# Designing Hydrogel-Based Bone-On-Chips for Personalized Medicine
## 4 Hydrogel-Based Bone-On-Chips
### Cell Interactions  
The selection and the number of different cell types to introduce in a BOC system includes, but it is not limited to, cells normally residing in the bone tissue, while the interaction with other tissues requires the use of different cell sources. The cellular component of the bone matrix is usually represented by osteoblasts or osteocytes--while MSC and osteoclast precursors are included to study bone remodelling or to create a bone marrow niche. MSCs are routinely obtained from bone marrow or adipose tissue, and used as osteoprogenitor cells in organ-on-chip mimicked the mineralizing microenvironment [95,102] or osteochondral bone formation [94]. On the other hand, MSCs were maintained in vitro to recapitulate the bone marrow microenvironment. Indeed, once seeded in a bone-marrow-on-chip with CD34+ cells (hematopoietic stem cells), it has supported white and red blood cells differentiation over 4 weeks of culture while improving the maintenance of CD34+ progenitors over traditional culture methods [16] (Figure 3A). A different approach from the traditional isolation and seeding of primary human cells has been proposed to recapitulate the physiology of bone marrow. Briefly, it consisted of a prior in vivo implantation of an hydrogel-based bone microenvironment. After 8 weeks, the hydrogel was populated by \begin{table}
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**Goal** & **Cells** & **Chamber** & **Hydrogel** & **Main findings** & **Ref** \\ \hline Bone marrow physiology & Subcutaneous implantation prior to in vitro culture & Cylindrical chamber for the explant and perfusion channels & Collagen I (3.0) with DBP, BMP2, BMP4 & Reconstitution ofhematopoietic niche physiology and response to radiation toxicity & [101] \\ \hline Bone marrow pathology & CD34+ cell BMSC HUVEC & Apical and basal channels separated by membrane & Mix of fibrin (5.0) and collagen I (0.2) & Prediction of clinically observed haematopoiesis and representation of Shuachman-Diamond syndrome & [16] \\ \hline  & SUP-B15 BMSC HOB & 4 channels & Collagen I (2.0 and 4.0) & Bone marrow microenvironment enhances leukemia cell survival during drug treatment & [103] \\ \hline  & leukemic cell (U937, HL60 or K562) HUVEC HS5 & Gel channel, leukemic channel and endothelial channel & Collagen I (2.5 and 5.0) & Angiogenesis in bone marrow promotes leukemia cell proliferation and survival & [104] \\ \hline Vascular bone tissue & HUVEC LF & 4 parallel channels separated by microposts & HA (0.05–0.5\%)/ fimbin (-) composite & HA enhances sprouting speed and increased lumen size, sprout length and number & [100] \\ \hline Cell migration & HOB & 1 channel & Collagen I (4.0) & Interstitial fluid flow alters hydrogel microstructure and cell migration & [84] \\ \hline  & HOB & 1 channel & Collagen I (4.0) & The inhibition of ECM degradation reduces cell motility & [85] \\ \hline  & hBM-MSC & 1 channel & Mix (1:1) of collagen (2.0) and Matrigel & Dose-dependent chemoattractant ability of different chemokines & [87] \\ \hline \end{tabular}
\end{table} 
Table 1: Cont. multiple hematopoietic cells and it was cultured in a microfluidic device after explatation to recapitulate a hematopoietic niche in vitro. Bone-marrow-on-chips and whole marrow from live mice showed comparable resistance to radiation, unlike stroma-supported cultures on a dish [101]. In view of modeling the last stages of osteogenesis, HOB can be isolated from trabecular or cortical bone tissue. It has been showed that HOBs cultured in BOCs experience the specific changes in cellular morphology, protein secretion and proliferation observed in vivo [88] (Figure 3B). Due to the technical complexity of isolating primary human osteocytes from the mineralized bone, the murine osteocyte cell line MLO-Y4 was recently used in microfluidic systems to investigate the mechano-regulatory action of osteocytes [91].