  The scarcity of organs for transplant has led to large waiting lists of very
sick patients. In drug development, the time required for human trials greatly
increases the time to market. Drug companies are searching for alternative
environments where the in-vivo conditions can be closely replicated. Both these
problems could be addressed by manufacturing artificial human tissue. Recently,
researchers in tissue engineering have developed tissue generation methods
based on 3-D printing to fabricate artificial human tissue. Broadly, these
methods could be classified as laser-assisted and laser free. The former have
very fine spatial resolutions (10s of $\mu$m) but suffer from slow speed ( $<
10^2$ drops per second). The later have lower spatial resolutions (100s of
$\mu$ m) but are very fast (up to $5\times 10^3$ drops per second). In this
paper we review state-of-the-art methods in each of these classes and provide a
comparison based on reported resolution, printing speed, cell density and cell
viability.

Tissues and Organs;Biological Physics