As of December 31, 2020, the Company had outstanding warrants to purchase 15,541 shares of common stock. The following table summarizes warrant activity as of December 31, 2020, 2019, and 2018:

	Warrants			Weighted average exercise price
Outstanding at December 31, 2017		2,099,199		$	5.21
Issued		15,750			3.06
Exercised		(101,310	)		1.30
Expired		(1,890,602	)		5.33
Outstanding at December 31, 2018		123,037			6.35
Issued		58,334			8.10
Exercised		(111,715	)		6.73
Outstanding at December 31, 2019		69,656			7.21
Issued		15,541			80.43
Exercised		(69,656	)		7.21
Outstanding at December 31, 2020		15,541		$	80.43

In connection with the entry into the Loan Agreement, the Company issued to Hercules a warrant to purchase a number of shares of the Company’s common stock, par value $0.0001 per share equal to 2.5% of the aggregate amount of the Term Loan Advances that are funded, as such amounts are funded. With the first advance of the 2020 Term Loan, Hercules Capital Inc. received warrants to purchase an aggregate 15,541 shares of the Company’s common stock at an exercise price of $80.43 per share, which was the volume weighted average price of the Company’s common stock over the ten-day trading period immediately preceding the initial closing, subject to certain limited adjustments as specified in the warrant. The warrants are exercisable for seven years from the date of issuance. The warrants were classified as a component of stockholders’ equity. The relative fair value of the warrants of approximately $0.9 million at the time of issuance, which was determined using the Black-Scholes option-pricing model, was recorded as additional paid-in capital and reduced the carrying value of the debt. The discount on the debt is being amortized to interest expense over the term of the debt utilizing the effective interest rate method.

In connection with the Company’s term loan facility with SVB and WestRiver which was completed in March 2019, the Company issued warrants for up to 70,000 shares of the Company’s common stock at a price per share equal to $8.10 based upon usage of the facility and were exercisable until March 4, 2026. The warrants were classified as a component of stockholders' equity of which 58,332 warrants were immediately exercisable and the remaining 11,668 warrants could be earned based on usage of the second tranche of the 2019 Term Loan. The additional warrants were initially immediately exercisable if and when the Term B Loan Advance was funded (however, the March 2019 Warrants were amended in connection with the First Amendment to the 2019 Term Loan, as discussed below). The relative fair value of the warrants of approximately $0.4 million at the time of issuance, which was determined using the Black-Scholes option-pricing model, was recorded as additional paid-in capital and reduced the carrying value of the debt. The discount on the debt was amortized to interest expense over the term of the debt utilizing the effective interest rate method. In July 2019, in connection with the First Amendment to the 2019 Term Loan, the March 2019 Warrants were amended to fix the number of shares that may be issued upon exercise of each such March 2019 Warrant at 29,167 shares of common stock, and SVB and WestRiver were issued new warrants (the “July 2019 Warrants”), which would have become exercisable upon funding of the Term B Loan Advance, to purchase 5,750 shares of the Company’s common stock at a price per share equal to $25.71. The July 2019 Warrants replace the portion of the March 2019 Warrants that could originally be earned by SVB and WestRiver upon funding of the Term B Loan Advance. Due to the Company not drawing down on the Term B Loan Advance by December 31, 2019, the July 2019 Warrants have been terminated. In December 2019, SVB exercised in full its portion of 29,167 warrants it received in connection with the 2019 Term Loan via a cashless exercise. The Company issued SVB 24,185 shares of its common stock in connection with the warrant exercise and did not receive any cash proceeds. In August 2020, WestRiver exercised in full its portion of 29,167 warrants it received in connection with the 2019 Term Loan via a cashless exercise. The Company issued SVB 26,270 shares of its common stock in connection with the warrant exercise and did not receive any cash proceeds. As of December 31, 2020, none of the warrants issued in connection with the First Amendment to the 2019 Term Loan were outstanding.

In connection with the First Amendment to the Loan and Security Agreement, SVB and WestRiver Innovation Lending Fund VIII, L.P. received warrants to purchase 15,750 shares of the Company’s stock at an exercise price of $3.06 per share, which are exercisable for seven years from the date of issuance (“2018 warrants”). The warrants were classified as a component of stockholders' equity. In August 2019, SVB exercised in full its portions of 7,875 warrants it received in connection with the 2016 Amended Term Loan via a cashless exercise. The Company issued SVB 6,969 shares of its common stock in connection with the warrant exercise, and did not receive any cash proceeds. In August 2020, WestRiver exercised in full the 7,875 warrants it received in connection with the 2016 Amended Term Loan via a cashless exercise. The Company issued SVB 7,579 shares of its common stock in connection with the warrant exercise, and did not receive any cash proceeds. As of December 31, 2020, none of the warrants issued in connection with the 2016 Amended Term Loan were outstanding.

In connection with the Registered Direct Offering, which was completed on December 11, 2017, the Company issued Common Warrants and Placement Agent Warrants to purchase 1,783,587 shares and 107,015 shares of common stock, respectively, with an exercise price of $5.25 per share and $6.6562 per share, respectively, which are exercisable from the date of issuance until December 11, 2018 (“2017 warrants”). The Common Warrants and Placement Agent Warrants were classified as warrant liability. See Note 3, “Fair Value Measurements”, for the fair value calculations of the warrant liability. The 2017 warrants had a 1 year term and expired on December 11, 2018.

In connection with the Company’s debt financing which was completed on November 9, 2016, the Company issued warrants to purchase 65,228 shares of common stock with an exercise price of $7.41 per share, which are exercisable upon issuance (“2016 warrants”) to SVB and Life Sciences. The warrants were classified as a component of stockholders' equity. In September 2019, SVB exercised its portion of 32,614 warrants it received in connection with the 2016 Original Term Loan via a cashless exercise. The Company issued SVB 24,328 shares of its common stock in connection with the warrant exercise, and did not receive any cash proceeds. In addition, in May 2020, Life Science exercised in full the 32,614 warrants it received in connection with the 2016 Original Term Loan via a cashless exercise. The Company issued Life Science 29,982 shares of its common stock in connection with the warrant exercise and did not receive any cash proceeds. As of December 31, 2020, none of the warrants issued in connection with the 2016 Original Term Loan were outstanding.

On November 3, 2014, the Company issued warrants to purchase 42,059 shares of common stock with an exercise price of $5.94 per share to the placement agent in connection with the issuance of certain convertible notes from September 2014 through November 2014 (the “September 2014 Notes”), which were exercisable upon issuance (“2014 warrants”). The warrants were initially classified as a liability in the consolidated financial statements, as upon a qualified financing, the warrant price would automatically adjust to a 10% premium to the conversion price of the September 2014 Notes in such mandatory conversion. The initial fair value of the warrant liability was $79,129 which was recorded as a discount to the notes and amortized over the term of the original September 2014 Notes. Upon the note amendment that occurred in September 2015, the discount was included in the carrying amount in the calculation of a loss on extinguishment. In connection with the automatic conversion of the September 2014 Notes upon the close of the Company’s IPO in November 2015, the warrant liability was reclassified to equity. The 2014 warrants had a five-year term and an expiration of November 2, 2019. All 42,059 warrants were exercised in 2019.

On October 29, 2013, the Company issued warrants to purchase 230,459 shares of common stock with an exercise price of $1.30 per share to the placement agent in connection with the June 2013 Notes, which were exercisable upon issuance (“2013 warrants”). The warrants were classified as a component of stockholders’ equity. The 2013 warrants had a five-year term. 129,149 warrants and 101,310 warrants were exercised in 2017 and 2018, respectively.

The initial fair value of the warrants were estimated using the Black‑Scholes option pricing model with the following assumptions:

Black-Scholes option valuation assumptions	2020 warrants			2019 warrants			2018 warrants			2017 warrants			2016 warrants			2014 warrants			2013 warrants
Risk-free interest rate		0.5	%		2.8	%		3.0	%		1.7	%		1.8	%		1.6	%		1.4	%
Dividend yield		—			—			—			—			—			—			—
Volatility		88	%		97	%		85	%		61	%		73	%		70	%		64	%
Weighted average contractual term	7 years			7 years			7 years			1 year			7 years			5 years			5 years