Mascara is probably the most difficult product in the color and decorative cosmetics category to formulate. It is difficult in that there are multiple facets of the product that must come together and work with one another as one unit. The formulation itself is relatively easy to put together. The brush also is not so remarkable. But the combination of formula and brush is what makes the product so difficult to put together as a saleable product. You can have a great formula but a terrible brush, or a great brush and a terrible formula. The trick is to get both a great formula and a great brush to work together to make a great mascara.
Since my background is in formulation and not packaging, I will only spend a short time on brushes. Brushes can vary greatly. The more diversity in the brushes, the more combinations that we can see on the market. Brushes can have solid or hollow bristles. Hollow bristles can deliver more product. The bristle configuration also allows for even deposit of product and also is responsible for building, lengthening and separating the lashes.