Yuchen Zhao

Jingqiu Ding

Jonathan Barés

Hu Zheng

Karola Dierichs

Achim Menges

Robert Behringer

Vibrational Collapse of Hexapod Packings

Columns made of convex noncohesive grains like sand collapse after being released from a confining container. However, structures built from non-convex grains can be stable without external support. In the current experiments, we investigate the effect of vibration on destroying such columns. The change of column height during vertical vibration, can be well characterized by stretched exponential relaxation when the column is short, which is in agreement with previous work, while a faster collapse happens when the column is tall. We investigate the collapse after the fast process including its dependence on column geometry, and on interparticle and basal friction.

Introduction

Jamming of granular matter is a wide-spread phenomenon [1,2]. One can see piles of sand and rocks, packing of apples and oranges in grocery stores. The jamming of soft, frictionless spheres has been studied in a detail [2], and yet new ways of jamming emerge constantly [3,4]. One of the recent explorations is the jamming of non-convex particles [5][6][7][8][9][10][11]. In a typical experiment, if one prepares a packing of the particles in a vertical tube and then removes the tube confinement, the cylindrical packing can retain its shape without confinement, and possibly sustain compression and shear [10]. This new phenomenon is not only interesting from the packing and jamming theory point of view [12][13][14][15], but also has significant implications, such as material and architectural design [16][17][18]. Open questions include: what are the physics that causes this jamming and how can one quantitatively describe the effect of particle shapes on the ability to jam and design new materials for applications based on particle level properties [19]. In particular, the work by Gravish et. al. [8] on staple packings is relevant here. Their experiment involves collapse of a column under external vibration. During collapse, the centroid height of the column follows a stretched exponential relation: [20]. Further, the entanglement between particles was quantified. It is interesting to ask whether the packing of other extreme shaped particles has a similar response to vibration.

h(t) = h(0) exp [-(t/τ) β ]
Here we studied the collapse of columns made with hexapod (star-shaped) particles under vibration of the base supporting the column (Fig. 1). In the first part of the paper, we present our experimental setup, preparation and data e-mail: yz172@phy.duke.edu characterizing the collapse of hexapod columns. In the second part, we confirm the applicability of a stretched exponential relation for collapse of hexapod columns under vibration. Although a stretched exponential describes the collapse dynamics where the column height is low enough, we observe deviation from this relation when the initial centroid height of the column is large enough. We also studied the effect of interparticle and basal friction on the stretched exponential relation. The final section provides a discussion and conclusions.

Experimental Set-up

We used two kinds of hexapods (Fig. 1 (a)), which are the same as those used in [10]. Each has the same hexapod shape: six equal-length arms with square cross-section lying symmetrically in the xyz direction. One kind is made of cast acrylic, which has a low friction coefficient, 0.4 ± 0.1, between any pair of particle arms. The other is made of laser-sintered white nylon PA2200, and has friction coefficient 1.0 ± 0.3. The arm tip-to-tip length is s = 20 mm and s = 25 mm for the acrylic and nylon hexapods, respectively. In the following we refer to the acrylic particles as low friction (LF) particles, and the nylon high friction (HF). Columns were prepared by first randomly pouring particles into a straight PVC tube (inner diameter D) from 40 cm above the base. The process was the same as [10]. Each tube was 30 cm tall. Then the tube was lifted vertically without disturbing the packing. The tube rested on either a Teflon surface, which had a friction coefficient 0.3 ± 0.1 between particles and the surface, or a sand paper surface, which had a higher friction coefficient such that particles only rolled when we tilted the surface. We refer to the Teflon and sand paper as low friction (LF) and high friction (HF) bases.

We prepared columns with different diameters D and initial heights, which were chosen to be around the critical height h c in our previous results [10]. Columns were placed on a vibration plate and we applied sinusoidal vibrations to induce collapse (See Fig. 2). The sinusoidal vibration had frequency f = 58.6±0.3 Hz and dimensionless vibration strength Γ = Aω 2 /g = 2.0±0.2, which were kept unchanged in all runs. On top of the plate, we glued either a Teflon sheet or sand paper as described earlier. The vibration generator consisted of an electromagnetic shaker that drove a 45 cm-by-45 cm square plate, that was screwed to a pair of square, aluminum tubes that strengthen the base. The driving sinusoidal signal was generated by a function generator, which was then amplified by a power amplifier. The shaker was cooled by air. We measured the vibration amplitude by imaging the vibrating plate via a highspeed camera (Photron FASTCAM SA5). The first peak of the vibration was large relative to later vibration, but contributed insignificantly to the packing compared to the total amount of energy from the vibration.

Electro--Magnetic Shaker

All the collapse dynamics were captured by high speed video imaging at 250 frame/s on the LF base and 60 frame/s on the HF base. Homemade Python codes were used to post-process the images and the centroid height h(t) of the packing in the images was found (Fig. 1). Because the packings were almost axially symmetric, the centroid height was close to the center of mass of the packing. We summarized the experiments in Table .1.

Results

Relaxation dynamics

In Fig. 1, we present several representative snapshots during column collapses on the LF base. Fig. 1 (b) to (e) describes the collapse process of an initially short column relative to its width. Particles on the edge of the column fell off within the first second, and formed a cone shape (Fig. 1(c)). Then, the cone shortened and widened slowly until there was almost a single layer of particles on the vibrating plate. Fig. 1(f)-(i) shows that a column with large h(0) collapsed differently. It took about 1.6 second for the column to gradually shorten and widen as in Fig. 1(f) and (g). After a cone was formed, the packing collapsed in a similar way as the initially short column. We also observed this difference with LF hexapod, although the first of the two-step process happened faster than HF, tall columns (data not shown).

In Fig. 3, we present the collapse dynamics of short columns and test the assumption of a stretched exponential relation for both HF and LF hexapods, and two different column diameters. By plotting the dimensionless centroid height h(t)/h(0) asln[h(t)/h(0)] vs. t. on a loglog scale, the assumption h(t) = h(0) exp [-(t/τ) β ] would be shown as a straight line. We found these collapses are well approximated by stretched exponentials. The exponent β is between 0.5 and 1, which is similar to previous work by Gravish et. al. [8]. However, the dependence of β on initial diameter or particle friction properties are not clear here. Pictorially, these collapse processes resemble Fig. 1(b)-(e). For HF hexapods, the collapse dynamics from two different diameters overlap within errorbars. The situation is not clear for LF hexapods. The characteristic relaxation times are τ = 3.0 s and τ = 1.3 s for HF and LF hexapods, respectively. These results agree with our previous study [10] showing that columns made of HF hexapod are more stable than LF ones.

Deviation from relaxation dynamics

As we increased h(0), however, we observed that the dynamics of h(t)/h(0) significantly deviated from a stretched exponential relation. As in Fig. 4(a), the data are well fitted to a straight line for low h(0), but gradually bend near time T b as the initial centroid height increases, and the collapse dynamics depends on h(0). Before T b , collapse is relatively fast, corresponding to a massive falling of those particles which constitute the top or middle of the column, while the bottom of the column retains its original shape (Fig. 1(g)). After T b , particles formed a cone-shaped pile and then slowly relaxed under vibration (Fig. 1(h,i)) as for short columns. We identify these bending points T b to be roughly the point where the cone-shape pile was formed and particles stopped rolling down from the top. One such snapshot at T b is Fig. 1(h). Such a two-step process happens for LF hexapods, as well as both hexapods on the HF base (not shown).

We do not have a specific form for the collapse before reaching T b . For data after the bending point, we took T b and h(T b ) at the bending point to be the new initial condition for the collapse, and fitted to a stretched exponential relation. The assumption behind shifting the starting point of the relaxation is that, there are two distinct collapse processes before and after T b . We observed experimentally that the collapse of a cone-shape pile (Fig. 1 (b-e,h andi)) could be stopped immediately after the vibration was turned off, while the collapse of taller columns could not (Fig. 1f andg, and it is also true for LF hexapod experiments). After T b , the pile configurations were more stable than the column. Our assumption is supported by the data shown in Fig. 4(b) for experiments N1. In Fig. 4(b) we fit the dynamics after T b for curve h(0)/s = 2.9. The choice of T b here corresponds to the moment when the cone was formed. We obtain similar fitting parameters as for the short columns (β = 0.8 and τ = 3.0 s) (Fig. 3), which indicates the collapse of the column after T b likely shares similar mechanisms as the collapse of short columns.

Effect of friction

Our assumption is further supported by experiments N3, e.g. HF particles on HF base, shown in Fig. 5 and insert. In the insert, the whole collapse dynamics are presented. The separation between fast collapse and slow relaxation is clear, and more apparent with increasing h(0), which agrees with the experiments on LF base. We define new time zeros at T b and fit the data to stretched exponentials, which are shown in the main figure. Overall, the good fits suggest that the collapse processes for cone-shape piles can again be approximated by stretched exponentials on the HF base. One difference is, that the characteristic relaxation times τ are 10 times larger than the ones for LF base, and the exponent β is systematically smaller, around 0.5 to 0.6. We attribute the increase of relaxation time to drag with the frictional base. A packing relaxes by expanding in the horizontal directions. On the frictional base, the horizontal expansion is retarded. Thus, under the same external vibration, the relaxation is slower.

Conclusions

In this paper, we quantitatively investigated the collapse dynamics of columns of hexapods (star-shaped particles) under constant sinusoidal vibration. For short and flat columns made of high or low friction particles, the dynamics of the centroid heights of the columns can be well fitted to stretched exponential relation for different column diameters. We also find that the inter-particle friction increases the characteristic relaxation time. The collapse dynamics departs from a stretched exponential when the column is tall initially. A tall column undergoes a two-step process, where initially it collapses fast into a cone-shape pile, and afterwards its relaxation is described using a stretched exponential fit with similar parameters as the short and flat columns. We also investigate the effect of friction on the base that supports the column. High friction surfaces increase the characteristic relaxation time significantly.

Figure 1 .

 1 Figure 1. (a) Nylon (left) and acrylic (right) particles and snapshots from two trials ((b)-(e) and (f)-(i)) in N1. The centroid height of the packing h(t) are measured and marked as a red dot in each image. (b)-(e): collapse of a column with h(0) = 11.6 mm. (f)-(i): collapse of a column with h(0) = 71.4 mm. Both columns were prepared with D = 102 mm and sat on a Teflon sheet base with low friction coefficient. A sinusoidal oscillation with frequency f = 58.6 ± 0.3 Hz and Γ = 2.0 ± 0.2 was applied.

Figure 2 .

 2 Figure 2. Schematic of the setup:The vibration generator consisted of an electromagnetic shaker that drove a 45 cm-by-45 cm square plate, that was screwed to a pair of square, aluminum tubes that strengthen the base. The driving sinusoidal signal was generated by a function generator, which was then amplified by a power amplifier. The shaker was cooled by air. We measured the vibration amplitude by imaging the vibrating plate via a highspeed camera (Photron FASTCAM SA5). The first peak of the vibration was large relative to later vibration, but contributed insignificantly to the packing compared to the total amount of energy from the vibration.

  h(t )/h(0)] h(0)/s=0.48, =0.82, =2.9 h(0)/s=0.56, =0.86, h(0)/s=0.4, =0.85, =0.90, h(0)/s=0.35,

Figure 3 .

 3 Figure 3. The negative ln[] of dimensionless centroid height h(t)/h(0) vs. time t on a log-log scale. Data from N1 and N2. Markers: triangle for frictional particles and circle for frictionless particles; filled symbols for D = 130mm and empty symbols for D = 102mm. Each data curve is an average of 2 or 3 repeated independent trials, with the errorbars indicate variation between trials. For fitting, we take 20 data points evenly spaced on the logarithmic time scale, and fit them into a straight line with the form: y(x) = βxβ ln(τ) which minimizes the least square error. The fitting parameters are in the legend of the plot, and the unit of τ is second. The change of number of points used in the fitting does not change only slightly change the fitting parameters but not our conclusions.

  h(t )/h(0)] h(0)/s = 0.48 h(0)/s = 1.1 h(0)/s = 2.1 h(0)/s = 2.9-ln[ h'(t )/h'(0)] ¡ = 0.81, ¢ = 3.5 h'(0)/s = 2

Figure 4 .

 4 Figure 4. (a): Relaxation of h(t) for columns with various h(0) and diameter D = 102 mm. Each data is an average of 2-3 independent, repeated trial. The data for h(0)/s = 0.48 is the same one used in Fig.3. The black star on the data h(0)/s = 2.9 indicates the T b , whose snapshot is shown in Fig.1(g). (b): Dynamics after the T b for the h(0) = data. Formally, We define h (t ) = h(t + T b ), (t >= 0) and fit a straight line into ln[h (t )/h (0)] vs t on a log-log scale, as in Fig.3.

7 ¡ 5 Figure 5 .

 755 Figure 5. Main: Collapse dynamics after T b for HF hexapod on HF base. Each color of triangles represents one h(0), as shown in the legend. Dashed lines indicate a stretched exponential relation. Insert: Collapse dynamics of these runs from the start of the sinusoidal vibration. The star markers indicate the choice of T b , which is around time of the cone-shape piles form.

Table 1 .

 1 Parameters used for vibration-collapse experiments: particle and base friction properties, column geometry.

	N Hexapod Bases D (mm) h(t = 0) (mm) D/s	h(t = 0)/s
	1	HF	LF	102, 130 12 → 72	4.1, 5.2 0.48 → 2.9
	2	LF	LF	102, 130 7 → 62	5.1, 6.5 0.35 → 3.1
	3	HF	HF	102	31 → 78	4.1	1.2 → 2.9
	4	LF	HF	102	14 → 66	5.1	0.7 → 3.3

Acknowledgments

We thank Dong Wang for useful discussions. Y.Z., and R.P.B. are funded by the U.S. National Science Foundation (grants NSF-DMR1206351 and DMS1248071), NASA (grant NNX15AD38G) and the W. M. Keck Foundation. K.D. would like to thank the ITASCA Education Partnership Program (IEP).


J Anita Lett

M Sundareswari

K Ravichandran

Amirdha Sher Gill

J Joyce Prabhkar

The Role of Cellulose in the Formulation of Interconnected Macro and Micoporous Biocompatible Hydroxyapatite Scaffolds 35

Keywords: bone tissue engineering, pure hydroxyapatite scaffolds, cellulose, porosity

In bone tissue engineering, ceramics are widely used as implant material to enhance bone growth formation or as drug release vehicle. In the existing work porous Hydroxyapatite scaffolds were prepared by polymeric replication method using Cellulose as a binding agent. The influence of binder on various sintering temperature were evaluated. The Hydroxyapatite scaffold sintered at 1150°C was characterized for phase purity, structural analysis and porosity measurements. Hence, it is possible to produce Hydroxyapatite scaffolds with highly inter connecting macro and micro pores with an apparent density of 0.944g/cm 3 corresponding to 75% porosity.

Introduction. Tissue engineering is a field where cells, bone/ scaffolds and signals/factors are mutually joined with the endeavour to re-establish, preserve and improve tissue and organ utility. Various scaffold resources have been investigated worldwide with both positive and negative outcomes. Factors for the failure of scaffolds include undesired scaffold degradation commodities affecting cellular functions, unaffected responses elicited by the scaffold materials themselves, lack of cell adhesion appropriate to non-suitable surface properties, controversies in the degradation rate of the scaffold and the growth rate of the fresh tissue, or mechanical mismatches connecting scaffolds and the tissue at the implantation location. Calcium phosphates are amongst the most widely used resources for bone tissue regeneration. They can be man-made as gels, pastes and solid blocks or even as porous matrices, with orthopaedics and dentistry being their main areas of relevance. Hydroxyapatite (HAP) are the most frequently used calcium phosphates, owed to their Stoichiometric ratio (Ca/P) ratios close to that of natural bone and also for their stability when in contact with physiological environment. HAP is a major constituent of bone resource and is resorbed after a long time in the body, due to its biocompatibility [START_REF] Woodard | The mechanical properties and osteoconductivity of hydroxyapatite bone scaffolds with multi-scale porosity[END_REF][START_REF] Harris | Open pore biodegradable matrices formed with gas foaming[END_REF][START_REF] Cyster | he influence of dispersant concentration on the pore morphology of hydroxyapatite ceramics for bone tissue engineering[END_REF][START_REF] Fu | Freeze Extrusion Fabrication of 13-93 Bioactive Glass Scaffolds for Bone Repair[END_REF]. The porous network or interconnected pores in HAP structure permit the tissue to penetrate, which further enhances the implant tissue attachment (Itoh et al). Several methods have been investigated to achieve the required porous scaffolds for instance, Sopyan et al has studied with pore-creating volatile particles, ceramic foaming methods and polymeric sponge process [START_REF] Sopyan | Porous hydroxyapatite for artificial bone applications[END_REF]. The polymeric sponge technique, which offers great flexibility, is particularly of interest due to its greater advantages such as opportunity to control the pore size, for several required complex shapes and straightforward process (Tian and Tian 2001).The polymeric sponge technique involves covering of open-cell polymeric foam with ceramic slurry followed by flaming out of polymeric foam in the course of sintering process which yields a duplication of the original polymer foam in the ceramic foam structure. However, the properties of the Hydroxyapatite scaffold prepared through the polymeric sponge technique are highly depend on the slurry properties together with homogeneity, rheology and dispersion (Zhang et al 2006). Monmaturapoj has reported the dispersant on the rheological behaviour of concentrated hydroxyapatite suspensions [START_REF] Monmaturapoj | Influence of preparation method on hydroxyapatite porous scaffolds[END_REF]. In this study, porous hydroxyapatite scaffolds were obtained using the polymer replication method, using cellulose as a binding agent and the morphology and its physio-chemical properties were studied. These scaffolds can be used as matrices for bone tissue engineering or as specific release vehicles [START_REF] Chu | MecCalcium biomineralization in the radular teeth of the chiton, Acanthopleura hirtosahanical and in vivo performance of hydroxyapatite implants with controlled architectures[END_REF]. Also, they may be functionalized with molecules such as collagen, chitosan, etc, in order to enhance their biological responses [8].

Material and Methodology.

Sample Preparation. The Hydroxyapatite powder with an average crystallite size of approximately 40 nm, was prepared according to our previous work by sol-gel [START_REF] Lett | Porous hydroxyapatite scaffolds for orthopedic and dentalapplications -the role of binders[END_REF].The template used to prepare scaffolds were polyurethane sponges with an average pore size of 400 microns. The Cellulose, used as a binder was made into a homogenous mixture by mixing for 2 hr with water at 50 ºC. To fabricate scaffolds, a ceramic slurry containing 6 g of Hydroxyapatite powder, binder, tensioactive agent (A40 V Dispex provided from BASF) and water [START_REF] Monmaturapoj | Influence of preparation method on hydroxyapatite porous scaffolds[END_REF] were blended again for 2 hours to disperse thoroughly. The polyurethane sponges were dipped in the slurries and the excess slurry was removed and dried at room temperature for 2 days, followed by drying in the hot air furnace at 110 º C until its completely dried. The polyurethane sponge used as template was removed by heating in a muffle furnace at 600 o C for 2 hrs, followed by the densification of scaffold by sintering at 1150 o C for 4 hours. Now, these prepared scaffolds were cut into cubes for further characterization.

Physio-Chemical Characterization. The microstructure of the hydroxyapatite scaffolds was characterized using scanning electron microscopy (FESEM: Supra VP35 Carl Zeiss, Germany) and the macro porous structure using optical stereo zoom microscope. The phase purity of the Hydroxyapatite scaffolds were determined by X-ray diffraction using a X'pertPro, Philips, The Netherlands with CuKα radiation over the 2θ range of 10°-80° with a step size of 0.05°. The functional group analysis of HAP scaffolds was carried out in the spectral range from 4000 to 650 cm -1 using a single beam Fourier transform infrared spectrometer (Agilent, Cary 630). The porosity and density measurements of the scaffolds were calculated by simple displacement techniques [8][START_REF] Lett | Porous hydroxyapatite scaffolds for orthopedic and dentalapplications -the role of binders[END_REF]. A scaffold of weight 'W' was immersed in a graduated cylinder containing a known volume (V1) of water until no air bubble emerged from the scaffold. The total volume of the water and scaffold was then recorded as V2. The volume difference (V2 -V1) was the volume of the skeleton of the scaffold. The scaffold was removed and the residual water was measured as V3.

The apparent density of the scaffold (ρ), was evaluated using,

3 2 V V W    (1)
The porosity of the open pores in the scaffold (ε), was evaluated using,

3 2 3 1 V V V V     (2)
Results and Discussions. The nano-HAP powder used for fabricating scaffolds processed an elongated cylindrical shape with the crystallite size of 40 nm. The scaffolds were approximately 10 mm x 10 mm x 10 mm in size. The morphology of the fabricated Hydroxyapatite is shown in Fig. 1, 2. It is found that the scaffolds replicated the pores in the sponges with a pore size of approximately 500 microns. Thus, highly porous Hydroxyapatite scaffolds were produced using the polymer replication method as seen in Fig. 2.With SEM observations (Fig. 2), pore diameters ranging from 400 to 500 μm were observed. On the other hand, micropores (Fig. 3) of size lesser than micron were also visualised in the pore walls and wall struts. The microstructures of the scaffolds (Fig. 2) could be observed that the scaffold had a compact structure and that the pores were evenly distributed. The open as well as interconnected pore network was an essential factor for the scaffold to permit cell growth and the transportation of nutrients and metabolic waste. Gotz et al. [START_REF] Gotz | Effect of surface finish on the osseointegration of laser-treated titanium alloy implants[END_REF] has reported that pore sizes around 300 µm were suggested for implants due to improved new bone and capillary formation. Hollister et al [START_REF] Hollister | Engineering craniofacial scaffolds[END_REF] conducted in vivo studies on HAP scaffolds with pore diameters ranging between 400 µm and 1200 µm and inferred no significant difference in bone growth for scaffolds of all pore sizes. The XRD patterns of the scaffolds prepared is shown in Fig. 3. All of the peaks matched with the JCPDS pattern 09-0432 for HAP, which suggested that no other phases were present, as shown in Fig. 3. All of the peaks were attributed to the HAP phase and no additional peaks were observed. The results indicated that the HAP did not decompose after sintering. The characteristic peaks at two theta 31.7° corresponding to 211 diffraction became narrower and sharper for sintering temperature 1150°C. These data confirms that the major phase as Hydroxyapatite and absence of impurity such as calcium phosphates and calcium oxide are clearly identified [22] .

Fig. 4. FTIR pattern of Sintered Scaffold.

The FT-IR spectra of the synthesized HAP scaffold prepared using cellulose as a binding agent is shown in Fig. 4. In the FTIR spectra, the bands at 3570 and 630 cm -1 [START_REF] Kazemzadeh Narbat | Fabrication of Porous Hydroxyapatite-Gelatin Composite Scaffolds for Bone Tissue Engineering[END_REF] were recognized to the hydroxyl stretching bands and bending bands of HAP, respectively. The broad absorption band from 3600 to 3300 cm -1 [START_REF] Kazemzadeh Narbat | Fabrication of Porous Hydroxyapatite-Gelatin Composite Scaffolds for Bone Tissue Engineering[END_REF] indicated the existence of the bending mode of absorbed water [START_REF] V´azquez | Revista Mexicana De Fi´Sica[END_REF]. The bands at 1093 and 1041 cm -1 were assigned antisymmetric ʋ3 [PO4 3-] P-O stretching mode and the ʋ1 P-O symmetric stretching mode was detected at 962 cm -1 [START_REF] Kazemzadeh Narbat | Fabrication of Porous Hydroxyapatite-Gelatin Composite Scaffolds for Bone Tissue Engineering[END_REF][START_REF] Kuriakose | A novel low temperature sol-gel synthesis process for thermally stable nano crystalline hydroxyapatite[END_REF][START_REF] Evans | Calcium biomineralization in the radular teeth of the chiton, Acanthopleura hirtosa[END_REF]. The bands at 603 and 569 cm -1 were attributed to components of the triply degenerate ʋ4 O-P-O bending modes.

The Scaffold prepared using cellulose showed high porosity of 75% with apparent density of 0.944 g/cm 3 . The mechanical properties are strongly subjective by apparent density [START_REF] Evans | Calcium biomineralization in the radular teeth of the chiton, Acanthopleura hirtosa[END_REF]. In trabecular bone, the apparent density ranges from 0.14g/cm 3 to 1.10g/cm 3 [START_REF] Evans | Calcium biomineralization in the radular teeth of the chiton, Acanthopleura hirtosa[END_REF]. Porosity is based on the presence of open pores. It was found that mechanical properties varied with binders used. Moreover, a few studies [START_REF] Li | The effects of sintering temperature andpressure on the sintering behavior of hydroxyapatite powder prepared by spark plasma sintering[END_REF][START_REF] Khalil | Consolidation andmechanical properties of nanostructured hydroxyapatite-(ZrO2 1 3 mol% Y2O3) bioceramics by highfrequency induction heat sintering[END_REF] have reported that the decomposition of HAP would reduce its mechanical properties. Hence, a symmetry is to be maintained between the porosity and apparent density for precise purposes, since higher mechanical strength corresponds to higher density, while a high porosity provides a surrounding favourable for living organism [START_REF] Gu | The Role of Cellulose in the Formulation of Interconnected Macro and Micoporous Biocompatible Hydroxyapatite Scaffolds[END_REF].

Summary. In this work, porous scaffolds were prepared using a polymeric sponge template method using cellulose as a binding agent. A well defined elongated cylindrical HAP crystals with negligible agglomeration was used to fabricate these scaffolds. The FESEM results exposed that the porous Hydroxyapatite scaffolds acquired macro pores and micro pores that emerges to be interconnected with a homogenous porous network (Fig. 3).The scaffolds comprises pure crystalline Hydroxyapatite phase and no additional phase were produced through the spongy technique as confirmed by XRD and FTIR. The scaffold prepared with cellulose in spite of high porosity (75%) appeared to report an apparent density of 0.944 g/cm 3 comparable with trabecular bone (0.14 g/cm 3 to 1.10 g/cm 3 ). Thus, it is possible to produce porous scaffolds with varied porosity and density. Such scaffolds can find its application for tissue engineering in non load bearing applications or even as a vehicle for the delivery of biological molecules. Currently, studies are being performed in order to incorporate collagen type I in these porous constructs, to improve their potential applications.

Fig. 1 .

 1 Fig. 1. Macro pores of Scaffold.

Fig. 2 Fig. 3 .

 23 Fig. 2 Micro pores of Scaffold.

© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/