The Company’s Series B, Series G, Series I-1 and Series I-2 non-redeemable convertible preferred stock has been classified as equity on the accompanying condensed consolidated balance sheets.

The authorized, issued and outstanding shares of non-redeemable convertible preferred stock as of March 31, 2021 consist of the following:

	Shares		Shares Issued		Liquidation		Carrying
	Authorized		and Outstanding		Preference		Value
					(in thousands)
Series B		5,000,000		250,000	$	445	$	—
Series G		5,000,000		5,000,000		5,000		5
Series I-1		2,000		814		814		—
Series I-2		4,310		4,310		4,310		—
Total		10,006,310		5,255,124	$	10,569	$	5

The authorized, issued and outstanding shares of non-redeemable convertible preferred stock as of December 31, 2020 consist of the following:

	Shares		Shares Issued		Liquidation		Carrying
	Authorized		and Outstanding		Preference		Value
					(in thousands)
Series B		5,000,000		250,000	$	441	$	—
Series G		5,000,000		5,000,000		5,000		5
Series I-1		2,000		814		814		—
Series I-2		4,310		4,310		4,310		—
Total		10,006,310		5,255,124	$	10,565	$	5

As of March 31, 2021, the Company was authorized to issue 120,000,000 shares of common stock, $0.001 par value per share, and 20,000,000 shares of preferred stock, $0.001 par value per share. The Company has designated 50 shares of Series D redeemable convertible preferred stock and a total of 10,006,310 shares of Series B, Series G, Series I-1 and Series I-2 non-redeemable convertible preferred stock.

In October 2014 and March 2016, the Company issued warrants exercisable for 62,047 and 11,159,995 shares of common stock, respectively, at an exercise price of $1.75 per share to certain placement agents and existing investors in connection with financing arrangements. In April 2020, the common stock warrants issued in October 2014 expired unexercised. The common stock warrants issued in March 2016 expired unexercised in March 2021. As of December 31, 2020, 3,948,569 common stock warrants issued in March 2016 were outstanding.

The Company adopted the 2006 Equity Participation Plan (as amended the “2006 Plan”), which provides for the grant of stock options, restricted stock and other equity-based awards. Awards for up to 100,000 shares may be granted to employees, directors and consultants under this Plan. The options granted under the 2006 Plan may be either qualified or non-qualified options. Options may be granted with different vesting terms and expire no later than 10 years from the date of grant. The 2006 Plan expired on November 16, 2016. Options and other equity-based awards granted prior to the expiration of the 2006 Plan will continue in effect until the option or award is exercised or terminates pursuant to its terms. No new awards may be granted under the 2006 Plan following its expiration.

In April 2010, the Company adopted the 2010 Equity Participation Plan, as amended (the “2010 Plan”), which provides for the grant of stock options, restricted stock and other equity-based awards. Awards for up to 9,700,000 shares may be granted to employees, directors and consultants under the 2010 Plan. The options granted under the 2010 Plan may be either qualified or non-qualified options. Options may be granted with different vesting terms and expire no later than 10 years from the date of grant. In June 2020, the Company amended the 2010 Plan to extend the term of the 2010 Plan until March 2030. No other material provisions were amended.

Transactions involving stock options issued to employees, directors and consultants under the 2006 Plan and the 2010 Plan are summarized below. Options issued have a maximum life of 10 years. The following tables summarize the changes in options outstanding and the related exercise prices for the Company’s common stock options issued:

						Weighted-
						Average
						Remaining
	Number of			Weighted-		Contractual		Aggregate
	Outstanding			Average		Term		Intrinsic Value
	Options			Exercise		(in years)		(in thousands)
Outstanding at December 31, 2020		4,255,371		$	3.41
Granted		—
Forfeited or canceled		(57,233	)	$	1.85
Expired		(21,139	)	$	289.50
Outstanding at March 31, 2021		4,176,999		$	1.99		7.02	$	—
Vested and expected to vest at March 31, 2021		4,160,749		$	1.99		7.02	$	—
Exercisable at March 31, 2021		3,769,236		$	2.05		6.90	$	—

The fair value of stock options granted is estimated at the date of grant using the Black-Scholes option valuation model. All options are amortized over the requisite service periods. For the three months ended March 31, 2021 and 2020, no stock options were granted.

Total stock-based compensation expense for the three months ended March 31, 2021 and 2020 was comprised of the following (in thousands):

	Three Months Ended March 31,
	2021		2020
Cost of sales	$	8	$	24
Research and development		23		49
Selling and marketing		17		22
General and administrative		186		323
Total	$	234	$	418

Unrecognized compensation expense related to stock options as of March 31, 2021 was $326,000, which is expected to be recognized over a weighted-average period of less than one year.

As of March 31, 2021, the Company had shares of common stock reserved for future issuance as follows: