Gross Profit and Gross Margin
Our gross profit is calculated by subtracting our cost of revenue and amortization of intangible assets from our revenue. We calculate our gross margin percentage as our gross profit divided by our revenue. Our gross profit has been, and we expect it will continue to be, affected by a variety of factors, including sales volume, current and potential royalties and excess and inventory obsolescence costs. Our gross profit may increase to the extent our revenue grows.
Sales and Marketing Expenses
Sales and marketing expenses consist of market research and commercial activities related to the sale of OviTex and OviTex PRS, along with the salaries and related benefits, including sales commissions and stock-based compensation for employees focused on these efforts. Other significant sales and marketing expenses include costs incurred with post-market clinical studies, conferences and trade shows, promotional and marketing activities, as well as travel and training expenses.
Over time we expect our sales and marketing expenses to increase in absolute dollars as we continue to expand our commercial organization to both drive and support our planned growth in revenue. It is unclear at this point, however, what long-term effect, if any, the ongoing COVID-19 pandemic will have on these expansion plans. We expect our sales and marketing expenses to decrease as a percentage of revenue primarily as, and to the extent, our revenue grows.
General and Administrative Expenses
General and administrative expenses consist primarily of salaries and related benefits, including stock-based compensation, for personnel in executive, finance, information technology and administrative functions. General and administrative expenses also include professional service fees for legal, accounting, consulting, investor and public relations, insurance costs and direct and allocated facility-related costs.
We expect that our general and administrative expenses will increase in absolute dollars as we execute our growth initiatives and expand our business and headcount to support these initiatives. It is unclear at this point, however, what long-term effect, if any, the ongoing COVID-19 pandemic will have on these expansion plans. We expect our general and administrative expenses to decrease as a percentage of revenue primarily as, and to the extent, our revenue grows.
Research and Development Expenses
Research and development expenses consist primarily of product research, engineering, product development, regulatory compliance and clinical development. These expenses include salaries and related benefits, including stock-based compensation, for employees focused on these efforts, consulting services, costs associated with our preclinical studies, costs incurred with our manufacturing partner under development agreements related to technology transfer, laboratory materials and supplies and an allocation of related facilities costs. We expense research and development costs as they are incurred.
We expect that our research and development expenses in absolute dollars will increase in the future as we develop new products and enhance existing products although it is unclear at this point what long-term effect, if any, the ongoing COVID-19 pandemic will have on these development plans. We expect research and development expenses as a percentage of revenue to vary over time depending on the level and timing of new product development initiatives.
Interest Expense
Interest expense consists of cash interest related to our credit facilities, non-cash interest attributable to the amortization of final payment fees and the amortization of deferred financing costs related to our indebtedness.