Recently, technologies to culture one or more cell types in three dimensions have attracted a great deal of attention in tissue engineering. Particularly, the improved viability, self-renewal capacity, and differentiation potential have been reported for stem cell spheroids. However, it is crucial to modulate spheroid functions with instructive signals to use multi-cellular spheroids in tissue engineering. We have been developing ECM-mimicking fibrous materials decorated with cell-instructive cues, which were incorporated within 3D stem cell spheroids to fine-tune their functions as modular building blocks for bottom-up tissue-engineering applications. In particular, we created composite spheroids of human adipose-derived stem cells (hADSCs) incorporating nanofibers coated with instructive signal of either transforming growth factor-β3 or bone morphogenetic growth factor-2 for chondrogenesis or osteogenesis of stem cells, respectively. The bilayer structure of osteochondral tissue was subsequently mimicked by cultivating each type of spheroid inside 3D-printed construct. The in vitro chondrogenic or osteogenic differentiation of hADSCs within the biphasic construct under general media was locally regulated by each inductive component. More importantly, hADSCs from each spheroid proliferated and sprouted to form the integrated tissue with interface of bone and cartilage tissue. This approach may be applied to engineer complex tissue with hierarchically organized structure.

Heat generation during bone cutting operations inorthopaedics may cause thermal damage to the bone. During the bone cutting, the maximum temperature occurs on the contact surface between the bone and tool. Because of the low thermal conductivity and diffusivity, the temperature gradient of the bone interior is very high around the cutting site and the measurement of maximum temperature is difficult at the contact surface. While many researchers tried to measure the temperatures, they may have underestimated the temperatures of bone on account of measurement limitations. To solve this problem, we investigated the temperature distribution model of the bone interior during the milling operation and verified the model with a cutting experiment.

During the bone milling, most of the cutting energy is converted into the heat energy near the contact surface between the bone and tool. If the cutting tool moves on the bone surface, we can assume that a heat source moves on the bone surface at the speed of the feed rate. To predict the maximum temperature, we performed a milling experiment with fresh bovine cortical femurs. The feed rate were 2~9.8mm/s, the cutting depth were 0.3~1mm and the rotational speed were 30,000~50,000RPM. No irrigation solutions were applied. To measure the local temperatures around the tool, two infrared thermometers were attached behind the bur at 10mmintervals from the bur center. We calculated the maximum temperatures and errors from the measured temperatures.

The predicted maximum temperature increment was 55~131& #8451; as the cutting conditions change. The mean errors and standard deviation errors were several degrees. The increased feed rate and decreased cutting-depth reduced the maximum temperature.

Our observed temperature is quite higher than those in the previous studies. Because of the high temperature-gradient(57& #8451;/mm), the thermocouple alone will likely yield large errors and generally underestimate the temperatures of the bone interior. With a thermal damage criterion of 50& #8451;(& #916; T=13& #8451;), thermal damage may reach 1mm in depth. To reduce the thermal damage, it is recommended to increase feed rate and decrease cutting depth.