Parameter Analysis

Parameter 1: Roller Ball Diameter

One big driver of our design was the diameter of roller balls that we used. We initially created 2 alpha prototypes with 9 mm and 0.5 mm diameter roller balls. We determined that the 9 mm was too big- it was comfortable but had poor scalp access and the 0.5 mm was tiny- it had great scalp access but was not comfortable. For our final design, we wanted to determine the optimal roller ball diameter.

To estimate the ideal roller ball diameter, we conducted user tests with different sized roller balls.

Fig 7, 8, 9. Pictures of the roller balls being manuevered on a scalp to test the "feel"

Average distance between hair follicles= 1 mm to 1.4 mm

While different people have different preferences, 3-5 mm seemed to balance good on scalp-feel with good scalp access. However due to manufacturability constraints, our proof-of-concept prototype has 9 mm diameter roller balls. Ours would be 4 mm while mass manufacturing. 

Parameter 2: Roller Ball Oil Dispensing Volume/Area

Procedure:

1. Take an individual roller ball and fill with a certain amount of hair oil. Record the exact amount filled. 

2. Using a napkin as the surface, roll the ball in straight lines until the oil has been depleted 

3. Measure the length and width of the oil lines and record area

4. Calculate volume per area to find dispersion 

Parameter 3: Materials

Our material selection for our proof of concept prototype had 3 major criteria: Mechanical Properties, Biocompatibility and Ecological Impact

Mechanical Properties 

• • The outer casing and support structures of the product need to be sturdy enough to withstand normal impacts during usage and operation, while also being light enough for the overall product to feel natural and easily maneuverable in the palm. For this purpose, we have selected PLA for these parts.

  • The reservoir needed to strike a balance between being flexible to accommodate the contours of the human scalp and creating an airtight seal between the roller ball housing and the reservoir, while also being rigid enough to not collapse or fail when exposed to the mechanical forces of manufacturing and regular usage. The team had initially decided on using molded silicone for the reservoir, however, it proved to be too soft to provide the requisite rigidity and hence pivoted to using NinjaFlex TPU, which was also a breeze to manufacture via 3D printing.

Biocompatibility 

• • While the outer casing's biocompatibility was not a major factor, the reservoir and the roller ball assemblies had to be made of materials known to not cause adverse reactions on contact with the scalp, which is even more sensitive compared to the rest of the skin. 

  • To this end, polypropylene was selected for the roller ball housings, while stainless steel roller balls were selected. 

  • The reservoir material (NinjaFlex) is a variant of TPU, which is widely used in medical instruments due to its biocompatibility.

Ecological Impact 

• • We selected each of the materials for the product, keeping in mind the ecological impact from both, an end-of-life standpoint and also considering the energy expenditure of fabricating a part from completely new or recycled materials.