Hear that? It's the sound of tens of millions of dollars flowing into mobile health.

In July alone we reported on $138 million in investment deals, which included a whopping $61 million for ClearPractice's parent company, $35 million for a sleep device maker, and a $25 million investment promised by Dr. Patrick Soon-Shiong in his new joint venture with Toumaz. Most investments, of course, were still only about a couple million in size.

During the past three days, we've also seen two important investment deals for August: $50 million for appointment setting app ZocDoc and $3 million for iPhoneECG developer AliveCor.

While the trickle of investment dollars appears to be picking up, a new survey from health apps incubator Rock Health finds that many digital health entrepreneurs characterize fundraising as their biggest challenge.

Rock Health mashed up data from Capital IQ, CrunchBase, NVCA, and a survey it conducted of 110 entrepreneurs working at digital health startups. Overall, Rock concluded as we have that venture capital interest in digital health is "skyrocketing" since 2010 was a record year that saw 138 healthcare technology investment deals, up from 52 such deals in 2005. Rock Health already counts 35 investment deals of $2 million for digital health startups in 2011, but many others see fundraising as a challenge.

Of the 110 startups interviewed, only 13 percent had secured angel funding. Only 2 percent had venture capital funding. Most said they were "bootstrapping" their startups on capital of $50,000 or less.

Another interesting finding from Rock's survey: Early stage digital health entrepreneurs are building business-to-consumer (B2C) companies despite the majority of investment dollars flowing to business-to-business (B2B) ventures. Of those surveyed, 60 percent of the startups plan to have consumers pay for their product, while 30 percent said hospitals would, about 30 percent said insurers would, some 26 percent expect employers to pay, and finally some 22 percent plan to have advertisers pay for it. Obviously, some respondents had picked multiple payers.

Besides funding the Rock survey found that "establishing a sustainable revenue model" and "talent acquisition" were among the most pressing issues facing their business today.

It was often said that a lack of regulatory guidance from the FDA was causing some investors to hesitate to invest in the growing number of mobile health startups. Hopefully last month's guidance document from the FDA goes a long way to ease those fears and investment dollars continue to flow.