The Company has stock-based compensation plans under which it grants stock options, RSUs, and PRSUs to officers, directors and third parties. Total stock-based compensation for the periods presented were as follows (in thousands):

	Three Months Ended
	March 31,
	2022		2021
Cost of sales	$	256	$	95
Research and development		972		498
Sales, general and administrative		8,956		3,881
Total	$	10,184	$	4,474

As of March 31, 2022, there was $57.9 million of unamortized compensation expense for RSUs and PRSUs to be recognized over a weighted average period of 1.81 years.

The Company issued approximately 1,225,000 shares of common stock, before net share settlement, upon vesting of RSUs and PRSUs during the three months ended March 31, 2022, and issued approximately 473,000 shares of common stock, before net share settlement, upon vesting of RSUs and PRSUs during the three months ended March 31, 2021.

Employees are eligible to participate in the ESPP approved by its shareholders. During the three months ended March 31, 2022, and 2021, no shares were issued under the ESPP.

The Company estimates the fair value of shares issued to employees under the ESPP using the Black-Scholes option-pricing model. The assumptions used to estimate the fair value of stock options granted and stock purchase rights under the ESPP are as follows:

	Three Months Ended
	March 31,
	2022			2021
Risk-free interest rate		0.07	%		0.10	%
Expected dividend yield		—			—
Expected term (years)		0.50			0.50
Volatility		50.29	%		78.51	%

The 2017 Common Stock Warrants (the “2017 PIPE Warrants”) have a five-year life and are exercisable by cash exercise only. During the three months ended March 31, 2022, and 2021, there were approximately 425,000 and 145,000 2017 PIPE Warrant exercises, respectively, for total cash proceeds of $0.9 million and $0.3 million, respectively. As of March 31, 2022, approximately 1,887,000 2017 PIPE Warrants remained outstanding.

The 2018 Common Stock Warrants (the “2018 PIPE Warrants”) have a five-year life and are exercisable by cash or cashless exercise. During the three months ended March 31, 2022, and 2021, there were approximately 126,000 and 2,148,000 2018 PIPE Warrant exercises, respectively, for total cash proceeds of $0.4 million and $0.3 million, respectively. As of March 31, 2022, approximately 8,354,000 2018 PIPE Warrants remained outstanding.

In conjunction with the Company’s 2018 acquisition of SafeOp, the Company issued warrants to purchase 2,200,000 shares of common stock at an exercise price of $3.50 per share (the “SafeOp Surgical Merger Warrants”), which have a five-year life and are exercisable by cash or cashless exercise. There were no exercises during the three months ended March 31, 2022. During the three months ended March 31, 2021, there were approximately 1,000,000 SafeOp Surgical Merger Warrant exercises for cash proceeds of $0.1 million. As of March 31, 2022, approximately 1,195,000 SafeOp Surgical Merger Warrants remained outstanding.

During the year ended December 31, 2018, in connection with the initial debt financing with Squadron Medical and a participant lender, the Company issued warrants (the “Squadron Medical Warrants”) to purchase 845,000 shares of common stock at an exercise price of $3.15 per share. An additional 4,839,000 Squadron Medical Warrants were issued at an exercise price of $2.17 per share during the second quarter of 2019, in conjunction with the Company’s draw on the expanded credit facility. In May 2020, an additional 1,076,000 Squadron Medical Warrants were issued at an exercise price of $4.88 per share in conjunction with the Company’s second amendment to the Squadron Medical debt for total Squadron Medical Warrants outstanding to Squadron Medical and the participant lender of 6,760,000. In conjunction with the second amendment, the expiration dates for all existing Squadron Medical Warrants were extended to May 29, 2027 to align all outstanding warrant expiration dates. No Squadron Medical Warrants have been exercised as of March 31, 2022.

In December 2017 the Company issued warrants to Mr. Patrick S. Miles, the Company’s Chairman and Chief Executive Officer, to purchase approximately 1,327,000 shares of the Company’s common stock for $5.00 per share (the “Executive Warrants”). The Executive Warrants have a five-year term and are exercisable by cash or cashless exercise. The Executive Warrants issued to Mr. Miles were accounted for as share based compensation, and the fair value of the Executive Warrants of approximately $1.4 million were recognized in full in the statement of operations for the year ended December 31, 2017, as the Executive Warrants were immediately vested upon issuance. No Executive Warrants have been exercised as of March 31, 2022.

A summary of all outstanding warrants for common stock as of March 31, 2022, were as follows (in thousands, except for strike price data):

	Number of Warrants		Strike Price		Expiration
2017 PIPE Warrants		1,887	$	2.00	June 2022
2018 PIPE Warrants		8,354	$	3.50	May 2023
SafeOp Surgical Merger Warrants		1,195	$	3.50	May 2023
2018 Squadron Medical Warrants		845	$	3.15	May 2027
2019 Squadron Medical Warrants		4,839	$	2.17	May 2027
2020 Squadron Medical Warrants		1,076	$	4.88	May 2027
Executive Warrants		1,327	$	5.00	December 2022
Other⁽¹⁾		131	$	5.74	Various through February 2026
Total		19,654

All outstanding warrants were deemed to qualify for equity classification under authoritative accounting guidance.