quarter of a given year. However, until recently, our iStent was the only MIGS device approved for sale in the United States by the FDA. Thus, for several years we had commercialized the iStent in the United States without any direct MIGS competitors. Other MIGS devices have now become available in the United States and globally, including our iStent inject, or are in development by third parties that have entered or could enter the market and which may affect adoption of or demand for our products. These MIGS products or other products that may be developed and receive regulatory approval, could achieve greater commercial acceptance, demonstrate better safety or effectiveness, clinical results, ease of use or lower costs than our iStent, iStent inject or our other pipeline products under development, which may reduce demand for our primary products, the iStent and iStent inject as well as for our products in development.
Cost of sales
Cost of sales reflects the aggregate costs to manufacture our products and includes raw material costs, labor costs, manufacturing overhead expenses and the effect of changes in the balance of reserves for excess and obsolete inventory. We manufacture our iStent products at our current headquarters in San Clemente, California using components manufactured by third parties. Due to the relatively low production volumes of our iStent products compared to our potential capacity for those products, a significant portion of our per-unit costs is comprised of manufacturing overhead expenses. These expenses include quality assurance, material procurement, inventory control, facilities, equipment and operations supervision and management.
Beginning in late 2013, cost of sales had included amortization of the $17.5 million intangible asset we recognized in connection with our royalty buyout agreement with GMP Vision Solutions, Inc. in November 2013. Amortization expense was $0.9 million and $2.6 million during the three and nine months ended September 30, 2018, respectively, and the intangible asset was fully amortized as of November 2018.
Beginning in 2015, cost of sales includes a charge equal to a low single-digit percentage of worldwide net sales of certain current and future products, including our iStent products, with a required minimum annual payment of $0.5 million, which amount became payable to the Regents of the University of California (the University) in connection with our December 2014 agreement with the University (the UC Agreement) related to a group of our U.S. patents (the Patent Rights). For the three months ended September 30, 2019 and September 30, 2018, we recorded approximately $1.5 million and $1.1 million, respectively, in cost of sales in connection with this product payment. For the nine months ended September 30, 2019 and September 30, 2018, we recorded approximately $4.2 million and $3.2 million, respectively, in cost of sales in connection with this product payment obligation. This ongoing product payment obligation will terminate on the date the last of the Patent Rights expires, which is currently expected to be in 2022.
Under the Protecting Americans from Tax Hikes Act of 2015 (PATH Act), the 2.3% federal medical device excise tax on U.S. sales of medical devices manufactured by us was suspended from January 1, 2016 to December 31, 2017, and, pursuant to HR 195 passed on January 22, 2018, was further suspended through December 31, 2019.
Our future gross profit as a percentage of net sales, or gross margin, will be impacted by numerous factors including commencement of sales of products in our pipeline, or any other future products, which may have higher product costs. Our gross margin will also be affected by manufacturing inefficiencies that we may experience as we attempt to manufacture our products on a larger scale, manufacture new products and change our manufacturing capacity or output. Additionally, our gross margin will continue to be affected by the aforementioned expense related to the UC Agreement.
Selling, general and administrative
Our selling, general and administrative (SG&A) expenses primarily consist of personnel-related expenses, including salaries, sales commissions, bonuses, fringe benefits and stock-based compensation for our executive, financial, marketing, sales, and administrative functions. Other significant SG&A expenses include marketing programs; advertising; post-approval clinical studies; conferences and congresses; travel expenses; costs associated with obtaining and maintaining our patent portfolio; professional fees for accounting, auditing, consulting and legal services; costs to implement our global enterprise systems; and allocated overhead expenses.
We expect SG&A expenses to continue to grow as we increase our global sales and marketing infrastructure, our clinical education and general administration infrastructure in the United States. We also expect other nonemployee-related costs, including sales and marketing program activities for new products, outside services and accounting and general legal costs to increase as our overall operations grow. The timing of these increased expenditures and their magnitude are primarily dependent on the commercial success and sales growth of our products, as well as on the timing of any new product launches and other potential business and operational activities.