, a pressure ulcer should probably be called a pressure/shear/micro-climate ulcer, or a “PSM” trauma.

We’ll concentrate here on the shear component of PSM trauma, because it is often misunderstood or mixed in with “friction” or “chafing” or excessive “rubbing” or “abrasion.” All of these can be involved in the application of shear as a root cause or as an effect, but they are not shear itself.

Then we’ll talk about how a particular technology can address this cause of PSM trauma and provide effective intervention.

Understanding Pressure, Friction & Shear

Pressure that occurs for too long can lead to blood vessels being occluded, and can result in vertical distortion of tissues between a support surface and a non-compressible structure, such as bone. Since the damaging effects are not immediate, regular changes of position can stop localized pressure build-up (Figure 1b).

Applied shear force or shear stress, on the other hand, produces immediate and potentially more harmful shear strain – i.e., distortions -- to both blood vessels and cell tissues. Shear strain that happens around our skin where it meets the support surface when we sit or lie down will distort the cells of the skin. Hence, it is important to select the support surface carefully, because the degree to which the support surface can absorb these distortions will reduce the distortions to the skin’s cells (Figure 1c).

Static friction resists when two objects attempt to slide on top of each other. Friction acts as a force transfer mechanism when a shear force is applied.

Dynamic friction occurs when the shear forces reach the point that the friction between two surfaces cannot hold them together any longer. The surfaces then start to slide on each other. Even when it is somewhat lower, dynamic friction can still initiate damage to the cells of the skin. The materials of the support surface will have a major influence on the potential damage caused to the skin when movement occurs across the surface.

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In Figure 1, a gel body with micro inclusions, image "a" shows no external force. Image "b" shows vertical force (load compression). Image "c" shows shearing force (distortion = shear strain).

How Does Shear Stress Interact with Tissue?

According to the literature, pressure applied to human soft tissue compresses the blood vessels. In doing so, it reduces blood supply to and from the cell. The result may be cell death occurring by ischemia.

Shear strain, however, deforms the cell wall and interferes with its biological function, which is transporting nutrients from the outside in, and transporting biological waste from the inside out. The most likely result of this disturbance is cell death by dysfunctional metabolism.

In biological tissue we find both occurrences of shear: Shear stress (the force applied to biological cells, fibers, etc.) and shear strain (the resulting deformation of structures, in particular of cell walls and cells).

Both, if applied in intolerable values and/or for long periods without relief, are traumatic for the biological cell. Death of cells by pressure, friction, shear and microclimate is called a pressure sore. Official statistics show the magnitude of the problem in the United States (decubitusulcervictims.com/pressure-sore-statistics).

Therefore, the need to reduce or avoid pressure and shear as much as possible is clear and justified.

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Figure 2: Shear stress and its effect on a hexagonal cellular object (cross section), in particular on cell wall deformation (strain).

Why Do We Need Friction?

Friction is a holding force between two objects, which are more or less pressed together by a normal (vertical) force. So friction is a necessary safety component in human support surfaces and should therefore not be completely eliminated on all surface segments simultaneously. Spot elimination of friction in “at-risk” zones, as would be required to remain safe, has been called “strategic friction management.”

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Figure 3: Low-friction pairing.

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Figure 4: High-friction pairing.

Here’s an example of high friction: A hand, palm-down, that’s pushed down on a highly polished surface (e.g., a nice table, a glass plate) and pushed forward at the same time significantly resists sliding. Why is that so? We can set a heavier mug on that same table and push it around more easily! Why does the hand “stick” to the table and the mug not so much?

It is because there is a higher friction between skin and the table than between the mug and the table.

A tangential force applied to skin, which is already weight bearing (think of a seated person) will therefore build and create stress and strain — at least in the skin surface, and potentially in the tissue below the skin. This is because all the biological tissue below the skin is connected to the skin on one side and to a bone on the other side.

So If All This Is Natural, What Can We Do About It?

Why don’t we find a way to eliminate friction? What would happen if we took away most or all of friction?

We certainly would not have a static situation anymore, because friction, the counter force, is much smaller than the applied shear force. We’d create a kinetic situation (sliding), and the stress and strain on the tissue (now not adhering to the support surface any more) might be significantly reduced or possibly eliminated.

Shear stress and strain basically could not build in this case, because friction, the necessary force transfer mechanism, would not be available.

Here’s another way of looking at it.

A child dressed in shorts — and therefore, with bare legs — tries to slide down a slippery metal slide on the playground. For some reason, this does not work. The child sits on the sloped slide and cannot manage to slide down because her bare skin sticks to the slide surface due to high friction. The result is a lot of pulling (shear stress and strain), squeaking, heat and related pain on the posterior skin and muscle tissue of the child’s legs.

Now in a similar scenario, imagine the child is dressed in a pair of long pants. The child easily slides down the slide, because the pants covering the skin have considerably reduced the friction (and the shear) in the contacting area. So this is a kinetic situation under basically identical circumstances. The only thing that changed was the friction, and that happened by introducing a friction reducer.

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Figures 5a and b: Low-friction pairing (pants/slide) in first figure vs. high-friction pairing (skin/slide) in second figure. Who wins? The resulting final speed is impeded in Figure 5b by the high-friction skin surface on the slide.

The static situation — while a shear force was applied — created a lot of shear stress, strain and pain. Introducing a friction reducer removed all the shear stress, shear strain and pain. In general, applying shear force creates more stress and strain in the tissue in a static situation than in a comparable kinetic situation. Static friction is higher than kinetic friction (and more so, as long as no long pants are introduced as an additional friction reducer, as shown in the example).

What if we could translate that model into any situation where shear stress and strain should be reduced from the very start? What if we had a slippery surface available to us for the purpose of reducing pain and trauma to tissue? What if we had smart friction reducers, which significantly reduce initial friction in a support surface?

We do. We have the low-friction material called GlideWear.

The Case for GlideWear

Tamarack Habilitation Technologies Inc. of Blaine, Minn., has developed a material that reduces friction significantly, much more so than any pair of pants or other textiles. GlideWear is described as a “very slippery” dual-ply textile, a breathable material with a low coefficient of friction (CoF) of 0.2 between its two layers. The superficial skin-to-fabric CoF may be higher, but it is the lowest CoF in a multi-layered situation, which determines the effective CoF. Think of a chain: The chain breaks at its weakest link, and a multilayered stack slides at the lowest CoF.

This surface is so slippery, though, that it can be placed only in the bony at-risk zones — if used overall in the contact surface, it could reduce the sitter’s ability to maintain seat support. GlideWear is so different in its friction-reducing properties from anything that has been known in human application so far, that you have to feel it in order to believe it.

Since it is not intended to move a whole person over long distances by sliding them along as one would on wheels, GlideWear can and should be applied in the form of strategic friction management. This does not allow for unlimited transfers as on a transfer board. It covers basically all micro-movements and limited movements to occur between a person and their support surface while being seated (or supported in a supine position).

What Is Strategic Friction Management?

Strategic friction management applies low friction only in spots where shear stress is harmful — for example, in bony at-risk zones or in areas of existing irritation or trauma. It preserves friction at all other “no-risk” zones.

In at-risk, bed-bound patients, strategic friction management calls for applying low friction in the zone of possible heel or sacrum/coccyx contact areas, not under the rest of the body when that is ulcer-free.

In prosthetic sockets, it means applying low friction at so-called bony prominences.

For clients with pressure ulcers in seat surface and/or lower back surface areas, the application area is in the zone of the ischial tuberosities, the coccyx, the greater trochanters and the sacrum only. It is not applicable in the posterior, aspects of the mid thighs or the healthy lateral gluteus.

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Figure 6: The GlideWear zone: Strategic friction & shear management. Portion “a” shows dual-layer shear reduction zone glides with user’s skin. Portion “b” shows single-layer stability zone that enhances user’s positioning control.

GlideWear “eliminates” the friction-transfer mechanism to a significantly high degree and therefore prevents the shearing in the soft tissue from happening.

GlideWear Take-Aways

-- Pressure sores are caused by three extrinsic factors: pressure, shear, microclimate.

-- This story focuses on understanding of soft tissue shear.

-- Shear stress or shearing is the force that causes layers or parts to slide on top of each other.

-- In human soft tissue, shear is the force that tends to move interconnected soft tissue layers in opposite directions, distorting them at the same time.

-- Shear strain is the distortion/deformation of tissue under the influence of shear stress.

-- Friction is a mechanical property of two materials that resists as the surface of one material slides over the top of the other. It is a property that depends on the material pairing.

-- When friction is the dangerous counter force under the application of ever-occurring shear forces, then either the shear or the friction would have to be reduced to improve the situation.

-- Shear forces are a natural occurrence.

-- Shear forces are present even in resting (static) situations and cannot be avoided easily.

-- Friction as part of a paired material property can be altered by conscious decision.

-- When friction is altered by altering one of the paired partners, then shear strain is altered.

-- Strategic friction management concentrates on lowering friction in at-risk zones and preserving normal friction in zones not at risk.

-- GlideWear is a textile with a very low coefficient of friction, actually with the lowest available in the global market. It is highly appropriate for strategic friction/shear management in human/object interfaces of any kind.

-- Excessive shear interferes with cell metabolism – a probable reason for trauma and cell death.

-- Not only pressure, but also shear needs to be managed to protect tissue.

Cushion & Backrest Comparo

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