issue engineering involves the development of biologi-cal

Chapter 7, Problem 7.84

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issue engineering involves the development of biologi-cal substitutes that restore or improve tissue function.Once manufactured, engineered organs can be implantedand grow within the patient, obviating chronic shortagesof natural organs that arise when traditional organ trans-plant procedures are used. Artificial organ manufactureinvolves two major steps. First, a porous scaffoldis fab-ricated with a specific pore size and pore distribution, aswell as overall shape and size. Second, the top surfaceof the scaffold is seeded with human cells that grow intothe pores of the scaffold. The scaffold material isbiodegradable and is eventually replaced with healthytissue. The artificial organ is then ready to be implantedin the patient.The complex pore shapes, small pore sizes, andunusual organ shapes preclude use of traditional manu-facturing methods to fabricate the scaffolds. A methodthat has been used with success is a solid freeform fab-ricationtechnique whereby small spherical drops aredirected to a substrate. The drops are initially moltenand solidify when they impact the room- temperaturesubstrate. By controlling the location of the dropletdeposition, complex scaffolds can be built up, one dropat a time. A device similar to that of 7.78 isused to generate uniform, 75-?m- diameter drops at aninitial temperature of Ti?150?C. The particles are sentthrough quiescent air at T??25?C. The droplet proper-ties are ??2200 kg/m3, c?700 J/kg?K.LTi = 150CAirT= 25C, VExit nozzleDroplet generatorTissue scaffold (a) It is desirable for the droplets to exit the nozzle attheir terminal velocity. Determine the terminalvelocity of the drops.(b) It is desirable for the droplets to impact the structureat a temperature of T2?120?C. What is the requireddistance between the exit nozzle and the structure, L