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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2016
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]  
Summary of Stock Option Activity
Stock-based compensation has been recognized in operating results as follows:
 
 
Years ended December 31,
 
2016
 
2015
 
2014
In cost of revenue
$
122,301

 
$
39,676

 
$
—

In research and development expenses
2,420,696

 
1,499,009

 
796,008

In selling and marketing expense
2,111,366

 
768,146

 
145,239

In general and administrative expenses
2,910,156

 
1,639,196

 
1,128,947

Benefit from restructuring
(60,203
)
 
—

 
—

Total stock-based compensation
$
7,504,316

 
$
3,946,027

 
$
2,070,194
Estimated Fair Value of Stock Options - Assumptions
The estimated fair value of stock option awards was determined on the date of grant using the Black-Scholes option valuation model with the following assumptions during the years indicated below:
 
 
Years ended December 31,
 
2016
 
2015
 
2014
Risk-free interest rate
0.93% - 1.89%
 
1.35% - 2.15%
 
1.42% - 2.1%
Dividend yield
0%
 
0%
 
0%
Expected volatility (range)
80% - 134%
 
73% - 77%
 
81% - 86%
Expected volatility (weighted-average)
103%
 
75%
 
85%
Expected term (in years)
5.5 years
 
6.0 years
 
5.8 years
Stock-Based Compensation Recognized
A summary of stock option activity and of changes in stock options outstanding is presented below:
 
 
Number of Options
 
Weighted-Average Exercise Price Per Share
 
Intrinsic
Value
 
Weighted-Average
Remaining
Contractual Life
Balance outstanding, December 31, 2013
4,287,545

 
$
5.18

 
$
5,896,329

 
6.7 years
Granted
1,410,038

 
$
4.42

 
 

 
 
Forfeited
(784,111
)
 
$
7.08

 
 

 
 
Balance outstanding, December 31, 2014
4,913,472

 
$
4.66

 
$
2,808,083

 
7.6 years
Granted
2,688,500

 
$
7.02

 
 

 
 
Exercised
(265,166
)
 
$
3.25

 
 
 
 
Forfeited
(388,176
)
 
$
7.75

 
 

 
 
Balance outstanding, December 31, 2015
6,948,630

 
$
5.45

 
$
5,903,466

 
7.8 years
Granted
3,246,250

 
$
5.02

 
 

 
 
Exercised
(1,335,271
)
 
$
3.81

 
 
 
 
Forfeited
(3,330,981
)
 
$
5.63

 
 

 
 
Balance outstanding, December 31, 2016
5,528,628

 
$
5.49

 
$
—

 
7.7 years
Vested and exercisable, December 31, 2016
2,415,461

 
$
5.18

 
$
—

 
6.4 years
Summary of Restricted Stock Unit Activity
A summary of the restricted stock unit activity is presented below:
 
Number of Shares
 
Weighted Average
Grant Date Fair Value
Per Share
 
Intrinsic
Value
Non-vested restricted stock units outstanding, December 31, 2015
—

 
$
—

 
 
Granted
402,000

 
$
4.06

 
 
Vested
(95,000
)
 
$
4.27

 
 
Forfeited
(35,000
)
 
$
3.99

 
 
Non-vested restricted stock units outstanding, December 31, 2016
272,000

 
$
3.99

 
$
571,200