Stockholders' equity (Tables)

3 Months Ended

Mar. 31, 2016

Stockholders' Equity Note [Abstract]

Warrant and option activity

	For the three months ended
	March 31,
	2015			2015
Beginning balance outstanding		26,397,410			23,933,922
Warrants issued during the year:
Expired during the year		(546,667	)		(73,530	)
Exercised during the year		(12,270,853	)		—
In connection with issuance of convertible notes		—			750,004
In connection with tender offer exercise		26,479,217			—
Ending balance outstanding		40,059,107			24,610,396

Options and warrants outstanding

The numbers and exercise prices of all options and warrants outstanding at March 31, 2016 are as follows:

Shares Outstanding	Weighted Average Exerecise Price		Expiration Fiscal Period
85,833		9.00	2nd Qtr, 2016
625		9.00	3rd Qtr, 2016
475,142		7.54	4th Qtr, 2016
34,749		7.99	1st Qtr, 2017
1,265,850		6.86	2nd Qtr, 2017
536,500		1.14	3rd Qtr, 2017
1,162,088		6.45	4th Qtr, 2017
10,000		6.60	1st Qtr, 2018
750,000		0.25	4th Qtr, 2018
35,063,685		0.38	1st Qtr, 2019
883,336		0.91	2nd Qtr, 2019
1,113,000		1.16	3rd Qtr, 2019
2,608,000		1.15	4th QTR, 2019
505,000		0.92	1st Qtr, 2020
75,000		0.65	2nd Qtr, 2019
1,707,780		0.19	4th Qtr, 2020
6,836,016		0.12	1st Qtr, 2021
133,334		7.05	4th Qtr, 2020
53,245,938

Stock based compensation assumptions used

The following weighted average assumptions were utilized for the calculations during the three months ended March 31, 2016 and 2015:

	For the three months ended
	March 31,
	2016			2015
Expected life (in years)	2.87 years			2.87 years
Weighted average volatility		126.09	%		108.81	%
Forfeiture rate		17.49	%		13.95	%
Risk-free interest rate		.96	%		.96	%
Expected dividend rate		0.00	%		0.00	%