## 2 Results and Discussion
### Drug Release from the Polyurethane/Hydroxyapatite Composites  
CLO was incorporated into the PU-HA composites by immersing the material in an aqueous drug solution of a known concentration. The drug content in the PU-HA composites was 1% wt. _In vitro_ CLO release from the PU-HA composites was conducted in a phosphate buffer solution (PBS) buffer at 37 \({}^{\circ}\)C for 1-8 weeks. The kinetic rates of CLO released from the obtained biomaterials at pH 7.4 are shown in Figures 7 and 8. Two factors could influence the release of CLO from the obtained PU-HA composites, namely the \(M_{n}\) of the PCL diols used in PU synthesis and the \(P\) of the composites. **PU-HA-1** and **PU-HA-2** released CLO faster compared to **PU-HA-3** and **PU-HA-4** or **PU-HA-5** and **PU-HA-6** (Figures 7 and 8). The percentage of CLO released was about 81% for **PU-HA-1**, 64% for **PU-HA-3** and 36% for **PU-HA-5** after eight weeks of incubation. In comparison, the percentage of CLO released after eight weeks of incubation was around 71% for **PU-HA-2**, 44% for **PU-HA-4** and 27% for **PU-HA-6**. It was found that the rate of CLO release increases with increasing the \(P\) and decreasing the \(M_{n}\) of PCL used in PU synthesis. The \(P\) of the PU-HA composites decreases with increasing HA content. It was already known that the hydrolytic stability of PU-HA composites and CLO release from matrices depend on numerous factors, such as composition, kind of hard or soft segments, the crystallinity, and the size and form of the crystallite of the PU, _etc._ However, it seems that in our study the main influence on this property has a kind of polyols type soft segments. The degradation tests of the obtained PUs or PU-HA composites and the kinetic rates of the CLO released were conducted in the same manner and under the same conditions. The results directly comparing CLO release with the \(M_{v}\) of the PUs (Table 4) or the mass loss (\(WL\)) of the PU-HA matrices studies follow the same trend (Figures 9 and 10). _In vitro_ degradation of the synthesized PUs was controlled by the change of the mechanical properties and the \(M_{v}\). The \(M_{v}\) of the obtained PUs were determined after four and eight weeks of degradation (Table 4). The changes in the \(M_{v}\) for the obtained PUs were around 4.7%-6.0% after four weeks and 7.2%-11.4% after eight weeks. **PU-1** degraded faster in comparison to **PU-2** and **PU-3**. The above parameters are in a good agreement with the results of the kinetic rates of CLO release from the obtained PU-HA composites. Furthermore, the kinetic rates of CLO release are in agreement with the change in the mechanical properties of the PUs and the _in vitro_ degradation of the produced PU-HA composites. After eight weeks' degradation process, the **PU-1** obtained from **PCL-1** retained around 76% of the original value of _F\({}_{S}\)_, 84% of the value of _Sh\({}_{H}\)_ and 81% of the value of \(\varepsilon\) (Tables 1 and 5). The changes of these parameters were clearly smaller for the PUs obtained from **PCL-2** and **PCL-3**, which confirms earlier reports of the higher hydrolytic stability of PUs containing longer polyester units. **PU-3** retained around 88% of the original value of _F\({}_{S}\)_, 93% of the value of _Sh\({}_{H}\)_ and 91% of the value of \(\varepsilon\) (Table 5). _In vitro_ degradation of the obtained PU-HA composites was controlled by the _WL_ of the materials. The results are shown in Figures 9 and 10. The _WL_ values of **PU-HA-1** and **PU-HA-2** were 22% and 12% after eight weeks of degradation, respectively. However, for **PU-HA-5** and **PU-HA-6** the _WL_ was 9% and 3% (after eight weeks). The _WL_ values of PU-HA increased slowly with increasing hydrolytic degradation time. These results correlate well with the change in the mechanical properties of the PUs.