Functional mechanics 

Tilting force:

Creating a free body diagram, we see the following forces. 

Noting that normal force goes to zero when the patient loses contact with the bed, the normal force goes zero, we get the following equation for the required pulling force.

Where W is the weight of the patient and Fpull is the required force exerted by the nurse. 

However, preliminary trials suggest that because of the patient body folding, there is an additional force reduction with and without the wedge, which will be represented as an still unknown constant alpha. 

To minimize the effect of phi and maximize the potential force reduction, a high angle is recommended of 50-60 degrees.

Material

Based on research of similar products, the main material for the wedges would be some form of PVC (Vinyl) or TPU (Polyurethane CPE). PVC is low cost, flexible, durable, and sterilizable. TPU has many of the same characteristics as PVC, but is more environmentally friendly.

Coeff of friction

The coefficient of friction of the materials we are considering is not readily available. We are doing tests to get a baseline coefficient on non-medical grade materials.

Sterilizability/Biocompatibility 

All of the materials considered are sterilizable and/or disposable and are safe to use as medical mattress coverings.

Durability

PVC is often used for one time use medical devices but is still considered durable and is resistant to chemical stress cracking. TPU appears to be better for repeated uses.

 

Fall-off prevention strategy

The system includes detachable side bumpers to help prevent patients from falling off the bed. We assume that the hospital has rails that can be raised to form a more rigid barrier in the case that there is a larger concern for patient safety.

Attachment to bed 

Most on-market patient positioning or boosting systems are attached to the bed by straps or by tucking a tail/apron underneath the bed. Our system would utilize one of those systems.

Length of Device

Pronepad should be the length of the patient’s torso and thighs. The head of the patient will be supported by the respiratory therapist, and the torso and thighs hold most of the patient’s weight. The average person has a torso length of 20.5 inches and a thigh length of 19 inches. Because we want to accommodate more than the average person, Pronepad’s length will be 48 inches long.