Concept Selection: A New Compressive Knee Sleeve

The TRIZ method was used to produce a number of design concepts for our product, a knee sleeve that will provide more compressive support than the products currently on the market. Contradictions were discovered between various properties and aspects of the overall knee sleeve concept. Customer needs that were collected from a survey were also considered when developing the contradictions. A TRIZ matrix was used to produce different types of solutions, and these were analyzed and applied to our product. This resulted in a list of possible device design concepts.

In order to eliminate concepts and select a winning concept, the weight of the various sleeve properties and customer needs were examined. It was clear that the device has to be fairly simple, be able to exert good compressive stress on the knee, and be durable and strong. Some of the more complicated concepts were eliminated. Other concepts eliminated were those that seemed to be too similar to what is already on the market or those that did not seem feasible to manufacture. One example of an elimination is an idea that involved a corn starch type dilatant fluid that would solidify when stress was applied. We discovered that these types of fluids tend to solidify through vibrations and would not respond as needed in a knee sleeve. Another idea involved sets of sensors on the sleeve that could “sense” when an impending action was going to occur to put stress on the knee and correspondingly adjust itself. This idea would have been far too expensive and probably not feasible. One idea that was eliminated involved the sleeve type design by using adhesive supports that would be placed on parts of the knee. This concept was flawed in that it would not provide adequate support and it would be very difficult to find an adhesive that would be comfortable yet effective at staying on the skin.

Another factor that was examined was how many different contradictions a certain idea solved. It was clear that a design concept that satisfied multiple sets of contradictions was superior to a concept that only solved one contradiction. After using these elimination techniques and analysis methods, it was apparent that one design stood above the rest. This concept employs a thin band that is pulled over the knee. There are six small band attachment points on the knee with nubs. Bands can then be placed on top of the primary sleeve and attached between different points to provide support to different areas of the knee. This idea incorporates several of the solution types from the TRIZ analysis. It is segmentation design because a good percentage of the support is coming from bands that can be attached on top of the main sleeve. It is secondary cushioning because the top support bands can take over when the stress on the knee overcomes the base sleeve.

The functionality of our knee brace is based on many factors. According to customer demands, the brace should be durable, comfortable, and supportive. Satisfying each of these demands and resolving the engineering conflicts through TRIZ methodology led us to our final design. Our brace consists of two parts: a comfortable inner sleeve and supportive outer bands. By relegating the bulk of the support aspect of the brace to the outer bands, many different types of support can be produced based on the arrangement of the tightening bands.

As seen in Figure 1, the design incorporates side stabilizers, preferably made of the same material as the base inner sleeve, to provide a firmer support to the medial and lateral sides of the knee. Incorporated in the side stabilizers are attachment points with nubs, three on each side. These nubs will protrude out of the base sleeve, but not excessively to deter the customer from wearing the sleeve underneath articles of clothing. The base sleeve will have no apertures (circle drawn in figure to show relative locations of nubs to patellar region); the patella will be closed as to prevent any irritation that may arise with tightening bands that overlap the patella.

Figure 1: Concept design of base knee sleeve with six attachment points

Representations of two differing tightening bands are seen in Figure 2. Figure 2(a) displays a thin band with circular apertures along the entire length which allows the wearer to selectively choose tightness over portions of the knee joint for comfort. Apertures need to be smaller than the size of the nubs so that the band can stay in place and have minimal loss of support from stretching during physical activity. Figure 2(b) represents a tightening band that is used for tightening across the patella. The second band (Fig. 2b) incorporates a design that provides greater comfort to the wearer while in motion; having a thin band over the knee (Fig. 2a) may irritate the wearer because the band may shift up and down across the patella during activity. Our design also incorporates attachment sites at the back ends of the tightening bands, in which excess portions would attach to the base inner sleeve to prevent dangling. One of the many customer demands is to wear the sleeve underneath clothing. Using this additional concept keeps excess portions of the bands from dangling freely and allows the customer to wear the sleeve easily without excess bunching.

(a) (b)

Figure 2: Concept design of tightening bands

There are several logical performance indices for our knee brace system. For comfort, it is logical to minimize the mass of each layer. However, care must be taken to not exceed that layer’s tensile strength. The basic mass equation used is

m = ρV

The sleeve can be treated as a thin-walled cylinder for our purposes. This approximation changes the volume term in the above equation to

V = At = πDLt,

Which creates a final mass equation of

m = ρ(πDLt).

Now, the stress on the knee must be taken into account. There are two definitions of stress that are of interest to us.

σ = εE and F = Aσ.

Rearrangement to solve for the force exerted on the sleeve and substitution yields

F = AεE.

The free variable in this case is the thickness of the sleeve, t. Thickness is included in our constraint by strain, ∆t/t. Substitution and rearrangement to solve for the free variable, t, yields

t = ∆t AE/F.

From before, we know that σ = F/A, or σ^-1 = A/F. Re-substitution yields a thickness equation based on stress, elastic modulus, and change in thickness.

t = ∆tE σ^-1

This equation can then be substituted into the mass equation, giving a final mass equation of

m = (ρEσ^-1)( πDL∆t).

Based on this, the figure of merit for the sleeve, M, is

M = σ(ρE)^-1

This figure of merit is also valid for the outer tightening bands. The only difference is the geometrical term A. A band is better approximated with a rectangular cross section.

Maximization of this term will minimize mass, accounting for the stress on the brace introduced by placing it on the knee. This figure solved for some common materials is shown in Table 1.

	ρ (g/cm^3)	σ (MPa)	E (GPa)	M
Low-C Steel	7.87	350	205	0.22
Gold	19.32	120	77.2	0.08
Neoprene	1.5	28	0.0035	5333.33
Natural Rubber (vulcanized)	0.95	28	0.0015	19649.12
Acrylonitrile-Butadiene Rubber	1.35	6.89	0.003	1701.23
Fluoroelastomer	1.81	15.6	0.0076	1134.05
HDPE	0.962	21.9	0.99	23.00
Kevlar (DuPont 29)	1.44	2920	70.3	28.84
Carbon Fiber (29700 Denier)	1.81	3800	228	9.21
Nylon	1.16	40	3.3	10.45
Diamond	3.51	8680	1200	2.06

Table 1: Performance Indices of Common Materials

A cursory examination of this table suggests that elastomers will have the highest values of M and are the best materials for this particular index. Before final material selection, the other functions of the brace should be considered. Durability can be modeled considering modulus of resilience. Heating of injured joints can ease symptoms, so heat loss should be considered. Once all of the functions of the brace are defined, a final material can be chosen.