Question

what biological principles are involved in scrubbing of calluses during a foot spa

Answer 1

Calluses are areas of skin that have thickened and become hard due to repeated friction, irritation, or any other external pressure. They occur in areas which are always in contact with a hard surface, like the feet because of frequent walking.

Although calluses are not harmful, they tend to cause other skin problems like infections or ulcerations.

The calluses are formed by the accumulation of keratinocytes that are undifferentiated terminally and present in the outermost layer of the skin.

The calluses, though dead, have an extensive network of proteins that are cross-linked and hydrophobic keratin intermediate filaments that has many disulfide bonds.

Therefore, at a foot spa, scrubbing off the calluses is done by using keratolytic agents like salicylic acid which is usually done with a pumice stone or a callus shaver.

Answer 2

Calluses occur due to the continuous rubbing of skin area , although they it self can't be dangerous but can cause infection etc. These are formed due to more divisions of keratinocytes  and their accumulation on the outer side of the skin, They can also contain dead keratinocytes filaments as well as the protein .
So in order to remove the calluses in the foot spa the  keratolytic agent is used along with callus shaver.