Most successful research fields will go through different stages of development before maturation and eventually gain general acceptance.

In the course of this development, it is important to periodically examine current progress, refocus goals, and explore new directions in the field. We believe the field of Cell-Printing (CP) has reached a stage when such an exercise is beneficial for all researchers involved.

A number of the CP techniques have reached certain successes in the laboratory and it is time to examine their current capabilities and limitations, and establish future goals and direction. This is the aspiration of the proposed book.

CP techniques have been developed to dispense cells in a controlled manner. In the first publication of successful mammalian CP, the author envisioned: "Potentially, multiple cell types can be placed at arbitrary positions with micrometer precision in an attempt to recapitulate the complex 3D cellular organization of native tissues."1 Since that time, many CP techniques have achieved the capability of placing multiple cell types at arbitrary positions with micrometer precision in two-dimensions (2D). This is an important achievement and a major milestone. However, the second part of the author s vision continues to elude us.

To recapitulate the complex 3D cellular organization of native tissues using CP is to conduct tissue engineering (TE). To engineer any tissue is a major endeavor in science, technology, and engineering. TE using CP requires 3D processing. A few CP techniques have demonstrated some 3D printing successes. But none have demonstrated the ability to print multiple cell types at arbitrary positions with micrometer precision in 3D. To achieve this capability will probably require new ideas, new materials, and advances in tissue biology as well as new technologies. Printing tissues and organs is a capability we should and need to achieve based on its potential application in science and especially medicine. The proposed book will be a venue for researchers from diverse backgrounds to showcase their work, address barriers ahead,

and brainstorm new trails towards achieving this capability.

TE is just one important goal to pursue for CP, and by no means the only one. CP techniques found application in other areas, for example, BioLP has been shown to produce protein arrays, sort cells, and microdissect malignant tissue. Developing applications beyond TE for CP techniques helps sustain CP development by attracting resources and recognition to the field. The proposed book will solicit ideas for potential applications for CP as well as review the applications

developed thus far.

The proposed book will consist of a collection of chapters from researchers in areas of CP and related fields. The chapters will be separated into three sections. The first section will be a review of the capability and development of established CP techniques and an introduction to any new CP techniques. The second section will focus on topics relating to achieving true 3D CP: ideas, strategies, materials, and technologies. The final section will focus on the applications of CP, both those already realized and those that hold potential for the future.

Cell and organ printing has become a hot topic of scientific pursuit. Since several early publications between 2000-2003 that demonstrated proof-of-concept, cell and organ printing has blossomed into a rich area for scientific exploration that is being performed by researchers across the globe. Research has thoroughly demonstrated that living cells can be printed via a number of actuations including electrospray, extrusion via micropens and ejection through photothermal, thermal or optical mechanisms. This topic has come of age and it is ripe for exploring the underpinnings of the research to date. We have included research that uses printing technology to deposit or guide cells for tissue engineering applications and for completeness, we have also included chapters describing bacteria printing, biomolecular printing that could be used to build growth factors or recruitment macromolecules into scaffolds, tissue microdissection, as well as live cell printing. The breadth of approaches includes 3D freeform fabrication, ink jet, laser guidance and modified laser direct write techniques. We hope that this book is not the final word but the first word, defining how these tools have been used to take the first steps towards the ultimate goal of creating heterogeneous tissue constructs. Only time will tell whether cell printers will truly become organ printers, but the technologies described in this book hold promise to achieve what the field of regenerative medicine requires - functional 3D scaffolds with multiple cell types differentiated into functional tissue!