Preferred Stock, Common Stock and Stockholders' Equity (Tables)

12 Months Ended

Dec. 31, 2020

Equity [Abstract]

Summary of Stock Option Activity

A summary of the Company’s stock option activity under the 2007 Equity Incentive Plan and the 2013 Plan is as follows:

	Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (Years)		Aggregate IntrinsicValue
Outstanding at December 31, 2019		3,114,371		$	1.71		8.32	$	2,143,756
Granted		1,172,000		$	1.58		9.22	—
Exercised		(199,111	)	$	1.08		2.78	$	484,188
Expired/Forfeited/Exchanged		(10,001	)	$	2.42		6.93	$	30,620
Outstanding at December 31, 2020		4,077,259		$	1.70		8.09	$	5,134,768
Vested and expected to vest at      December 31, 2020		4,077,259		$	1.70		8.09	$	5,134,768
Exercisable at December 31, 2020		1,943,106		$	2.32		7.32	$	1,967,591

Summary of Estimated Fair Value of Stock Option Award

The estimated fair value of each stock option award granted was determined on the date of grant using the Black Scholes option-pricing valuation model with the following weighted-average assumptions for option grants during the years ended December 31, 2020 and 2019:

	Year Ended December 31,
	2020	2019
Risk free interest rate	0.39% - 0.96%	1.80% - 2.55%
Expected option term	5.5 - 6.0 years	4.27 - 6.0 years
Expected volatility of common stock	99.73% - 103.99%	90.34% - 112.58%
Expected dividend yield	0.0%	0.0%

Summary of Estimated Fair Value of Shares to be Acquired under Employee Stock Purchase Plan

The estimated fair value of the shares to be acquired under the ESPP was determined on the initiation date of each six-month purchase period using the Black-Scholes option-pricing valuation model with the following weighted-average assumptions for ESPP shares to be purchased during the years ended December 31, 2020 and 2019 as follows:

	Year Ended December 31,
	2020	2019
Risk free interest rate	0.13% - 1.11%	1.89% - 2.52%
Expected term	0.5 years	0.5 years
Expected volatility of common stock	69.72% - 111.98%	130.36% - 170.68%
Expected dividend yield	0.0%	0.0%

Summary of Recognized Stock-Based Compensation Expense

The Company recognized stock-based compensation expense to employees and directors in its research and development and its general and administrative functions during the years ended December 31, 2020 and 2019 as follows:

	Year Ended December 31,
	2020		2019
Research and development	$	316,892	$	710,155
Selling, general and administrative		1,311,666		663,803
Total stock-based compensation expense	$	1,628,558	$	1,373,958

Summary of Common Stock Reserved for Future Issuance

Common stock reserved for future issuance consists of the following at December 31, 2020 and 2019:

	December 31,
	2020		2019
Stock options issued and outstanding		4,077,259		3,114,371
Authorized for future option grants		1,402,433		1,587,155
Warrants to purchase common stock		1,841,879		2,713,561
Authorized for employee stock purchase plan		244,574		263,065
Total common stock reserved for future issuance		7,566,145		7,678,152