As exit options become scarce, angel investors are injecting more cash into fewer startups, a recent report shows.

According to the University of New Hampshire's Center for Venture Research, a total of 23,100 ventures have received angel funding so far this year, down 3.8 percent from the first half of 2007.

Over the same period, the total value of investments has increased by 4.2 percent to $12.4 billion, the report found.

Software, health-care and energy businesses are attracting the largest share of angel funding, while investments in biopharmaceutical firms are dropping off.

Nearly half of all angel investments this year have gone into seed and start-up stage funding.

So far this year, only six venture-backed companies have filed an initial public offering, the lowest number since the 1970s.